A number of toxic metals are present in the environment. Metals of major toxicological concern are Lead, Mercury and Cadmium.
Lead (Pb) cannot be completely eliminated from our environment. Small concentrations of lead are present everywhere.
In soil, lead levels range from 5-25 mg per kg; in ground water from 1-60 μg per litre (little lower in natural surface water); and in air below 1 μg per m .
In most people, the lead intake from food is 100-300 μg per day. However, in some countries average lead levels in the blood of children have been found 4-6 μg per 100 ml. Lead poisoning of children is a matter of serious concern. Like calcium, lead is deposited in bones and during pregnancy it is slowly released from the mother’s bones to the blood reaching the fetus.
The inhibitory effect of lead to the enzymes involved in the synthesis of heme is well known. A moderate exposure to lead with its level in the blood about 50 μg per dl causes anemia. However, other effects may be observed at lower levels of exposure. The impaired heme synthesis may result in anemia, which may be due partly to the greater fragility of the erythrocyte membrane (WHO, 1977).
The nervous system is also a target organ of lead and encephalopathy may occur with blood lead level over 80 µg per dl. At lower levels of 40-50 μg per dl children may exhibit hyperactivity, low attention span and decreased IQ. (Emhardt et al., 1981) which is due to impaired function of neurotransmitters and Ca ion. Lead also affects the nervous system of occupational workers.
Lead is also known to cause renal failure after long term exposure. However, some uses of lead, e.g. lead-acid battery used in motor vehicles and lead use in consumer electronics are still going on or are slowly increasing; human population is advised to be cautious against lead poisoning.
Mercury, like lead, provokes most concern because it is toxic to the nervous system and the fetus and small babies are more sensitive to its adverse effects. A number of anthropogenic activities are responsible for raising Hg levels in the environment. The Hg level in the ambient air is extremely low; its level in unpolluted water is about 0.1 μg per 1. The level of Hg in food, except fish, is also very low (5-20 μg per kg.); but most fishes contain higher levels (200-1000 μg per kg in swordfish and tuna).
The consumer population is exposed to mercury toxicity due to food grain treated with mercury fungicide or fish contaminated with methyl mercury (10-40 ppm). About 90% methyl mercury is absorbed across the human gastrointestinal tract. Cases of mercury poisoning due to eating mercury contaminated fish have occurred throughout the world. The toxic effects of Hg are related to the nervous system. The earliest symptom is Paresthesia and at higher levels of exposure, ataxia, deafness, dysarthria and eventually death may occur (Clarkson, 1987).
Some studies have concluded that methyl mercury poisoning would be unlikely if the daily intake corresponds to a blood level of 20 by per dl and a hair level of 50 μg per g (WHO, 1976). The divalent mercury salts are also harmful after contact or ingestion. Mercury has also been used to fill up dental cavities. The critics of this practice claim that the amalgam is linked to arthritis, multiple sclerosis, and mental disorders. In view of so many uses of mercury and the problems associated with them, it is advised that the use of mercury should be reduced as much as possible.
Generally environmental cadmium levels are lower than those of lead and people ordinarily have lower exposure. The cadmium level in water is very low, about 1 fig per 1, except in polluted areas. Its level in the air is in the range of few nanograms per m3, but may be several milligrams per m3 in certain workplaces. The environmental levels are raised by smelting and industrial uses. Food grains and cereal products are the main sources of cadmium. Humans are also exposed to Cd through cigarette smoking.
Cadmimum bioaccumulates in the liver and kidney and the amount of Cd stored in kidney increases with age. Kidney damage is the most prevalent chronic effect of Cd exposure. The Cd poisoning in Japan has once resulted in itai itai disease, which was due to eating of rice produced from soil containing very high levels of Cd (Nomiyama, 1980). Whenever present, high Cd levels in soil become a matter of worry since plants take up Cd more readily than other metals. Because tobacco plants are able to concentrate Cd, cigarette smoking increases the risk of Cd exposure.
The acute effects of Cd toxicity result mainly from local irritation; after inhalation, the lesions include pulmonary edema and chemical pneumonitis; after ingestion the clinical effects are vomitting and abdominal pain. After long term exposure, Cd causes kidney lesions. Other effects of Cd exposure include hypertension and prostate cancer in occupational workers. In all the developed countries efforts are on to control Cd emissions so that people are not exposed to high levels of Cadmium.
Other metals of concern are Arsenic, Chromium, Beryllium, Nickel and Selenium. Their acute and chronic exposures have been observed to cause serious problems such as skin and lung cancer and black-foot disease (due to As poisoning); hypersensitivity, renal problems and lung cancers (due to Cr exposure); berylliosus(lesions in lung alveoli due to Be exposure ); nasal cancer and cancers of the kidney, lung and stomach (due to M exposure ); and hair loss, nail pathology and teeth decay (due to over exposure to Se).
In animals, an overexposure of Se induces more severe effects including liver necrosis, anemia, disorders of reproductive function and enlargement of pancreas. Excessive intake of cobalt results in polycythemia, an over production of erythrocytes. Overexposure to Fe may cause liver dysfunction.
Occupational exposure to Mn results in pneumonitis and encephalopathey. Occupational exposure to Zn203 fume results in ‘metal fume fever’ (Goyer,1996). Metals such as Co, Cu and Fe are essential metals required in the proper development of erythrocytes. However, certain metals have been used in medicine.