According to reference , heavy metals are naturally occurring elements that have a high atomic weight and a density at least 5 times greater than that of water. Their multiple industrial, domestic, agricultural, medical and technological applications have led to their wide distribution in the environment; raising concerns over their potential effects on human health and the environment. Their toxicity depends on several factors including the dose, route of exposure, and chemical species, as well as the age, gender, genetics, and nutritional status of exposed individuals. Because of their high degree of toxicity, arsenic, cadmium, chromium, lead, and mercury rank among the priority metals that are of public health significance. These metallic elements are considered systemic toxicants that are known to induce multiple organ damage, even at lower levels of exposure. They are also classified as human carcinogens (known or probable) according to the U.S. Environmental Protection Agency, and the International Agency for Research on Cancer.
Toxic metals are not usually found in elemental form, but occur ubiquitously in nature as more or less soluble salts or hydroxide compounds. High concentrations can be found in water, in sediments, and in foods derived from plants or animals. This chapter includes the following group of elements that are toxic to humans: aluminum (Al), arsenic (As), lead (Pb), cadmium (Cd), mercury (Hg), and thallium (TI). Their physiological function is largely unknown.
Toxic metals accumulate in the organs to varying degrees following high-dose exposure as a result of differences in absorption, distribution, and excretion. The toxicologic kinetics of these metals vary from element to element, but are based mainly on the properties of individual compounds, such as their water solubility .
Suspected acute or chronic intoxication:
- Absorption of a high dose of a toxic element (e.g., in a pesticide, dye, or cleaning product) either accidentally or with suicidal or criminal motives
- Occupational-related exposure in areas metal production and metal processing
- Chronic exposure in highly contaminated regions.
Intoxication can be measured by determining the blood concentration and the urinary excretion of the metal. Occupational and environmental medicine take only an upper threshold but not the toxic limit into account when evaluating exposure. Deficiencies are not clinically relevant. Because metals accumulate in red cells, their concentrations are usually higher in whole blood than in serum or plasma. Plasma should be used in preference to serum to avoid the risk of contamination by elements that have accumulated within cells being released during coagulation. Blood should be collected using metal-free, lithium heparinate blood collection tubes.
The analysis of hair and nail samples can be of some use in toxicological or forensic investigations since toxic elements accumulate in these samples to varying degrees.
In the routine laboratory setting, arsenic, lead, cadmium, mercury, and thallium are often analyzed using direct electrothermal atomic emission spectrometry (ETAES). When ETAES is used to determine the concentration of mercury or arsenic, mercury is first enriched using the cold vapor technique and arsenic is first enriched using the hydride technique. Inductively-coupled mass spectroscopy (ICP-MS) is highly sensitive.
Each toxic metal has unique features and physic-chemical properties that confer to its metal-induced toxicity and carcinogenicity. Because toxic metals accumulate in organs, organ biopsies and renal excretion respectively provide the most powerful and second most powerful confirmation of exposure. However, organ biopsies are seldom ethically acceptable.
Well regulated uptake and excretion mechanisms for essential trace elements are sometimes also used by toxic metals. In aluminum, lead, nickel, or cobalt intoxication, the divalent metal ion transporter of the enterocytes and/or its internal transport and storage protein metallothionein are used, which can reduce the absorption of iron and copper and lead to anemia . The presence of lead inhibits the incorporation of iron into the heme molecule. The injury caused by toxic doses of metals with unknown functions can therefore be due to interactions with essential trace elements.
The direct effects of toxic metals are due to the triggering of oxidative reactions, the formation of complexes with proteins (e.g., via sulfhydryl groups) or the binding to active sites of enzymes and inhibition of enzyme activity. Pathological effects appear primarily in the form of micro angiopathies that cause typical toxic symptoms such as glomerular nephropathies, encephalopathies, stomatitis, and intestinal disturbances ().
The degree of tissue damage depends on whether the element is absorbed by an organ, whether it is excreted again quickly or stored, and where it is stored. Mercury and cadmium accumulate primarily in the kidneys, whereas lead accumulates in the bones and arsenic accumulates in the liver. Fertility disorders associated with exposure to heavy metals are also becoming increasingly common .
Hair analysis is not recommended for the determination of toxic elements since the correlation between the hair concentration and organ content is low for many elements. Furthermore, validated reference methods and generally accepted reference intervals do not exist for hair analysis and the results show limited reproducibility. Therefore, hair analysis is only useful for obtaining a rough estimate of exposure .
2. Brätter P, Heseker H, Kruse-Jarres JD, Liesen H, Negretti de Brätter V, Pietrzik K, Schümann K. Mineralstoffe, Spurenelemente und Vitamine: Leitfaden für die ärztliche Praxis. Gütersloh; Bertelsmann Stiftung 2002: 88.
5. Köppel C, Hallbach J, von Clarmann M, Ibe K. Klinische Interpretation. In: Gibitz HJ, Schütz H (eds). Einfache toxikologische Laboratoriumsuntersuchungen bei akuten Vergiftungen. Mittlg DFG Senatskommission für klinisch-toxikologische Analytik 1995/XXIII: 135–9.
6. Deutsche Forschungsgemeinschaft. Maximale Arbeitsplatzkonzentrationen und biologische Arbeitsstofftoleranzwerte. DFG-Senatskommission zur Prüfung gesundheitsschädigender Arbeitsstoffe, Mittlg XXI, Weinheim: Verlag Chemie, 1985.
Al is a ubiquitous element, comprising 8.1% of the earth’s crust. It is the third most abundant element, following oxygen and silicon, and the most abundant metal (even more abundant than iron).
Al tends to donate its three valence electrons to form colorless Al3+cations. These cations are hydrated in water [Al(H2O)6]3.. Al reacts quickly with air and water to form aluminum oxide and hydrogen. The elemental Al does not exist in pure state but is always combined with other compounds such as hydroxide, silicate, sulphate, and phosphate. The most important Al-containing silicon minerals are potassium feldspar (orthoclase) K(AlSi3O8), sodium feldspar (albite) Na(AlSi3O9), calcium feldspar (anorthite) Ca(Al2Si2O8), potassium mica (muscovite) KAl2(AlSi3O10)(OH,F)2 and cryolite Na3AlF6. The wide distribution of Al ensures the potential for causing human exposure and harm.
Suspected Al toxicity:
- Evaluation of occupational exposure in the context of diseases affecting the lower respiratory tract and lungs
- Monitoring individuals exposed to Al, such as workers in aluminum-processing plants
- Monitoring dialysis patients on Al medications (phosphate binders).
- Serum, plasma: blood collection with metal-free collection devices: 1 mL
- 24 h collection of urine or random specimen of urine with reference to creatinine excretion: 5 mL
Conversion: μg/L × 0.0371 = μmol/L
Al occurs naturally in the environment, foodstuffs, and drinking water, but it is also used in processed foods, materials (aluminum containing food packaging, aluminum foils, cooking utensils and banking trays), cosmetic products and drugs .
Only about 0.1% of orally ingested Al is absorbed from the gastrointestinal tract. The tolerable weekly intake set by the European Food Safety Authority is 1 mg aluminum/kg body weight in adults and up to 2.3 mg/kg in children. Exceeding these values does not mean that there is an acute health hazard. The levels are designed to be precautionary and long-term values for the general population .
Although Al has no biological function, a correlation exists between its local concentration in tissues and the degree of dysfunction it causes. In clinically healthy individuals with normal exposure to Al, the highest concentrations of Al are found in the spleen, lungs, liver, and bones. Other tissues contain 0.3–0.8 mg/kg wet weight, which is around 100–300 times higher than the Al concentration in the plasma .
If renal function is normal, Al does not accumulate in the body even following increased intake. Plasma concentrations of up to 10 μg/L are still considered normal; however, the typical signs of Al intoxication can be absent at significantly higher concentrations.
The US Food and Drug Administration published a recommendation in 2004 regarding the contamination of parenteral nutrition. According to this recommendation, patients receiving parenteral nutrition should receive no more than 5 μg Al/kg body weight per day.
The urine accounts for > 95% of excreted Al. Reduced renal function increases the risk of Al accumulation in the very young, elderly and diseased individual.
Urinary Al is the main criterion used to evaluate occupational Al exposure. A concentration of 300 μg/L (11.1 μmol/L) is the biological tolerance value. It is evaluated in combination with a clinical evaluation of neurotoxic symptoms. Workers can be exposed to high levels of Al in the workplace in the following situations, for example:
- Extraction of metallic Al from bauxite or kaolin
- Production of corundum from bauxite with inhalation of Al dust and vapor
- Al processing in many branches of industry (e.g., Al welding).
The plasma Al concentration can be used as a criterion for assessing the Al burden (). Toxicity is known to occur at concentrations > 100 μg/l, although neurotoxic symptoms may occur at > 50 μg/L. Studies on Al welders revealed that the content in welding fumes correlated with Al concentrations in plasma and urine.
There is no correlation of Al exposition and the urinary Al excretion in patients with decreased renal function. In dialysis patients, for example, there is no correlation between the accumulation of Al in the bones, neurotoxicity and the plasma concentration . High acute exposure to Al is basically due to the use of Al compounds in medicines (antacids, phosphate binders, buffered aspirin, vaccines, allergen injections). Inadvertent Al contamination has been reported in infusion solutions, mainly human albumin . In addition to occupational exposure, chronic Al exposure can result from:
- Storing and preparing acidic foods in Al-containing cooking utensils
- Feeding infants with soy-based formula
- Regularly drinking water from Al-rich soils.
