After reading this article you will learn about the toxicity of various important metals. The metals whose toxicity has been discussed are: 1. Copper 2. Silver 3. Gold 4. Calcium 5. Barium 6. Magnesium 7. Strontium 8. Zinc 9. Cadmium 10. Mercury 11. Aluminium 12. Lead 13. Arsenic 14. Selenium 15. Chromium 16. Manganese 17. Iron 18. Cobalt 19. Nickel 20. Vanadium.
1. Copper (Cu):
Copper occurs as native metal and in the form of sulphide ores in nature. Liver and brain are rich in copper. The concentration of copper in soils ranges from 2 ppm to 100 ppm (on dry weight basis) with a mean value of 20 ppm. In unpolluted water, copper concentrations have been estimated between 0.0006 ppm and 0.4 ppm with a median value of 0.01 ppm. Oysters contain 137 ppm Cu and black pepper contains 53 ppm.
ADVERTISEMENTS:
Copper is an industrial health hazard. Copper poisoning occurs mostly due to homicidal ingestion of copper containing solutions or consumption of acidic beverages stored in containers made of copper. Excess of Cu (470 mg) in human body is toxic and causes hypertension, sporadic fever, uremia, coma and even death. Cu also produces pathological changes in brain tissues.
2. Silver (Ag):
In occurs as free metal, in ores and in ores of other metals. The normal human body has about 1 mg silver. Mushroom contains about 300 ppm. Ag and other food products contain about 0.9 ppm Ag (Wheat bran etc.)
Excess of Ag (100 mg) in human body causes changes in blood cells, gastroenteritis, diarrhoea, and fall in blood pressure and decrease in respiration.
3. Gold (Au):
It occurs free in nature and in minerals. The adult human body contains < 10 mg Au, about 50% of which is accumulated in bones.
Soluble gold compounds are toxic. Au toxicity in man causes fever, violent diarrhoea, gastritis, colitis.
4. Calcium (Ca):
It occurs as limestone, gypsum and apatite Ca is an essential macro element which perform functions in bone structure, muscle contraction, blood clotting etc. Calcium is present in bones, teeth and in body fluid.
ADVERTISEMENTS:
Excess calcium has teratogenic action in chicks when about 0.1 mg of Ca gluconate is injected into egg.
The Ca salts are non-toxic except at very high doses (1000 mg for 20 days). An excess of Ca ions depresses the functioning of muscles and nervous tissues. In man hypercalcemia causes coma and death if serum Ca level rises to 16 mg/100 ml.
5. Barium (Ba):
It occurs in nature as sulphate, burite or heavy spar (BaSO4). It is present in all living organisms’ especially marine plants and animals. The human body contains about 22 mg Ba, 66% of it is present in bones.
Barium salts are used in the manufacture of alloys and in the manufacture of glass, ceramics, television picture tubes etc. Ba (OH)2 is used in sugar refining, while BaSO4 is used as a lubricant in drilling oil wells.
ADVERTISEMENTS:
Ba toxicity (more than 100 mg in human beings and other mammals) causes excessive salivation, colic, vomiting, diarrhoea, tremors, muscular paralysis and paralysis of nervous system.
6. Magnesium (Mg):
It is an essential nutrient for living organisms as it forms part of the structure of the body and plays a critical role in cell metabolism. Chlorophyll, a green pigment present in leaf contains Mg.
Magnesium toxicity (in high doses 400 mg) causes nausea, muscular weakness and paralysis in human beings and mammals. New born infants develop hypermagnesemia if their mothers have been treated with MgSO4.
7. Strontium (Sr):
It occurs in nature as carbonate and sulphate. Earth’s crust contains 450 ppm Sr and sea water has about 8 ppm Sr. A human adult body contains about 320 mg of Sr.
ADVERTISEMENTS:
Strontium salts are non-toxic at low concentrations but high concentration (800 mg) results in respiratory failure and death in rats. In cats, injection of SrCl2 raises blood pressure due to stimulation of cardiac muscle but large doses produce apnea.
