Gourab Basak1, Kriti Singh2, Anamika Sahu3, Udit Jain4
1Assistant Professor, Department of Veterinary Public Health and Epidemiology, Apollo College of Veterinary Medicine, Jaipur, Rajasthan
2Project Associate, Centre for One Health, Guru Angad Dev Veterinary and Animal Sciences University, Ludhiana, Punjab, India
3Technical Officer, Voluntary Health Services, Chennai, Tamil Nadu
4Assistant Professor, Department of Veterinary Public Health, U.P. Pandit Deen Dayal Upadhyaya Pashu Chikitsa Vigyan Vishwavidyalaya evam Go-Anusandhan Sansthan, Mathura, Uttar Pradesh
Metals are a unique class of toxicants that occur mainly in nature, dispersed in rocks, ores, soil, water and air. These are available in different chemical forms because of their physiochemical, biological and anthropogenic activities. These are widely used in industry, agriculture and medicine. However, along with their essential uses, some impose health hazards to both humans and animals. Heavy metal poisoning is the accumulation of these heavy metals in toxic amounts especially in the soft tissues of the body. The degree of clinical findings varies according to the accumulation of the same in the body. Heavy metals like zinc, copper, chromium, iron, and manganese are required by the body in a very small amount but gradual chronic accumulation in the body leading to a concentration sufficient to cause poisoning. However, lead, mercury, arsenic and cadmium cause serious public health consequences. Mostly the transmission results through water, foods, medicines, improperly coated food containers, industrial exposure etc. Though not in the lime-light, heavy metals may act as silent killers to humans. The major and common affections of heavy metals are depicted in Figure 1.
Figure 1. Chief predilection consequences of heavy metals on health
Humans mostly get in contact with arsenic from industrial sources like smelting and microelectronic industries and contaminated drinking water (contaminated by herbicides, pesticides, fungicides, wood preservatives and paints). Overexposure of arsenic leads to headaches, drowsiness, confusion, seizures and even life-threatening complications. It also pertains to neurological symptoms like encephalopathy, peripheral neuropathy, demyelination, and pericapillary haemorrhages within the white matter. In case of skin lesions, the formation of mees’ lines (transverse white bands on the fingernails), oedema on soft layers of tissue underneath the skin, hyperkeratosis, hyperpigmentation, and exfoliative dermatitis are detected. Besides, gastroenteritis characterised by vomiting, abdominal pain, diarrhoea, bloody diarrhoea and sometimes, fever are common symptoms. Additionally, haemolysis, anaemia, hypotension, and garlic-like odour in breath are observed. In chronic cases, symptoms like inflammation of sensory and motor nerves (polyneuritis) and the mucosal membrane of the throat can be encountered.
Lead exposure leads to accumulation of the metal in bones among lead production workers, battery plant workers, welders, and solders mainly; but it may affect any organ system simultaneously. It is excreted in urine and faeces, and may also appear in hair, nails, saliva, sweat and breast milk. The effect of the toxicity depends on the age and amount of the exposure experienced by an individual. In children, the symptoms may vary and may not be noticeable until they develop over a period of three to six weeks. Overexposure can lead to change in behaviour such as being less playful, clumsier, irritable and sluggish (lethargic). Symptoms like headaches, vomiting, abdominal pain, lack of appetite (anorexia), constipation, slurred speech (dysarthria), altered kidney function, hyper-proteinemia, and anaemia can be witnessed. Neurological symptoms include ataxia, encephalopathy, seizures, convulsions, papilledema, impaired consciousness, mental retardation and selective deficits in language, cognitive function, balance, and behaviour. In addition, in adults, high blood pressure and damage to the reproductive organs may be encountered. Fanconi’s anaemia, a blood disorder can be acquired by acute lead poisoning.
Cadmium is used in electroplating, storage batteries, vapour lamps, and solders. It also infers a variety of clinical symptoms on overexposure. Apart from the general symptoms, viz., fatigue, headache, nausea and fever, other various systemic symptoms include abdominal cramps, diarrhoea, emphysema, pulmonary oedema, dyspnoea, tachycardia, anaemia, cyanosis, anosmia, renal tubular dysfunction, proteinuria and osteomalacia. Similarly, chromium which is used in cars, glass, pottery and linoleum cause lung and respiratory tract cancer and kidney disorders on repeated exposures apart from gastrointestinal symptoms, severe water-electrolyte disorders, acidosis lesions on the myocardium and liver. Cobalt, which is generally used in jet-engine manufacturing leads to nausea, anorexia, tinnitus, nerve damage, respiratory diseases, enlarged thyroid gland, heart and kidney damage.
Mercury, again an important heavy metal, affects lungs, kidneys, brain, and skin. It is excreted through urine and faeces, and the poisoning causes fatigue, depression, sluggishness, irritability, headache, respiratory symptoms including tightness or burning pain in the chest, fibrosis, behavioural and neurological changes, progressive cerebellar syndrome, abnormal involuntary movements like rapid or slow involuntary movements of fingers or toes which is termed as choreoathetosis. Additionally, cerebellar ataxia, dysarthria, erethism or mad hatter syndrome (a personality disorder because of chronic exposure to inorganic mercury), sensory impairments like constriction of visual fields, tunnel vision, blindness, and hearing loss may be encountered.
Heavy metal toxicity affects males and females equally depending on the exposure. Even some of these are toxic at very low concentrations whereas some executes acute toxicities and others chronic involving a battery of clinical symptoms affecting the vital organs of the body. The prognosis of heavy metal poisoning is good if diagnosis is made before any vital and/or permanent damage. Some of the major preventive ways are listed in the Figure 2.
Figure 2. Major preventive measures for nullifying heavy metal toxicities
Reduction of exposure and identification of high-risk groups and vulnerable populations as well as curing of such conditions constitute the primary prevention. In cases of acute poisoning, chelating agents, viz., dimercaprol, calcium disodium EDTA, and penicillamine are the choices for treatment as these agents after binding to the metals get excreted in urine. Non-occupational population comprising of children, pregnant women etc., get exposed via metals in food, water, consumable products and air; as a consequence of expanding environmental dissemination. Thus, control of source of contamination is an important step. Environmental or workplace monitoring and biological monitoring of exposed or high-risk people provide the effective data for remedial actions. Moreover, training and education of related personnel and the common public about health risks and the preventive approaches lead to the reduction and control of heavy metal exposure.