The toxic dose of Al is 5 g; the lethal dose is unknown . The accumulation of Al is in the order liver > kidney > brain when mice were exposed to Al sulphate in drinking water. The most important toxic effects of Al are neurotoxicity, bone disease, anemia, and pulmonary fibrosis . Ingestion of aluminum phosphide causes acute fatal poisoning. Diseases and symptoms associated with Al intoxication are listed in .
Blood should be collected using plastic heparin monovettes. Specimens collected in glass tubes must be processed and transferred to plastic containers within 1 h of collection to minimize alteration of Al concentration . Contamination from other sources of this ubiquitous element when collecting, storing, and processing samples must be avoided.
Chronic excessive intake of Al is also reflected in the hair. When hair is used as a specimen, however, contamination during sample preparation cannot be distinguished from toxic Al concentrations.
In a study many serum Al concentrations in the toxic range were not confirmed after retesting. Serum Al concentrations of 60 μg/l or greater were considered false positives and not indicative of chronic toxicity if another specimen retested within 45 days had a concentration below 20 μg/l.
Because Al occurs mainly in silicon compounds as water-insoluble complexes, its bio availability is greatly reduced. At pH 3–8, its amphoteric character allows it to be transformed from free Al3+ into [Al(OH)4]–. At pH 7.4, Al exists as barely soluble Al(OH)3, which is significantly more soluble at acidic pH. Because it can bind to oxygen, nitrogen, and phosphate atoms, Al reacts well with biological macromolecules such as proteins.
The body stores 30–50 mg of Al: 50% in the bones, 25% in the lungs, and 1% in the brain . The oral bio availability of Al is around 0.3% from water and 0.1% from the diet. The main absorption site for Al is the proximal part of the small intestine. The absorption of Al, including Al(OH)3, is inhibited by phosphates in food. Al bound to citrate, on the other hand, is absorbed more easily. This is why using antacids and drinking fruit juice at the same time can lead to a significant acute Al burden.
In the plasma, Al competes with iron to bind to the transport protein apotransferrin. Up to 90% of the Al is bound to transferrin; the rest is bound to citrate. Like iron, Al is taken up by cells via the transferrin receptor and bound to proteins in the cytoplasm. The tissue Al concentration is 0.3–0.8 mg/kg wet weight and increases with age. Al not incorporated into the tissues is eliminated by the kidneys. Uremic hemodialysis patients have increased plasma Al concentrations and increased amounts of Al in the bones and liver.
The brain has a lower Al concentration than many other tissues, because Al transport out of brain extracellular fluid occurs by mono carboxylate transporter or transferring mediated endocytosis and glutamate transporter . However, this system can become overburdened due to a Al intake, which leads to increased accumulation of Al in the brain (normal brain concentration is no more than 0.25–0.75 mg/g wet weight).
At cellular level, Al accumulates in mitochondrial membranes, in the reticuloendothelial system of the liver and spleen, and in lysosomes.
Al can displace iron from its functional bonds in enzymes and coenzymes. There is some overlap between the symptoms of Al intoxication and the symptoms of iron intoxication (hemosiderosis).
The toxic effects of Al exposure are thought to be due to reduced mitochondrial function caused by oxidative stress. This has a negative impact on the respiratory chain, in particular, complex I and ATP synthesis. Nucleotide content is also reduced by the inhibition of nucleotide translocase and the increased lipid peroxidation in Al intoxication causes mitochondrial swelling.
1. Tahán JE, Granadillo VA, Romero RA. Electrothermal atomic absorption spectrometric determination of Al, Cu, Fe, Pb, V and Zn in clinical samples and certified environmental reference materials. Anal Chim Acta 1994; 295: 187–97.
3. European Food Safety Authority. Safety of aluminum from dietary intake, scientific opinion of the panel of food additives, flavourings, processing aids and food contact materials. ESFA J 2008; 1–34.
12. Hellstroem HO, Mjöberg B, Mallmin H, Michaelsson K. No association between aluminium content of trabecular bone and bone density, mass or size of the proximal femur in elderly men and women. BMC Muskuloskelet Disord 2006; 7: 69.
13. Boukhari LV, Nacher M, Goulle JP, Roudier E, Elguindi W, Laquerriere G. Plasma and urinary aluminium concentrations in severely anemic geophagous pregnant women in the Bas Maroni region of French Guiana: a case control study. Am J Tro Med Hyg 2010: 83: 1100–5.
14. Kiesswetter E, Schäper M, Buchta M, Schaller KH, Rossbach B, Kraus T, Letzel S. Longitudinal study on potential neurotoxic effects of aluminium: II. Assessment of exposure and neurobehavioral performance of Al welders in the automobile industry over 4 years. Int Arch Occup Environ Health 2009; 82: 1191–1210.
15. Fritschi L, Hoving JL, Sim MR, Del Monaco A, MacFarlane E, McKenzie D, et al. All cause mortality and incidence of cancer in workers in bauxite mines and alumina refineries. Int J Cancer 2008; 123: 882–7.
17. Mehrpour O, Alfred S, Shadnia S, Keyler DE, Soltaninejad K, Chalaki N, Sedaghat M. Hyperglycemia in acute aluminium phosphide poisoning as a potential prognostic factor. Human & Experimental Toxicology 2008; 27: 591–5.
As is a metallic element that occurs naturally in the earth’s surface at 1.5 to 2 ppm, mostly in inorganic form. However, As is not uniformly distributed throughout the world. Arsenic generally exists in low concentrations in many rock types but is frequently associated with metal ore deposits (e.g., Au, Ag, Cu and Fe) . Because surface and ground water are often in contact with ores or tailings, waters near former melting or smelting sites often contain elevated As concentrations. Although most As compounds have no smell or taste, heat can cause As to sublimate to a gas with a garlic odor.
- In oxidizing conditions As will usually exist as compounds of H3AsO4 called arsenates (+5)
- In mildly reducing conditions, As is generally present as H3AsO3 compounds called arsenites (+3)
- In moderate reducing conditions, As often combines with S and Fe to form As sulfides or FeAs, which are virtually insoluble in water and immobilized in the environment
- In strongly reducing environments, elemental As (0) or H3As (–3) can exist, but such conditions are rare.
- Exposition to As containing drinking water
- Inhalation of arsine gas
- Oral intake of As compounds either accidentally or with suicidal or criminal motives
- Suspected intoxication due to occupational exposure.
Atomic absorption spectrophotometry with flame less hydride technique. Neutron activation analysis is scientifically the most suitable method because it does not require any special sample preparation and it has very low detection limits.
- Whole blood: 3 mL
- 24 h urine specifying the collected volume: 5 mL
* Expressed for 1,5 L/24 h; Conversion: μg/l × 13,3 = nmol/l
In As-prone areas, many people suffer from skin disorders, respiratory diseases, disorders of the nervous system, obstetric disorders, diabetes, cardiovascular diseases, as well as cancers of various organs .
The US Agency for Toxic Substances and Disease Registry ranked As top among hazardous substances in their 2015 priority list (). As is widely dispersed around the world. As is released into the environment through anthropogenic pollution. As contamination in groundwater occurs in more than 107 countries worldwide and is major contributor to inorganic As intake for the global population, especially for Asians, who consume rice as a staple food. Large populations in America, Bangladesh, China and India are exposed to arsenite and arsenate via drinking water .
Air: As concentrations of 1 to 3 ng/m3 has been detected in remote areas, yielding estimated daily As intakes of 20–200 ng. In urban areas without substantial industry emissions were 20–30 ng/m3, yielding estimated daily As intakes of 400–600 ng .
Soil: As may consist in numerous forms in soil. It may be complexed with organic material, it may be bound as an inorganic oxyanion to soil cations, or it may exist in its mineral form. If the soil overlays an ore deposit rich in sulfides, the arsenic level may reach several hundred mg/kg, but more typical values tend to be in the range of 0.1 to 40 mg/kg in most soils .
Diet: the As content of various foods per kg dry weight are as follows: apples 0.04–1.72 mg, rice up to 3.53 mg, potatoes up to 1.25 mg, beef 0.008 mg, pork 0.22–0.32 mg, crabs 27–52.5 mg. The large amounts of arsenic in seafood are due to organically bound arsenic in the form of harmless arsenobetaine and arsenocholine .
Arsenic in water: inorganic As occurs in drinking water as arsenite or arsenate. The mean As concentration in surface water is 0.001 mg/L. In Germany, and subsequently, in the WHO guidelines for drinking water, an upper limit of 0.01 mg/L was specified for the As concentration in water, to ensure that a potentially carcinogenic daily dose of 0.2 mg is not exceeded . Surface water contains mainly arsenate, whereas groundwater contains mainly arsenite. Many regions of the world, such as Bangladesh, have higher water As concentrations. Here, water is mostly extracted from groundwater through 4 million shallow tube-wells from deep rock layers, and 8%, 35%, and 58% of water samples contain 0.3 mg, more than 0.05 mg, and more than 0.01 mg of As per liter of water respectively. In Bangladesh, As occurs mainly as arsenite. More than 20,000 deaths occur each year and the health of 50 million people is threatened as a result of these elevated As concentrations . In the USA, 5% of water sources exceed the upper limit specified by the WHO. More than 350 thousand people are exposed to As concentrations of more than 0.05 mg/L and more than 2.5 million are exposed to concentrations of more than 0.025 mg/L .