8. Zinc (Zn):
It occurs in nature as blende, sulphide, silicate, zinc spar, zincite (ZnO) etc. The mean concentration of zinc in the earth’s crust has been estimated to be 70 ppm. Sea water contains 9-21 ppb Zn. The human body contains 300 mg Zn, 65% of it in muscle, 20% in bone, 6% in plasma, 2.8% in erythrocytes and about 53% in liver.
Zinc salts are relatively non-toxic but heavy doses (165 mg) for 26 days because vomiting, renal damage, cramps. The most common sources of zinc poisoning in human beings are metal fumes and illness arising from the ingestion of acidic food prepared in zinc galvanised containers.
Zinc stereates are the cause of penumonitis. Inhalation of air containing ZnO at 1-34 mg/m3 causes metal fume fever and penumonitis in human beings.
9. Cadmium (Cd):
Although it does not occur free in nature and there is no specific ore from which it is mined. The finding is significant because many people are exposed to cadmium via cigarette smoke and contaminated food and water.
Human beings are also exposed to the harmful cadmium while manufacturing and recycling nickel cadmium batteries. Cadmium is obtained generally as byproduct from refining of zinc and copper. It is present in trace amounts in plants and marine animals. Wheat and rice proteins contain appreciable amount of Cd.
It is used in industry in protective coating for iron, copper and steel. Telephone wires are made up of Cu-Cd alloys. Ni-Cu batteries are used in electronic equipment’s.
Cigarette-smoke inhalation is a source of exposure to Cd. The total Cd in cigarette smoke varies from 10 to 18 µg per 20 cigarettes. This represents 70% of Cd content of cigarette tobacco (Vandi, 1969). Cadmium dust, fumes and mists pollute the atmosphere in Zinc, Cu and lead industries.
Cadmium is toxic to all men and animals. Cd toxicity (50 mg) causes vomiting, diarrhoea, abdominal pain and loss of consciousness. It also causes growth retardation, bone deformation, impaired kidney functioning, impaired reproductive function, hypertension, tumor formation and teratogenic effects. If we inhale Cd through nose, it causes choking, coughing, bronchitis and damage to renal tissues.
According to a recent study by the US based National Institute of Environmental Health Sciences (NIEHS), cadmium disturbs the human DNA repair system that is important to prevent cancer. Marine diatoms can use cadmium, cobalt or zinc interchangeably for optimal growth, but this is the first enzyme from a marine diatom that carries cadmium in its metal binding site in native form.
In order to replace dying cells, they need to duplicate their DNA. During the replication process, lots of errors are made, which are regularly rectified by a process, known as post replication mismatch repair (MMR). Cadmium affects MMR, which can lead to cancer, reproductive problems and birth defects.
In a recent study cadmium, a toxic metal often used in batteries, has for the first time been found to a have a biological use – as a nutrient that is used by plankton in the ocean. Other trace metals found in the ocean may also be used in enzyme secreted by sea creatures. This could play an important role in the cycling of trace metals in sea water which, in turn, has implications for global carbon cycling and climatic change.
10. Mercury (Hg):
Mercury is present in hair dyes, cosmetics, dental analgams and lighting. Mercury also comes from coal fired plants and chlor – alkali industry.
It occurs as native metal mixed with its ores. The human body contains about 13 mg. Hg. about 70% of which is present in fat and muscle tissues. Hg containing insecticides and fungicides causes Hg toxication.
Kidney > liver > intestinal and colon walls > brain > heart > lungs > respiratory mucosa > muscles > skin. The kidney retains maximum concentration of Hg. In human body Hg was found in brain, liver and in serum proteins. Tissue retention is greater for CH3 Hg+ than for Hg salt. The toxic action is due to crowding of Hg+2 ions around the immediately available thiol groups of proteins and delay in distribution of these ions among rest of thiol group throughout body.