The toxicity of As is highly influenced by its oxidation state, solubility, exposure dose, frequency and duration, the biological species, age and gender, as well as on genetic and nutritional factors /, /. In general, trivalent As compounds are more toxic than their pentavalent equivalents and inorganic compounds are more toxic than organic compounds such as methylated arsenic compounds. Although arsenites (AS+3) have a higher affinity than arsenates (As+5) for binding to disulfide, which plays an important role in the toxicity of the arsenites, a positive correlation also exists between toxicity and solubility. Arsenates are more soluble than arsenites. To eliminate the more toxic arsenites from groundwater, for example, these must first be oxidized using oxygen. Arsenic trioxide, arsenic hydride (colorless, with a garlic-like odor), and 2-chlorovinyldichloroarsine are highly toxic . Arsenic hydride does not occur in the natural environment but is intentionally or unintentionally produced as a result of human activities. It is also produced by bacteria.
Elemental As and organic As compounds are only mildly toxic, e.g. arsenobetaine, which exists in appreciable quantities in marine organisms.
In the environment, As+3 is converted into (As+5) and vice versa chemically or biologically. Bacteria and phytoplankton transform arsenate to arsenite and vice versa. Many organisms also form organic As compounds by methylation, which takes place mainly in soil and water. In the human body, (As+5) compounds are converted into (As+3) compounds, which in turn are methylated to form less toxic metabolites and excreted renally.
In groundwater, As exists as arsenite or arsenate. Arsenate that is ingested in drinking water is converted relatively quickly into arsenite. This either takes place non-enzymatically, with glutathione as an electron donor, or is catalyzed by the enzyme glutathione S-transferase.
Arsenic exerts its toxic effects through impairment of cellular energy regeneration by the inhibition of mitochondrial enzymes, and through uncoupling of oxidative phosphorylation. Most toxicity of arsenic results from its ability to interact with sulfhydryl groups of proteins and enzymes, and to substitute phosphorous in a variety of biochemical reactions .
The determination of As in the blood and urine can be used to evaluate both acute and chronic As intoxications. Repeated measurements of these indices can be used to depict long-term exposure level and changes in exposure over time. The Health Effects of Arsenic Longitudinal Study (HEALS) found a high degree of correlation between urine values and blood values in individuals with chronic As exposure. Urinary arsenic concentration was a useful biomarker for tracking arsenic exposure and did not fluctuate greatly over time .
Acute As toxicity is usually diagnosed by increased urinary As in excess of 50 μg/L (665 nmol/L) or 100 μg (1,330 nmol) in a 24-hour urine, and a shorter time span before examination, if no seafood has been ingested . As speciation of inorganic or organic forms of As is often as important as total quantification, because of their different toxicity and mobility. As speciation in biological samples is an essential tool to gain insight into its distribution in tissues and its specific toxicity to target organs . Many organic As compounds that are present in appreciable amounts in marine organisms are only mildly toxic. Consumption of this “fish arsenic” does not cause intoxication . The diet in the days before the examination must also be taken into consideration, because a diet high in marine organisms can lead to relatively high urinary As, which may be mistaken for chronic intoxication. For example, the urinary As concentration following a seafood meal was 291 ± 267 μg/L, while a few days later, it was only 9 ± 12 μg/L .
If intensive treatment is not started immediately, oral doses of As2O3 of 70–180 mg are fatal within 1 hour . When an individual is exposed to sub fatal doses, the length of time frame for the appearance of symptoms depends on the dose involved, the route of exposure, and the health of the individual . The clinical symptoms are vomiting, abdominal pain, and diarrhea. Neurological effects of As may develop within a few hours after ingestion, but usually are seen 2–8 weeks after exposure. It is usually a symmetrical sensorimotor neuropathy, often resembling the Guillain-Barré syndrome. The predominant clinical features of neuropathy are paresthesia, numbness and pain, particularly in the soles of feet .
In chronic long-term exposures, individuals begin to display characteristic signs of As toxicity at oral intakes of about 20 μg/kg per day of body weight, but some humans can ingest over 150 μg/kg per day without any apparent illness . Chronic As intoxication can result from drinking water, industrial accidents, occupational exposure, or environmental exposure.
The initial clinical symptoms of long-term exposure to low levels of As (arsenicosis) are skin discolorations, chronic indigestion, and stomach cramps. Longer-term effects include skin, lung, kidney, liver, cardiovascular system, peripheral vascular system, and reproductive system, as well as neurological diseases and carcinomas. The European Chemicals Bureau and US Environmental Protection Agency classified As as a carcinogen in 2007.
Although the deleterious effects of high As concentrations in drinking water (over 300 μg/L) have been documented, the chronic disease-causing effects of low to moderate concentrations (10–300 μg/L) cannot be attributed to As alone. Lifestyle factors such as smoking, body mass index, and genetic factors that influence arsenic metabolism must also be taken into consideration . Diseases caused by acute and chronic As intoxication are listed in .
The treatment of As intoxication focuses mainly on accelerating elimination while reactivating blocked enzymes by administering an antidote (dimercaptopropanol, BAL).
Chronic high As intake is also reflected in the hair. Since the determination of As in hair samples has not yet been sufficiently validated, measured values cannot be used to reliably determine As levels in the organs.
Following intake, soluble As salts are absorbed rapidly in the small intestine with an absorption rate of 95% (this percentage is probably lower for mono methylated and dimethylated compounds). As is then distributed rapidly throughout the body and can be found in different organs, in particularly, the liver. As undergoes hepatic bio methylation to form mono methyl arsenic (MMA) and dimethyl arsenic acids (DMA). The uptake of trivalent arsenic compounds is mediated by aquaglyceroporin and pentavalent arsenates are transported into cells by phosphate transporters. As accumulates in hair, nails, and skin and also reaches the placenta, breast milk, and brain (via the cerebrospinal fluid) /, , /.
As is detoxified in the liver by the transformation of inorganic As into organic As and the reduction of pentavalent As into trivalent As. The reduction is catalyzed by glutathione and other thiols, which are reducing agents or can be catalyzed by the enzyme glutathione S-transferase. Methylation of the trivalent arsenic compounds then occurs, catalyzed by As (III) methyl transferase, which uses S-adenosyl methionine (SAM) as a methyl donor. A methyl group is added to arsenite, which leads to the synthesis of pentavalent MMA. Pentavalent MMA is immediately reduced again by glutathione to form trivalent MMA. Trivalent MMA is then methylated to form DMA. Not all methylation steps end with DMA; other metabolites such as trimethyl arsenite or trimethyl arsinoxide are also produced.
As-containing compounds are excreted by the kidney. DMA is the main metabolite. Typically, 60–80% of As is excreted in the urine as DMA, 10–20% is excreted as MMA, and 10–30% is excreted as inorganic As. Specific single nucleotide polymorphisms of As(III) methyl transferase lead to a 50% reduction in the excretion of DMA.
Many metals, such as As, cadmium, lead, and mercury, have an affinity for sulfhydryl groups and can change the protein structure. This reduces the activity of enzymes that are involved in energy metabolism, DNA synthesis, and DNA repair.
As, instead of phosphate, is incorporated into energy-rich compounds such as ATP, which reduces the amount of energy available for metabolic processes (e.g. for glucose uptake by cells, gluconeogenesis, fatty acid oxidation, and glutathione synthesis).
10. Chen Y, Parvez F, Gamble M, Islam T, Ahmed A, Argos M, et al. Arsenic exposure at low-to-moderate levels and skin lesions, arsenic metabolism, neurologic functions, and biomarkers for respiratory and cardiovascular diseases: Review of recent findings from the Health Effects of Arsenic Longitudinal Study (HEALS) in Bangladesh. Toxicology and Applied Pharmacology 2009; 239: 184–92.
21. Ashan H, Chen Y, Zablotska L, Argos M, Hussain AI, Momotaj H, et al. Arsenic exposure from drinking water and risk of premalignant skin lesions in Bangladesh: baseline results from the Health Effects of Arsenic Longitudinal Study. Am J Epidemiol 2006; 163: 1138–48.
23. Wang CH, Hsiao CK, Chen CL, Hsu LI, Chiou HY, Chen SY, et al. A review of the epidemiologic literature on the role of environmental arsenic exposure and cardiovascular diseases. Toxicol Appl Pharmacol 2007; 222: 315–26.
25. Chou WC, Chung YT, Chen HY, Wang CJ, Ying TH, Chuang CY, et al. Maternal arsenic exposure and DNA damage biomarkers, and the associations with birth outcomes in a general population from Taiwan. PLOS one 2014; 9: issue 2, e86398.
26. Karagas MR, Stukel TA, Morris JS, Tosteson TD, Weiss JE, Spencer SK, et al. Skin cancer risk in relation to toenail arsenic concentrations in a US population-based case-control study. Am J Epidemiol 2001; 153: 559–65.
Lead is a soft metal that is white and shiny when freshly cut. Pb occurs naturally in the earth’s crust at 2 × 10–4 percent. Pb forms divalent or tetravalent compounds and its predominant oxidation state is +2. Pb(IV) derivatives are strong oxidizing agents. The most important mineral is PbS (lead glance). Pb also exists in the form of carbonate, chromate, molybdate, phosphate, and tungstate.
Suspected acute or chronic lead intoxication:
- Accidental oral ingestion of lead compounds
- Workers with high levels of occupational exposure, for example, workers in smelting plants, lead refineries, and the lead-processing industry
- Inhabitants of highly contaminated regions
- Patients with clinical symptoms suggestive of acute or chronic lead intoxication
- Whole blood (lithium heparinate or EDTA): 1 mL
- Urine, to investigate Pb burden in Pb mobilization test (): 5 mL
As a result of environmental protection measures in the workplace and the ban on leaded fuel, environmental and blood Pb concentrations have decreased significantly in western industrialized nations. This is supported by data from the German Environmental Survey and the National Health and Nutrition Examination Survey in the USA.
Blood Pb concentrations of below 4 to 380 μg/L (0.02 to 1.82 μmol/L) were measured, with a geometric mean of 30.7 μg/L (0,15 μmol/L). Reference values were lowered based on this survey. The following factors influenced the Pb concentration in the survey :
- Age (increased concentrations were measured in the age groups 18–19 years to 50–59 years)
- Hematocrit (a positive correlation was found between blood Pb concentration and hematocrit)
- Frequency of consumption of beer, sparkling wine, and fruit wine (Pb concentrations increased with frequency of consumption)
- Pb concentration in domestic drinking water.
Threshold Pb levels above which chronic intoxication can occur:
- For women of childbearing age and children: Values ≥ 100 μg/L (0.48 μmol/L).
- For men and women over 45 years of age: values ≥ 150 μg/L (0.72 μmol/L).
Data from the National Health and Nutrition Examination Survey study in the United States show a decline in the geometric mean Pb blood level in the general population from 131 μg/L (0.63 μmol/L) in 1976–1988 to 16 μg/L (0.08 μmol/L) in 1999–2002. Factors linked to higher Pb concentrations were older age, male sex, smoking, Pb in old paint and water pipes, drinking, lower socioeconomic status, urban residence, and housing in older buildings. According to the Bio monitoring Study of Lead in New York City, the geometric mean of the blood Pb in the general population was 17.9 μg/L (0.09 μmol/L). Smokers had a concentration of 24.9 μg/L (0.12 μmol/L), construction workers had a level of 28.6 μg/L (0.14 μmol/L), and Chinese immigrants had mean values of 24.9 μg/L (0.12 μmol/L).
Pb dust and Pb compounds are absorbed primarily via the respiratory tract (70–100%) and, to a lesser extent, via the gastrointestinal tract (5–20%) . If the particle size is less than 1 μm, inhalation accounts for around 90% of absorption. Enteral absorption depends on the physicochemical characteristics of the Pb (metallic, inorganic or organic compound) and whether it contains other cations such as Ca2+, Fe2+ , and Zn2+. The absorption rate of Pb is 11% for adults. Lipophilic Pb, such as the antiknock agent tetraethyl lead, is absorbed well by the skin. The Pb concentration rises fast after Pb exposure, but within 100 days Pb is redistributed to soft tissues and bones.
The absorbed Pb is accumulated in the body in three compartments:
- About 95% is stored in the bones of adults. The half-life is 4–20 years. Pb is stored for longer in cortical bone than in trabecular bone.
- Below 5% of Pb is free in plasma, and the rest is bound within the erythrocyte to the enzyme δ-aminolevulinic acid dehydrogenase (ALAD). The half-life of Pb in the blood is 35 days.
- Below 5% of Pb is localized in soft tissues. The half-life of Pb in soft tissues is about 30 days and in the brain 2 years.
Metallic Pb is eliminated by the kidneys (75%), in the bile (15%), and via the hair and nails (10%).
The clinical symptoms of Pb intoxication are anemia, neuropathy, nephropathy, gastrointestinal disorders, reproductive disorders, and cardiovascular effects. In the Nurses’ Health Study, early menopause (< 45 years) was more common in women with the highest tibial Pb concentrations than those with the lowest Pb concentrations (odds ratio 5.3) .
Determination of Pb in whole blood
Because of the short mean biological half-life in blood the Pb concentration reflects an ongoing exposure. However, if exposure to Pb has been going on for a long period of time, giving rise to a high body burden of Pb, blood level of Pb will remain elevated for an extended period of time. Normal blood Pb concentrations do not rule out Pb intoxication .
Supplementary investigations and findings
- Reduced ALAD activity in erythrocytes (less than 10% of normal)
- Increase in free erythrocyte protoporphyrin (over 500 mg/L whole blood)
- Increase in renal excretion of δ-aminolevulinic acid (over 20 mg/L urine)
- Increased renal excretion of coproporphyrin III (over 0.5 mg/L urine)
- Basophilic stippling of erythrocytes (more than 100 stippled cells per 1 million erythrocytes)
- Abnormalities in complete blood count: hypochromic anemia, anisocytosis, poikilocytosis .
Pb mobilization test
To measure the Pb burden, one can perform a mobilization test using a chelating agent . Urine examinations do not provide information about exposure quantity, unless a Pb mobilization test is performed. Blood Pb and the EDTA-test are correlated under steady-state circumstances, especially if the exposure is recent. This is because most of the chelatable Pb comes from the blood-soft tissue compartments, and to a lesser extent from trabecular bone with higher bio availability compared with the cortical bone. The exchange of Pb is greater during increased bone turnover such as pregnancy, immobilization, or hyperparathyroidism .
Non-invasive measurement of lead: Due to the high contamination risk, hair analyses cannot be used for diagnostic purposes. A non-invasive way to measure stored Pb is by in-vivo X-ray fluorescence of the bone. Stored Pb can be estimated by determining the Pb concentration in exfoliated milk teeth.
Forced Pb excretion using chelating agents such as CaNa2EDTA, dimercaptopropane sulfonate (DMPS), or 2,3 dimercaptosuccinic acid (Succimer) increases renal Pb excretion by a factor of 25–30. The chelating agents are used for Pb intoxication when the blood concentration is above 400 μg/L (1.9 μmol/L) . The German Society for Occupational and Environmental Medicine recommends forced excretion if the blood concentration of Pb exceeds 150 μg/L (0.72 μmol/L) for women or 250 μg/L (1.2 μmol/L) for men . The United States Centers for Disease Control and Prevention recommends forced Pb excretion in children for Pb concentrations ≥ 450 μg/L (2.16 μmol/L).
Metal-free blood collection devices that contain lithium heparin or EDTA should be used.
Hair mineral analysis
Chronic high Pb intake is reflected in the hair. However, contamination during sample preparation cannot be distinguished from toxic Pb concentrations in the hair.
The blood Pb concentration increases in exsiccosis with increased hematocrit.
Incomplete collection, contamination, precipitation of Pb salts, and absorption of Pb onto vessel walls with polar properties lead to falsely low results.
Non-occupational Pb exposure occurs as a result of oral ingestion or inhalation of contaminated food or air. Pb is absorbed by the lungs and the intestine, depending on the food component. Calcium, zinc, phosphate, and phytate inhibit its uptake. Fasting, for example, can increase the absorption rate of Pb by up to 60% . Children can absorb 30–75% of the Pb ingested, while adults absorb about 11% of ingested Pb.
One of the major mechanisms by which lead exerts its toxic effect is through biochemical processes that include lead’s ability to inhibit or mimic the actions of calcium and to interact with proteins. Within the skeleton, lead is incorporated into the mineral place of calcium. Pb has a high affinity for the sulfhydryl group of cysteine, the amino group of lysine, the carboxyl group of glutamate and aspartate, and the hydroxyl group of tyrosine. Of all the cellular structures, the mitochondria are most sensitive to Pb.
Circulating Pb is quickly taken up from the plasma into erythrocytes and bound to δ-aminolevulinic acid dehydrogenase (ALAD). The binding capacity of erythrocyte ALAD for Pb is so high that it cannot be saturated, even in severe Pb intoxication. From the red blood cells, Pb is distributed to the liver, kidneys, and brain. With a half life of around 30 days, Pb is released from these tissues and is eliminated by the kidneys or deposited in the skeleton as Pb phosphate. The Pb content of the skeletal system increases with age and constitutes 70% of the total Pb burden in children and 90% in adults.
Renal excretion of Pb is delayed in severe renal insufficiency; this must be taken into account during forced Pb elimination using chelating agents. The movement of Pb out of bones is increased when bone metabolism is activated by hyperparathyroidism, immobility, or pregnancy, for example.
Enteral and red blood cell uptake of Pb are decreased in the presence of iron, probably because both elements are transported into the cell using the same metal ion transporter. For this reason, iron deficiency potentiates Pb absorption.
Pb inhibits the synthesis of heme, mainly due to the effect of Pb on the mitochondrial membrane. Heme acts as both an oxygen carrier in the red blood cells and a respiratory pigment in the cytochrome c system of all cells. Heme synthesis begins and ends in the mitochondria; intermediate steps take place in the cytoplasm. Pb disrupts heme synthesis by inhibiting ALAD and ferrochelatase . ALAD is a cytoplasmic enzyme, while ferrochelatase is a mitochondrial enzyme.