Mercury toxicity is a world-wide problem as mercury and its salts are industrial health hazards. Hg is found in water due to disposal of industrial mercury waste. Excess of Hg (more than 100 mg) may cause headache, abdominal pain, diarrhoea, hemolysis, digital and tremors. Hg vapour causes acute tightness and pain in chest causing great difficulty in breathing. Hg compounds affect central nervous system.
Hg compounds create hyper coagulability in blood. Excessive Hg causes death due to binding retention of Hg2+ in renal tissues. Women suffer from Minamata disease due to tetragenic effect which were partially present in foetus of women.
In 1907, the Chisso Corporation built a factory for manufacturing petrochemicals and plastics in a small fishing village of Japan situated along the shore of Minamata Bay. From 1932 to 1968, the corporation dumped an estimated 27 tonnes of mercury compounds into the bay.
The HgCl2 was converted to an organomercury compound CH3HgSCH by anaerobic bacteria in the mud on the sea bed. This is concentrated in the food chain.
First, it is taken up by the plankton, which is eaten by the fish, and other sea food, which in turn is eaten by man. The first signs of the effects of this dumping came to surface in 1925 when the fish population started declining.
But it was only during the mid-1950s that the effects were seen in animals and human beings, who consumed the fish. Birds started dying. People began to experience numbness in their limbs and lips. Their speech slurred and their vision got constricted. Some even suffered from brain damage.
Most victims complained of severe convulsions, intermittent loss of consciousness, repeated lapses into crazed mental states and they finally slipped into coma. With the onset of a very high fever, they would die. As of 1992, the number of people officially diagnosed as having Minamata disease totalled 2252, of which 1043 died.
Neurotoxicity has been observed in human beings from indirect CH3Hg + intoxication by eating pork raised on Hg contaminated food.
The mechanism of reaction is that Hg binds thiol ligands of functional proteins and therefore inhibits or inactivates these proteins.
11. Aluminium (Al):
Al occurs as bauxite, spinel, muscovites and orthoclase. The earth’s crust contains about 8% Al in undecomposed rock fragments. A man’s body contains about 60 mg Al, 35% of which is in the skeleton and about 20% in lungs. Al is not an important element for mammals.
Many of Al compounds undergo hydrolysis and form colloidal aluminium hydroxide or colloidal aluminium phosphate and are removed from blood by phagocytosis. However, a small portion of Al ions is transported into brain.
Al compounds are non-toxic unless given in very high doses (more than 4000 mg). It creates gastro-intestinal irritation and shaver’s disease in lungs.
12. Lead (Pb):
Lead is used as pigment in paints. Lead occurs as its sulphide, cerussite (PbCl2) and galena. Pb is also present at 15 ppm in earth’s crust. In sea water 5 ppb lead is present. It is found in all living organisms.
The permissible limit of lead is 2.5 kg/kg in vegetables and food products. Thus it is distributed in food and in the environment. A human body contains about 120 mg Pb, 96% in the bones. The concentration of Pb increases with age and it may reach to a limit of 400 mg. It is not essential for mammals.
Under specific conditions Pb is stimulatory causing enhanced protein synthesis, DNA synthesis and cell replication. Any metabolic disturbance resulting in osteolysis will liberate Pb from its skeletal storage.
Pb is deposited mostly in bones and some in soft tissues. Pb is also retained by mammals in liver, kidneys, muscles etc. About 800 mg Pb creates toxicity in human.
The toxicity symptoms are mild anemia, brain damage, vomiting, loss of appetite, convulsions, uncoordinated body movements and stupor, eventually producing coma and death (when Pb > 500 mg.).
Lead toxicity is due to concentration of diffusible Pb in soft tissues. Another possible mechanism for Pb toxicity is due to formation of metallothionein.
13. Arsenic (As):
Arsenic compounds may be organic and inorganic. Inorganic arsenic is more toxic than organic arsenic and trivalent form is about 60 times more toxic than the pentavalent form, and is more soluble in water.