Inhibition of ALAD
In the first step of heme synthesis, this enzyme catalyzes the condensation of two molecules of δ-aminolevulinic acid (δ-ALAS) to porphobilinogen (see also ). The half maximal inhibition of ALAD takes place at a blood Pb concentration of 160 μg/L (0.77 μmol/L) and 90% inhibition at a concentration of 550 μg/L (2.64 μmol/L) . As a metabolic consequence of ALAD inhibition, δ-ALAS accumulates and its urinary excretion increases. The increased δ-ALAS concentration in the tissues causes neurological symptoms. ALAD activity depends on the integrity of the sulfhydryl groups and on the availability of zinc, because each molecule of ALAD contains one molecule of zinc. The inhibition of ALAD can be reversed by increasing the availability of zinc.
Inhibition of ferrochelatase
This enzyme catalyzes the final step in heme synthesis, the incorporation of iron into the protoporphyrin ring (see ). If this step is inhibited, iron-free protoporphyrin is incorporated into the heme-binding pockets in the red blood cells. If insufficient iron is available, zinc protoporphyrin is incorporated. Because protoporphyrin binds relatively firmly to hemoglobin, it does not diffuse out of the red blood cells into the plasma and skin. In contrast to patients with erythropoietic protoporphyria, therefore, patients with Pb intoxication do not develop skin photosensitivity . The increase in protoporphyrin in the erythrocytes depends on the blood Pb concentration. If the Pb concentration exceeds 500 μg/L (2.4 μmol/L), the protoporphyrin concentration exceeds 2,500 μg/L erythrocytes .
Inhibition of red cell osmotic resistance
The reduction in the osmotic resistance of the erythrocytes is due to inhibition of Na+/K+–ATPase in the erythrocyte membrane by Pb, which leads to a significant efflux of K+ from the erythrocytes. Their basophilic stippling results from the precipitation of mitochondrial remains and ribosomal DNA. This occurs because Pb inhibits pyrimidine 5’-nucleotidase in red blood cells. After the cell nucleus is ejected, the enzyme cleaves the remaining nucleotide chains into small fragments .
2. Stellungnahme der Kommission Human-Biomonitoring des Umweltbundesamtes. Aktualisierung der Referenzwerte für Blei, Cadmium und Quecksilber im Blut und Urin von Erwachsenen. Bundesgesundheitsbl, Gesundheitsforsch, Gesundheitsschutz 2003; 46: 1112–3.
5. Mc Kelvey W, Gwynn RC, Jeffery N, Kass D, Thorpe LE, Garg RE, et al. A biomonitoring study of lead, cadmium, and mercury in the blood of New York city adults. Environ Health Perspect 2007; 115: 1435–41.
11. Lommel A, Dengler D, Janßen U, Fertmann R, Hentschel S, Wessel M. Bleibelastung durch Trinkwasser. Teil 1: Einfluss auf den Blutbleispiegel junger Frauen. Bundesgesundheitsbl, Gesundheitsforsch, Gesundheitsschutz 2002; 45: 605–12.
12. Lommel A, Dengler D, Janßen U, Fertmann R, Hentschel S, Wessel M. Bleibelastung durch Trinkwasser. Teil 2: Effekte verschiedener Präventionsstrategien. Bundesgesundheitsbl, Gesundheitsforsch, Gesundheitsschutz 2002; 45: 613–7.
13. Vaidyanathan A, Staley F, Shire J, Muthukumar S, Kennedy C, Meyer PA, et al. Screening for lead poisoning: a geospatial approach to determine testing of children in at-risk neighborhoods. J Pediatr 2009; 154: 409–14.
15. Dietrich KN, Ware JH, Salganik M, Radcliffe J, Rogan WJ, Rhoads GG, et al. Effect of chelation therapy on the neuropsychological and behavioral development of lead-exposed children after school entry. Pediatrics 2004; 114: 19–26.
18. Brätter P, Heseker H, Kruse-Jarres JD, Liesen H, Negretti de Brätter V, Pietrzik K, Schümann K. Mineralstoffe, Spurenelemente und Vitamine: Leitfaden für die ärztliche Praxis. Gütersloh; Bertelsmann Stiftung 2002: 107–9.
19. Karimooy HN, Mood MB, Hosseini M, Shadmanfar S. Effects of occupational lead exposure on renal and nervous system of workers of traditional tile factories in Mashad (northeast of Iran). Toxicology and Industrial Health 2010; 26: 633–8.
26. Paglia DE, Valentine WN, Dahlgren JG. Effects of low level lead exposure on pyrimidine 5’-nucleotidase and other erythrocyte enzymes. Possible role of pyrimidine 5’-nucleotidase in the pathogenesis of lead-induced anemia. J Clin Invest 1975; 56: 1164–9.
Cd belongs to subgroup II of the periodic table (the zinc group). It is an underground heavy metal that occurs naturally in the earth’s crust at 1 × 10–5 percent. The concentration of Cd in soil is around 0.1 ppm. In nature, Cd is mainly found as an accompanying element of zinc e.g., as zinc blende (sphalerite) or zinc spar (smithsonite). Pure Cd minerals such as Cd blende (greenockit, CdS), monteponite (CdO), and otavite (CdO3) are less common.
Blood sampling with metal-free blood collection assembly.
- Whole blood (lithium heparinate): 5 mL
- 24 h urine: 10 mL
- Random urine (calculation of the Cd/creatinine ratio): 10 mL
Cd is hazardous both by inhalation and ingestion and can cause acute and chronic intoxications. Cd dispersed in the environment persists in soils and sediments for decades. The most toxicological property of Cd is its long half-life in the human body, in particularly in kidneys and other vital organs such as the lungs or the liver. In addition to its extra ordinary cumulative properties, Cd is also a highly toxic metal that can disrupt biological systems, usually at doses that are much lower than most toxic metals .
When taken up by plants, Cd concentrates along the food chain and accumulates in the body of people eating contaminated foods. Cd is also present in tobacco smoke, further contributing to exposure.
The main routes of exposure to Cd are via inhalation or cigarette smoke, ingestion of food and employment in primary metal industries. Cd is used in the following industrial products: pigments (Cd yellow), nickel-Cd batteries, stabilization of polyvinyl chloride in the plastics industry, coating for machinery parts and alloys, and control rods for nuclear reactors. Cd oxide in cigarette smoke and polluted air has relatively high bio availability.
Chronic Cd intoxication is suspected in areas with Cd-contaminated soil, especially via inhalation of Cd smoke.
Exposure to Cd is commonly determined by measuring Cd concentration in blood and Cd excretion in urine. Blood Cd reflects the recent Cd exposure. Cd in urine indicates accumulation, or kidney burden of Cd.
Chronic Cd exposure can cause renal disease, anemia, osteoporosis, and bone fractures. Cd is also a potent carcinogen . Chronic Cd exposure is usually related to food intake, especially in combination with smoking. Animal offal, shellfish, mussels, and oysters have the highest Cd content. The normal daily Cd intake from food is 10–20 μg. Tobacco smoking is an additional source of exposure for smokers. Each cigarette contains 1–2 μg Cd, so the amount of Cd in a packet of cigarettes corresponds to the amount of Cd ingested daily in food.
The normal absorption rate of orally ingested Cd is 5% but this increases to 15% in patients who are deficient in iron. The absorption rate of Cd fumes or particles via the respiratory tract is 10–50%; the absorption rate from tobacco smoke is around 10%.
The body Cd burden is negligible at birth, increases continually until the age of 60–70 years, and then declines. At low levels of exposure, bone is the main target and is more severely affected than the kidneys. The correlation between low Cd concentrations and proteinuria is weak .
Cd accumulates in the liver and kidneys since the binding protein metallothionein is synthesized in these organs. As most renal Cd is bound to metallothionein, the form of Cd responsible for renal damage is the highly toxic Cd2+ ion that avidly reacts with cellular components . In individuals with low levels of environmental exposure to Cd, the kidneys can contain up to 50% of the total body Cd. The amount of Cd excreted daily in urine is very low, representing somewhat 0.005–0.01% of the total body burden . There is a well-documented dose-response relationship for Cd. The amount of Cd stored in the kidneys can be estimated non-invasively by determining the urinary Cd excretion as a surrogate marker. A urinary Cd concentration of 5 μg/g creatinine is considered the upper tolerable limit for occupational exposure to Cd in many countries. The earliest manifestation of Cd-induced renal damage considered as critical consists in increased urinary excretion of micro proteins like α1-microglobulin, retinol-binding protein and β2-microglobulin.
The Cd concentrations in whole blood and urine can be used to estimate the body Cd burden. Hair analysis allows only a rough estimate, since the correlation between the hair Cd concentration and levels in organs such as the liver and kidney is weak.
Different reference values for Cd are used for samples, depending on the geographic region from which the group of interest originates as well as the method of determination.
Hair mineral analysis
Chronic high Cd exposure is reflected in the hair. However, contamination during sample preparation cannot be distinguished from toxic Cd content in the hair. The content in the hair provides only a rough estimate of levels in the organs.
Cd does not have any physiological functions and the body has not developed any special mechanisms for its transport or homeostasis. It is transported to tissues using mechanisms that evolved for essential metal ions such as Zn2+, Fe2+, Mn2+ and Ca2+. Many of the effects of Cd in the body result from its interaction with these metals, in particular Zn. Cd and Zn bind to macromolecules in the body, mainly via sulfur (S), oxygen (O), and nitrogen (N) atoms. Although both metals have a high affinity for metallothionein, proteins, and enzymes with sulfhydryl groups, Cd has a higher affinity than Zn .
Cd has an enteral absorption rate of 3–7%; however, this can increase by a factor of 3–4 if iron deficiency anemia is present. The respiratory absorption rate of Cd is around 40%.
In the blood, Cd is transported by erythrocytes or bound to albumin. It is then taken up by hepatocytes, where it induces the synthesis of metallothionein, a low molecular weight protein that is rich in cysteine and has a high affinity for divalent metals. The toxicity of Cd is reduced significantly by binding to metallothionein . The metallothionein-Cd complex is released into the blood stream reaching the kidney where it is filtered through the glomeruli and taken up by the renal proximal tubule cells by pinocytosis.
Within renal tubular cells, the metallothionein-Cd complex is degraded but the metallothionein-Cd complex degrades and releases free Cd, which recombines with metallothionein newly synthesized by tubular cells. Once tubular metallothionein is saturated with Cd it is no longer able to remove all the free Cd in the tubular cells, which may result in damage of the kidney tubules. The damaged tubule is no longer able to sequester Cd, and therefore increased excretion of Cd is observed in the urine.
Most of the Cd in the body is found in the liver, although it remains there for a short time only. In the longer term, it is stored in the kidneys. There, it has a half life of 17–30 years. Only small quantities of Cd are excreted in the urine.
Oxidative stress plays an important role in Cd toxicity . It leads to glutathione (GSH) depletion and the reduction of protein-bound sulfhydryl groups, which results in increased production of reactive oxygen species (ROS) such as the super oxide anion (O2–.), hydrogen super oxide (H2O2), and hydroxyl radicals (OH.) (see also ). Because Cd is a redox-stable metal, radicals can only be formed indirectly.
One of the mechanisms of cellular damage is disruption of the antioxidant system, of the hepatocytes in particular. Hepatocytes contain large amounts of glutathione, which is reduced as the result of Cd binding. This leads to Cd-induced hepatotoxicity and reduced amounts of glutathione in other organs such as the kidneys.
Mitochondria are thought to be particularly vulnerable to the damaging effects of Cd. Mitochondrial damage is likely, given that dysfunctional mitochondria are central to the formation of excess reactive oxygen species (ROS), and are known key intracellular targets for Cd. When mitochondria become dysfunctional, for example, through long term exposure to environmental toxicants like Cd, they produce less energy and more ROS. The imbalance between these ROS and the natural anti-oxidants creates the condition of oxidative stress . It has been speculated that Cd causes single-strand DNA damage and disrupts the synthesis of nucleic acids and proteins .
1. Miura N. Individual susceptibility to cadmium toxicity and metallothionein gene polymorphisms: with references to current status of occupational cadmium exposure. Industrial Health 2009; 47: 487–94.
2. Stoeppler M, Brandt K. Contributions to automated trace analysis. Part V. Determinations of cadmium in whole blood and urine by electrothermal atomic-absorption spectrophotometry. Fresenius Z Anal Chem 1980; 300: 372–80.
3. Stellungnahme der Kommission Human-Biomonitoring des Umweltbundesamtes. Aktualisierung der Referenzwerte für Blei, Cadmium und Quecksilber im Blut und Urin von Erwachsenen. Bundesgesundheitsbl, Gesundheitsforsch, Gesundheitsschutz 2003; 46: 1112–3.
6. Usuda K, Kono K, Ohnishi K, Nakayama S, Sugiura Y, Kitamura Y, et al. Toxicological aspects of cadmium and occupational health activities to prevent workplace exposure in Japan: a narrative review. Toxicology and Industrial Health 2010; .
15. Mc Kelvey W, Gwynn RC, Jeffery N, Kass D, Thorpe LE, Garg RE, et al. A biomonitoring study of lead, cadmium, and mercury in the blood of New York city adults. Environ Health Perspect 2007; 115: 1435–41.
19. Haswell-Elkins M, Satarug S, O´Rourke P, Moore M, Ng J, McGrath V, et al. Striking association between urinary cadmium level and albuminuria among Torres Strait Islander people with diabetes. J Expo Sci Environ Epidemiol 2007b; 17: 298–306.
20. Nawrot TS, van Hecke E, Thijs L, Richart T, Kutznetsova T, Jin Y, et al. Cadmium-related mortality and long-term secular trends in the cadmium body burden of an environmentally exposed population. Environ Health Perspect 2008; 116: 1620–8.
Hg is a heavy metal and the only metal that is liquid at room temperature. In the solid phase, Hg is soft and flexible; in the gas phase, it exists in atomic form. Hg binds easily to sulfur and halogens and, at temperatures above 300 °C, reacts with oxygen to produce Hg(II)oxide (HgO). Hg exists in monovalent and divalent compounds, and forms Hg alloys (amalgams) with many metals. Hg is an naturally occurring element and a common environmental contaminant, accounting for only 1 × 10–5 percent of the earth’s crust. The most important Hg ore is cinnabar (cinnabarite, HgS). For the medical assessment of Hg intoxication, the form in which the Hg was absorbed is important:
- As elemental Hg, a highly poisonous vapor
- As inorganic Hg in the form of a salt, e.g. as Hg2Cl2 (calomel)
- In organic form, as a carbon compound such as methylmercury (MeHg), which is less toxic than mercury vapor.
Suspected chronic mercury intoxication:
- Individuals with dental amalgams
- Daily consumption of fish
- Hg inhalation at high levels of occupational exposure
- Neurological and neuropsychiatric symptoms.
Suspected acute Hg intoxication:
- Symptoms such as dyspnea, chest pain, nausea, vomiting, and swollen joints, in the context of occupational exposure to Hg.
Principle: The mercury ions of the specimen are adsorbed quantitatively on SDS coated chromosorb due to its complexation with 2-mercaptobenzoxazole, while the retained Hg2+ ions are then stripped from the column with minimal amounts of 2 molar nitric acid in acetone. The eluting solution is sent to cold vapor atomic absorption spectrometry for evaluating Hg2+ ion content .
- Heparin or EDTA whole blood: 3 mL
- 24 h urine (specify volume): 5 mL
- Random specimen of urine (excretion with reference to creatinine): 5 mL
- Hair samples
Exposure to Hg in any general, non occupational population occurs mainly through the diet, and in any cases, through the consumption of fish. The consumption of fish on the global scale is widespread, however the benefits of fish diet must be balanced against any negative neurodevelopmental effects that may be by mercury exposure. Hg bio accumulates up the food chain, therefore large predatory species such as tuna, shark, and swordfish may have high concentrations of Hg in their tissue. Elemental Hg does not degrade or breakdown in the environment, and it tends to cycle through ecological and biological systems.
A further source of Hg exposure in the population is dental amalgam.
Anthropogenic exposure to Hg is generally to Hg vapor and can occur in dentistry, mining, smelting, combustion of coal and industrial processes (manufacture of electrical equipment and medical instruments). Thiomersal, a Hg-containing preservative, is a component of some vaccines but has been phased out from most routine childhood vaccines . Hg is also used in medicinal products, bleaches, and Ayurvedic medicines ().
- Absorption of elemental Hg (Hg0) is negligible through the oral route. The half life of Hg0 in the blood is 40–60 days.
- Oral absorption of inorganic Hg compounds is poor to moderate depending on the precise form. Dental amalgam consists of Hg0 (50%), parts of silver, tin, copper and zinc. The half life of inorganic Hg in the blood is 40–60 days.
- Hg vapor is well absorbed by the respiratory route (nasal and oral mucosa, lungs). In the blood, 50% of the Hg vapor dissolves in the plasma and the other 50% distribute to erythrocytes. For this reason, and because Hg is lipophilic, Hg vapor in the blood rapidly reaches the organs and crosses the blood-brain barrier to reach the brain. The half life of Hg vapor in the blood is 3–6 days.
- Oral absorption of organic Hg is nearly complete. The half life of organic Hg in the blood is 70 days. The intestinal absorption rate of MeHg is almost 100% and 90% is bound to erythrocytes following absorption. MeHg has a half life of 60–90 days in the blood. MeHg is oxidized to the toxic Hg2+ ion in the organs. MeHg, which binds preferentially to the amino acid cysteine in fish and other seafood, is less toxic than Hg vapor.
Once absorbed, Hg is distributed throughout the entire body, but has a particular affinity for the central nervous system and the kidneys. Its elimination is limited and takes place via renal excretion and feces.
Acute Hg intoxication with elemental Hg or inorganic compounds is rare . Symptoms of acute Hg vapor intoxication include shortness of breath, chest pain, dyspnea, paroxysmal cough, nausea, vomiting, diffuse joint swelling, and rash. Deaths are observed at doses above 3–5 mg/kg body weight. Clear toxicity results in urinary levels of 50 to 100 μg Hg/L. See also .
Occupational inhalation of Hg vapor (Hg0) has declined sharply in recent years . Natural emissions of the element are 6–7 times greater than industrial emissions . The exposure amount in Western industrialized nations is around one-third of the quantity of Hg that can be safely absorbed (daily oral intake rate or reference dose of methylmercury 0.2 μg/kg of body weight according to the United States Environmental Protection Agency and the WHO). A good half of this can be attributed to fish consumption and the other half to dental amalgam. The Environmental Protection Agency Public has developed public health actions levels at 1.1 μg/g of hair. Because imported fish species marketed in the USA contain increasing amounts of methylmercury, the provisional tolerable weekly intake in children of methylmercury established by the Joint FAO/WHO Expert Committee of Food Additives is 1.6 μg/kg body weight.