Both trivalent form and pentavalent form are commonly found in the ground water of both India and Bangladesh. It occurs in nature as a brittle metal. It is present in water at 50 ppb and earth’s crust at 2 ppm. A human body contains about 18 mg. As which is distributed in tissues.
Arsenic accumulates with age in human and animal tissues such as spleen aorta and hair.
Arsenicosis or arsenic toxicity develops in the human body after two to five years. Initially, the skin begins to darken. This stage is called diffuse melanosis. This happens first in the palms. This disease leads to spotted melanosis, when darkened spots begin to appear on the chest, back and limbs.
At a later stage leucomelanosis sets in; the body begins to show black and white spots. It is common in persons who have stopped drinking contaminated water but previously had spotted melanosis. The blood contains about 25 mg as. As compounds are absorbed through skin. It binds globin of hemoglobin in erythrocytes.
As toxicity (greater than 25 mg) creates vomiting, diarrhoea, nausea, severe irritation of nose and throat, abdominal pain, skin eruptions, inflammations and even death.
Keratosis is the middle state of arsenicosis. The skin, in portions, becomes hard and fibrous and looks as if the body has broken out into hard boils or ulcers. Diffuse or nodular keratosis in the palm of the hand or sole of the foot is a sign of moderately severe arsenic toxicity. Rough dry skin, often with palpable nodules on hands, feet and legs indicate severe toxicity. This can lead to the formation of gangrene, and cancer.
Arsenic poisoning also causes liver and spleen enlargement and cirrhosis of the liver, myocardial degradation and cardiac failure, peripheral neuropathy affecting primary sensory functions, diabetes mellitus, goitre and skin cancer. Three types of cancers can develop primarily from keratosis.
These are:
(a) Bowen’s disease, which is a form of squamous cell carcinoma.
(b) Basal cell carcinoma, and
(c) Squamous cell carcinoma.
14. Selenium (Se):
Se is a toxic element of VIth group. It is found in nature in various forms.
So toxicity is due to interference with sulphur metabolism and function, it also effects enzyme systems.
In the right amount, selenium facilitates the normal functioning of the immune system and the thyroid gland. Its deficiency can lead to cardiac problems. But if the ingestion levels are excessively high, the element can cause a disease known as selenosis.
Signs of Se poisoning (more than 4 mg) are fever, nervousness, vomiting, falling of blood pressure and finally death. It also causes liver, kidney and spleen damage with loss of nails and hair. It causes blindness in animals. Cats are most susceptible followed by rats and hamsters.
Bureau of Indian Standards prescribes a limit of 0.01 ppm of selenium in drinking water. For soil, the safe limit is 0.5 ppm. Any amount above this would lead to plants containing more than 5 ppm of the element.
When selenium levels in plants goes beyond 5 ppm, it becomes toxic for consumption. If the amounts are in excess of 100 ppm, white patches start to appear on the plants and this condition is known as snow white chlorosis.
15. Chromium (Cr):
Main sources of chromium are paint industry, alloy industry, glass industry, electroplating diffluent and tannery wastes, where K2Cr2O7 is used for hide cleaning and leather treatment. It occurs in nature as chrome iron ore (FeOCr2O3). It is also present in soil and plants at 1-2.5 ppb in sea water and about 200 ppm in earth’s crust. A human body contains 6 mg Cr. It a cumulates in lungs with age.
According to WHO, the level of hexavalent chromium in potable water should not be higher than 0.05 mg/L.
It is used in industries in the manufacture of steel, jet engines, tools, paints, photography, electric cells, rubber goods and matches.
Chromium bound to β-globulins is distributed in lungs, heart, brain, liver, testes and spleen.
Trivalent Cr is least toxic of trace elements while hexavalent Cr (greater than 70 mg) is very toxic. It causes cancer, anuria, nephritis, gastro-intestinal ulceration and affects central nervous system.