The signs and symptoms of chronic intoxication vary with the form of Hg and route of exposure but include nausea, stomatitis, increased salivation, skin and nail discoloration, metallic taste, vomiting, and abdominal pain. Chronic inhalation leads to sensory peripheral neuropathy and impairment of the central nervous system with personality change, irritability, intention tremor, ataxia, and lack of concentration. Renal effects include both tubular and glomerular damage. Long-term, chronic exposure of pregnant women as the result of regular fish consumption or the release of Hg from dental amalgam can cause disruption of the developing brain of the fetus and is associated with neuropsychological changes after birth.
The consumption of fish containing high levels of MeHg or of contaminated seeds has led to epidemics of chronic intoxication and many deaths. MeHg mainly damages the central nervous system. See also .
Whole blood, urine, and hair specimens can be used to determine Hg. Each of these specimens is particularly suited to determining exposure to elemental, inorganic, or organic (mostly MeHg) Hg.
Urinary Hg determination is used mainly to detect inorganic Hg e.g. from dental amalgam. Excretion rates of 50–100 μg/L provide clear evidence of toxic exposure to inorganic Hg; however, lower values do not rule this out.
Whole blood Hg determination is the preferred approach if exposure to organic Hg is suspected, since organic Hg is excreted preferentially in the feces rather than in the urine.
MeHg in particular accumulates in the hair and its determination in 1 cm segments of hair is used as a surrogate marker to determine monthly Hg exposure.
Individuals who were exposed less recently can be identified more effectively by determining Hg in the urine rather than in whole blood. In the blood, inorganic Hg is mainly found extracellularly and organic Hg is mainly found intracellularly. The chemical form of the exposure can therefore be determined from the distribution of Hg between erythrocytes and plasma. In inorganic Hg intoxication, the ratio of erythrocyte Hg to plasma Hg is less than 2; in organic Hg intoxication, the ratio is 10–20 . In individuals who were exposed less recently, Hg concentrations can be pathological in the urine and normal in whole blood. Furthermore, the values in both specimens are often within the reference interval.
The chemical form of the mercury form (organic/inorganic) can be determined from the distribution ratio of Hg between erythrocytes and plasma.
Evaluation of treatment
In the early phase acute Hg intoxication is managed with 2,3-dimercaptopropanol (BAL) by intramuscular route. Less severe inorganic Hg or MeHg intoxication are treated with the less toxic chelators 2,3 dimercaptosuccinic acid (DMSA) and 2,3-dimercaptopropane-1-sulfonic acid (DMPS) . Urinary Hg excretion should be determined at the start of treatment, upon each dose increase, and then every four weeks.
Hg is the only metal that is liquid at room temperature. Due to its high surface tension, metallic Hg forms small beads in fluid media. If Hg reaches the environment as a solid salt or a vapor, it is transformed into organic MeHg by microorganisms (in water, by plankton in particular) and enters the food chain in this way. Hg compounds are also used as medications for the eyes, ears, nose, and throat as well as bleaching agents, toothpaste, additives to vaccines, in-vivo allergy tests, antiseptics, herbicides, fungicides, and dental amalgam. The current Hg concentration in the biosphere is ten times higher than the concentration that was present during the pre-industrial era .
Oxidative stress is involved in the molecular mechanisms of toxicity of Hg. Once in the cell both Hg2+ and MeHg form covalent bonds with cysteine residues of proteins and deplete cellular antioxidants.
The key marker of exposure for MeHg neurotoxicity is the concentration of organic or inorganic Hg in the central nervous system (CNS). Hg enters the brain in its methylated form, but as long as it remains in that form, it can also exit the brain. Demethylated Hg persists as inorganic Hg, and accumulates. Its half life elimination is in the order of years. If exposure is brief, the proportion of inorganic Hg is 5–10%, but this value increases continuously with prolonged exposure . In Minamata disease, the Hg concentration in the brain was 0.3–75 μmol/L. In the adult brain, MeHg poisoning damages the so-called primary areas of the cerebral cortex, affecting the visual, auditory, somatic sensory, and motor cortex, as well as the hippocampus and the granule layer of the cerebellum, causing a remarkable loss of neurons in these brain regions .
MeHg vulnerability of the brain reflects the ability of lipophilic MeHg to cross cell membranes and concentrate in the cells of the CNS. The main route for MeHg transmembrane transport is the amino acid transport system for large amino acids. MeHg is oxidized to form highly toxic Hg2+ within the cells, which binds strongly to cell structures and is removed very slowly. Hg is more toxic than any other element, even arsenic, lead, and cadmium.
The molecular mechanisms of MeHg toxicity are based on its high affinity for thiol groups (-SH), which are found in enzymes, cytoskeletal proteins, and cysteine-containing peptides. The interaction between MeHg and thiol groups inactivates enzymes at cellular and subcellular level.
One cause of Hg neurotoxicity is oxidative stress due to reduced glutathione formation, which results in an increase in reactive oxygen species (ROS). This leads to DNA strand breakage, lipid per oxidation, and protein modification.
2. Stellungnahme der Kommission Human-Biomonitoring des Umweltbundesamtes. Aktualisierung der Referenzwerte für Blei, Cadmium und Quecksilber im Blut und Urin von Erwachsenen. Bundesgesundheitsbl, Gesundheitsforsch, Gesundheitsschutz 2003; 46: 1112–3.
3. Ouboter PE, Landburg G, Satnarain G, Starke SY, Nanden I, Simon-Friedt B, et al. Mercury levels in women and children from interior villages in Suriname, Sout America. Int J Environ Res Publ Health 2018; 15: 1007–19.
7. Brätter P, Heseker H, Kruse-Jarres JD, Liesen H, Negretti de Brätter V, Pietrzik K, Schümann K. Mineralstoffe, Spurenelemente und Vitamine: Leitfaden für die ärztliche Praxis. Gütersloh; Bertelsmann Stiftung 2002: 110–2.
14. Mc Kelvey W, Gwynn RC, Jeffery N, Kass D, Thorpe LE, Garg RE, et al. A biomonitoring study of lead, cadmium, and mercury in the blood of New York city adults. Environ Health Perspect 2007; 115: 1435–41.
22. Kommission „Human Biomonitoring“ des Bundesumweltamtes. Stoffmonographie Quecksilber: Referenz- und Human-Biomonitoring-Werte (HBM). Bundesgesundheitsbl Gesundheitsforsch Gesundheitsschutz 1999; 6: 522–32.
25. Kjellström T, Kennedy P, Wallis S, Mantell C. Stage 1: Preliminary tests at age 4. Solna: National Swedish Environmental Protection Board, 1986. Physical and mental development of children with prenatal exposure to mercury from fish. Report 3080.
28. Schoeman K, Bernd JR, Hill J, Nash K, Koren G. Defining a lowest observable adverse effect of hair concentrations of mercury for neurodevelopmental effects of prenatal methylmercury exposure through maternal fish consumption: a systematic review. Ther Drug Monit 2009; 6: 670–82.
Thallium is named after the green spectral line it produces on flame spectroscopy (Greek “thallos” means “green shoot or twig”). Tl is considered a heavy metal on account of its density. In its pure form Tl is bluish-white in color and very soft and malleable. Tl has 47 isotopes. Natural Tl is a mixture of two stable isotopes, 205Tl (70.5%) and 203Tl (29.5%) and has an atomic mass of 204. Tl appears as monovalent thallo- and trivalent thalli- compounds. The chemical properties of monovalent Tl are similar to alkali metals (potassium) while trivalent Tl behaves more like aluminum. When exposed to aluminum and moisture Tl oxidizes at the surface, forming a coating of Tl(I) oxide (TI2O) and, at higher temperatures it forms Tl(III) oxide (Tl2O3) .
Inorganic Tl(I) compounds are more stable than Tl(III) analogues in aqueous solution at neutral pH. Covalent organothallium compounds are stable in the trivalent form . Tl is particularly toxic in its Tl(I) compounds, such as sulfate (Tl2SO4), carbonate (Tl2CO3) and acetate (CH3COOTl). The sulfide (Tl2S) and iodide (TlI) are both poorly soluble and therefore, much less toxic. Tl salts are odorless, tasteless, and colorless .
Tl is generally found in the earth’s crust as salts and minerals. Its concentration is 0.3–0.6 mg/kg in the earth’s crust and 65 pmol/kg in seawater.
As a non-essential element, only Tl intoxication is important in human, animal, or plant metabolism.
- Acute poisoning due to the ingestion of water-soluble Tl salts in rodenticides or pesticides (banned in Europe), either accidentally or with suicidal or criminal motives
- Chronic occupational exposure associated with the mining and processing of iron, cadmium, and zinc-containing ores, cement production, and Tl processing
- Clinical symptoms of Guillain-Barré syndrome with hair loss.
- Direct electrothermal atomic emission spectrometry (ETAES) .
- Photometry of thallium-halogenide complexes in the urine .
Heparin or EDTA whole blood: 3 mL
24 h collection of urine (specify volume): 5 mL
In nature, Tl is found in trace amounts in many minerals, mainly in sulfides ores (Fe, Pb, Zn) which are commonly employed for the production of sulfuric acid. In the roasting process, Tl may arrive in the flue dust or in the lead chamber slime or it can remain in the pyrides cinder which is used in the cement industry /, /. The release of Tl into the environment by human activities is the result of gaseous emissions from cement factories, coal-fired power plants, and metal sewers. Although it is very toxic (MAC 0.1 mg/m3), Tl is not a potential environmental pollutant. The Office of Environmental Health Hazard Assessment (OEHHA) has developed a public health cut-off of 0.1 μg/l for Tl in drinking water and the maximum contaminant level (MCL) at which known adverse effects of human health are anticipated, is 2 μg/L .
Tl is used as a catalyst in alloys, optical lens, jewelry, low temperature thermometers, semiconductors, and scintillation counters. It is applied in clinical diagnostics as a contrast agent for cardiovascular and tumor imaging. Most gamma radiation detection equipment such as scintillometer and infrared radiation detection and transmission equipment contains Tl as an activator. Tl-arsenic-selenium crystals are essential filters for light diffraction in acousto-optic measuring devices . Tl salts are used as rodenticides and insecticides for depilation and as a rodenticide and insecticide in some developing countries, although the WHO recommended in 1973 that the latter use be discontinued.
The clinical picture associated with Tl intoxication is non-specific, variable, and depends on the dose and administration route.
Because Tl salts are usually odorless, tasteless, and colorless, they were a popular method of murder in the past. Tl acetate, Tl carbonate, and Tl sulfate are fatal. Average lethal dose for at orally administered Tl sulfate is 10–15 mg/kg body weight. Intoxication can also result from inhaling contaminated dust. Industrial and deliberate poisoning are now rare thanks to strict regulations. The maximum blood concentration is reached after around 2 hours, at which stage Tl can also be detected in the urine.
In the early stage acute Tl2SO4 poisoning caused by rodenticides and insecticides can present with symptoms similar to those of Guillain-Barré syndrome, acute porphyria, myocardial infarction, diabetic neuropathy, arsenic intoxication, acute systemic lupus erythematosus, carbon monoxide poisoning, or organophosphate poisoning .
- After oral ingestion of Tl, the poison causes an inflammatory reaction in the structures that were exposed first, resulting in glossitis, pharygooesophagitis, gastritis, enteritis and colitis.
- Often treatment-resistant nausea and vomiting occur during the day 3–4 day period following intoxication.
- Neurological symptoms appear between 2–5 days which are characterized by painful, rapidly progressing peripheral neuropathy that dominates clinically in the second and third week.
- Cardiac signs such as sinus tachycardia, irregular pulse, hypertension and angina-like pain occur in the second week after ingestion of Tl.
- During the first 2 weeks after exposure, there is albuminuria, erythrocyturia, leukocyturia and the presence of cylindrical casts. However, the renal function is not grossly impaired.
- Alopecia is the best known effect of Tl poisoning. Epilation begins about 10 days after ingestion, complete hair loss is seen in about 1 month. After 2–3 months, the hair will be restored to its former condition.
- The typical clinical picture unfolds by 2–3 weeks of acute poisoning. By then, precious time for therapeutic intervention is lost .
- About one month after poisoning, transverse white lines called Mee’s lines appear in the nail plate as results of complete erosion after of the proximal parts of the nails.
Chronic occupational exposure can occur in workers employed in the smelting of sulfide ores, the manufacture of cement, semiconductors, and special glasses.
The cardinal symptoms of chronic Tl intoxication are alopecia and leg hypersensitivity that is reminiscent of that seen in Guillain-Barré syndrome. Distal symmetrical axonal degeneration with secondary demyelination occurs . Other typical symptoms include nail dystrophy, skin changes, cardiovascular disorders, renal impairment, polyneuritis, muscle paralysis, tachycardia, and even cardiogenic shock.
Less severe intoxication with sublethal doses below 10mg/kg body weight develops insidiously, with initial symptoms such as constipation, upper abdominal pain, and back pain appearing after 1–2 weeks. Diffuse alopecia usually leads to a diagnosis. The blood Tl concentration is usually below 500 μg/L.
Tl+ ions are generally eliminated by urine, bile, saliva feces, milk and tears. The half-life is 1–3 days after low doses and between 1 and 1.7 days under clinical therapy after ingestion or exposure . Whole blood, urine, feces, hair, and nails are used as specimens to diagnose acute and chronic Tl intoxication.
In the acute phase, Tl intoxication must be distinguished from the following diseases:
- Acute porphyria, by determining porphobilinogen, δ-ALAD, and total coproporphyrin
- Guillain-Barré syndrome, by determining ganglioside antibodies
- Acute rheumatic disease, by determining antinuclear antibodies.
Tl concentration over time:
- Low concentrations of Tl can be normalized quickly in the blood due to the short half life of Tl in the body. In acute intoxication, the Tl concentration is usually above 500 μg/L to 2 mg/L and occasionally as high as 41 mg/L . Four weeks after acute exposure, however, the concentration is still greater than 20% of the concentration measured in the first week.
- In acute intoxication initial excretion of Tl in urine is high, but after 24–48 hours, fecal elimination may be important. Tl can generally be detected up to 2 months after acute intoxication .
- As of day 4, microscopic evidence of intoxication (melanin deposits) can be seen in the hair roots.
- During the first 2 weeks, albuminuria, erythrocyturia, leukocyturia, and cylindruria occur as the result of nephrotoxicity, but serum creatinine is only slightly to moderately increased. The complete blood count is usually normal but there may be a mild leukocytosis. Hepatic enzymes are elevated to no more than 5 times the upper reference interval value.
Patients with clinical symptoms of Tl toxicity have blood concentrations of 3–25 μg/L and urinary concentrations of 4–80 μg/L. Patients with significant intoxication have blood concentrations of up to 200 μg/L and usually have increased urinary concentrations, generally up to 500 μg/L, and occasionally higher. The hair Tl concentration provides only a rough estimate of levels in the organs.
The measures used depend on the type of intoxication. To eliminate the metal and interrupt the enterohepatic circulation, gastric lavage, forced diuresis, intravenous administration of potassium chloride, hemodialysis, and oral administration of potassium hexacyanoferrate II (Prussian blue) are used (usually in combination). Renal excretion mirrors total body Tl.
Blood collection tubes must not contain polystyrene beads.
The amount of Tl ingested daily in food is less than 2 μg. In acute intoxication, Tl is absorbed rapidly, and almost completely, through the gastrointestinal tract, oral mucosa, and skin. After absorption Tl is distributed from the blood to tissue, kidneys display the highest concentrations, followed by bones, stomach, intestines, spleen, liver, muscle, lung, central nervous system (CNS), as well as hair and nails. In the CNS Tl causes neurodegeneration, demyelation, and the end products of lipid peroxidation /, /. Tl crosses the placental barrier and also reaches breast milk. Tl is eliminated by the kidneys and intestine, whereby Tl is secreted directly from the blood into the intestinal lumen .
Tl is stored intracellularly, mainly in the mitochondria. Due to the presence of empty δ orbitals in electronic configuration, Tl has a high affinity for sulphur ligands. It can form complexes with and thus inactivate sulphydryl groups of proteins (to cysteine residues in particular) which are usually involved in reactions catalyzed by enzymes . Inhibition of enzymes with active sites containing cysteine residues promotes oxidative stress as a result of decreased glutathione formation .
Tl+ can mimic K+ because of the same ionic radius and inability of the cell membrane to differentiate between this two cations. Tl+ follows K+ distribution pathways and in this way alters K+ dependent processes. Tl+ can substitute K+ in the Na+-K+-ATPase because its affinity is ten times higher than of K+. This disrupts K+ transport into the cell and mitochondrial K+ transport. Due to its K+-like behavior, Tl+ interferes with many vital metabolic processes and is highly cardiotoxic and neurotoxic.
Tl+ interferes with riboflavin hemostasis forming an insoluble complex and intravascular sequestration of riboflavin. Riboflavin deficiency leads to structural and functional anomalies in proteins. As a result, skin keratinization is disrupted, which leads to hyperkeratotic lesions on palms and soles, acneform lesions on the face and icthyotic lesions on the legs.
4. Aderjan R, Daldrup T, Gibitz HJ, Schneider A. Thallium. In: Gibitz HJ, Schütz H (eds). Einfache toxikologische Laboratoriumsuntersuchungen bei akuten Vergiftungen. Mittlg DFG Senatskommission für klinisch-toxikologische Analytik 1995; XXIII: 422–35.
Clinical and laboratory findings
Clinical and laboratory findings
Pb mobilization test to assess Pb burden
The 95th percentiles are shown. Conversion: μg/L × 0.00483 = μmol/L
Clinical and laboratory findings
Clinical and laboratory findings
Conversion: μg/L × 4.99 = nmol/L. Blood reference values applied to individuals who consume fish up to three times a month. Urinary reference values applied to individuals without dental amalgam.
Type of exposure
Clinical and laboratory findings
Figure 11.6-1 Mechanisms of cellular damage to neurons and astrocytes by methylmercury (MeHg). MeHg inhibits uptake of glutamate and amino acids that are associated with the synthesis of astrocytic glutathione (GSH). Accumulation of glutamate in the extracelluler space and the resulting excessive activation of N-methyl-D-aspartate (NMDA) receptors can lead to cytotoxicity and cell death. Other proposed mechanisms include the MeHg caused dysfunction of mitochondria, impaired cytoplasmic Ca2+ homeostasis and the release of reactive oxygen species (ROS). Modified with kind permission from Ref. .