16. Manganese (Mn):
It occurs in nature as oxide (pyrolusite, manganite, brounite), silicate (tephroite) and carbonate (manganese spat). It is found in sea water at 1 ppb and also found both in plants and animals. The human body contains 12 mg, Mn, 45% of it is in bones and rest in soft tissues including brain. It is essential for mammals.
It is used in industry in the manufacture of alloys, dry cells batteries, fireworks, glass and ceramic industries.
Ingested Mn2+ is converted into Mn3+ in human body. Mn absorption is increased for iron deficiency. Mn salts are deposited in lungs where slow absorption takes place.
It is least toxic of essential metals. At concentrations higher than 100 ppm Mn is toxic to man and mammals both. It causes growth retardation, fever, sexual impotence, muscular fatigue and eye blindness. Chromic manganese poisoning known as manganism, leads to psychiatric disorders characterised by irritability, difficulty in walking, speech disturbances and compulsive behaviours.
Permanganate appears to be most toxic among Mn forms when given intravenously and MnO4– ion is less toxic by substantaneous route. Divalent Mn2+ in 3 times more toxic than Mn3+.
17. Iron (Fe):
It is most abundant element in earth’s crust. 5% occurs in sea water at about 3.5 ppb. A human body contains about 4.5 gm Fe, about 70% of which is in haemoglobin, 26% in proteins and 3.5% in myoglobin.
The degree of absorption in human beings and animals depends upon solubility and stability of compound or chelate. Fatty acids, amino acids, fructose etc. act as low molecular weight ligands for Fe. It is excreted mostly in faeces and in smaller amounts in urine and sweat.
Fe (More than 10 mg/kg level) causes rapid increase in respiration, pulse rate, congestion of blood vessels, hypertension and drowsiness.
18. Cobalt (Co):
It occurs in earth’s crust at 23 ppm and in sea-water at 0.1 ppb. Soil contains 100 ppm. It is present in plants, animals and in human body (1.2 mg).
It is used in making high speed tools, alloys, turbines etc.
CO (CO)8 is most toxic and causes lung problems. It causes pulmonary ederna, while nickel causes dermatitis, pneumonia, lung and nose cancer due to Ni (CO)4. Nickel also causes giddiness, headache, nausea and vomiting. Exposure to nickel is also known to lower body weight, damage of the heart and the liver and also irritate the skin.
The high doses (27 mg) cause paralysis, diarrhoea, low blood pressure, lungs irritation and bone defects.
19. Nickel (Ni):
It occurs in earth’s crust at 80 ppm and in sea water at 2-5 ppb. It also occurs in food stuffs- wheat (3-5 ppm), gelatin (4.5 ppm), cocoa (5 ppm), tea (7.6 ppm), baking powder (13 ppm), cabbage, mushrooms, beans. The human body contains about 10 mg Ni, about 18% in skin and rest in tissues.
The lungs retain Ni salts, specially from cigarette smoke. Ni dust is deposited in bronchial mucosa.
Small amount of Ni is non-toxic but higher amount (more than 30 mg) may cause changes in muscles, brain, lungs, liver and kidneys and can also result in cancer. Sometimes it causes tremor, paralysis and even death.
20. Vanadium:
Vanadium ranks 22nd in abundance in the earth’s crust, where it is present in concentrations about 135 ppm.
Vanadium is emitted into the air, water and terrestrial ecosystem from a number of sources which include waste effluents from vanadium metal industries, alumina plants, textile mills, iron and steel industries (which use vanadium as alloying metal) and chemical industries which use vanadium as a catalyst, fly ash of coal, phosphate fertilizers, animal wastes and sewage sludge.
The earliest signs of vanadium poisoning are nausea, vomiting and intensive cough. The tongue gets blue-black and a blue line appears on the gums. Other symptoms are: sore throat, loss of appetite, anemia, and blurred vision, difficulty in breathing, emaciation, diarrhoea, and severe-conjunctivitis often with suppuration, melancholia, dry paroxysmal cough, depression and albuminuria.