Heavy metal poisoning is the toxic manifestation of accumulated undesired levels of heavy metals in the body tissues, including blood.
Indications of heavy metal poisoning depend upon the extent of exposure. The early signs in mild cases of poisoning are vague and may be presented by exhaustion, headache, and inability to concentrate or think. In case of severe toxicity, affected person may experience indigestion, constipation, muscle pain, anemia, dizziness, tremors, and lack of coordination.
In cases of acute lead toxicity, commonly observed symptoms cover gastrointestinal problems, such as loss of appetite, cramps, nausea, vomiting, constipation. These symptoms may affect sleep, mood, fatigue, and sexual desire. Prolonged lead exposure causes more severe condition such as involvement of nervous system, blood-forming problems, and even death.
In the long-term, the effects of arsenic poisoning are debilitating and fatal. Symptoms found include gross pigmentation and hyperkeratinization of skin, dermatitis, wart formation, vasospasticity, bronchitis, rhinitis, conjunctivitis, reduced velocity of nerve conduction, encephalopathies, peripheral neuropathies and even death.
Effects of short-term mercury toxicity are nausea, vomiting, skin rashes, eye irritation, tachycardia and hypertension. In chronic exposure, damage to nervous system and kidneys may develop leading to symptoms of tremors, loss of vision, hearing problems and memory loss.
Following tests are required to determine the level of exposure and extent of damage done:
Children are the most vulnerable group to metal toxicity, in particular lead. It is critical that the source of exposure should be immediately removed. When radiographic evidence points to retained metal parts such as toys, coins and paint chips in the elementary tract, whole-bowel irrigation with polyethylene glycol electrolyte solution may be carried out.
Lead exposure through inhalation or ingestion is followed by its incorporation in the skeletal system which acts as a toxic reserve and is difficult to remove. Certain chelating agents are used to reduce or control toxicity of metals; the chelating agents bind with the metals and are then eliminated from the body. However, removal does not restore any neurocognitive damage that already developed . Chelating agents are administered orally, intravenously, intradermally or through suppositories .
Suitable supportive care is necessary depending upon the severity and symptoms of the case. Care is required to correct dysrhythmias, maintain electrolyte balance due to fluid loss, ensure proper rehydration and provide mechanical ventilation if necessary.
Treatment is directed to reduce damage to central nervous system, liver and kidneys and induce recovery.
Cases of heavy metal toxicity are rare in clinical practice and therefore their prognosis is difficult to follow. This poses a problem because untreated metal toxicity cases can lead to serious consequences such as multi-organ failure, cancer and even death.
Heavy metals have an increased affinity to bind with organs like brain, nerves and kidneys and thus affect their functions resulting in neurological and other serious problems. There is also an association between high blood pressure and higher lead contents and other heavy metals contents in the body. In cases of poisoning by some metals, neurological damage may lead to permanent disability. Studies have demonstrated relationship between the deposit of heavy metals (lead, mercury, cadmium and aluminium) in the body of children and the presence of learning disabilities and lower IQ.
In cases of long-term exposure to arsenic contents in water, the common risks are skin lesions such as hyperkeratosis, pigmentation and skin cancers. Exposure under the occupational or industrial situations leading to inhalation of arsenic contents can cause lung cancer. High dosage of arsenic exposure poses greater health risks.
The most common cases of heavy metal poisoning are due to exposure to lead. Following the ban on usage of lead content in paint, food cans and petrol in advanced countries, exposure to this metal has been reduced drastically . Occupational lead exposure however continues during processes such as mining, smelting, welding of lead-based painted materials, glass manufacturing and in operations involving battery manufacture. In areas near these operations, air, water and soil may be contaminated by lead, which may ultimately find its way into the human food chain causing risks to human health.
Arsenic poisoning can result from criminal as well as suicidal intent to its usage. Some medicinal preparations used in traditional systems of medicine can contain arsenic risking serious effects to health. For the general population, arsenic becomes a risk when food and water are contaminated. In places, where ground water is contaminated with high inorganic arsenic contents, drinking of such water leads to serious health problems.
Iron toxicity is largely caused due to excessive use of iron pills alone or in combination with multivitamin formulations. In children, iron toxicity is caused by accidental ingestion of iron pills .
Mercury poisoning may be caused by ingestion, inhalation and skin absorption . The most common source of mercury poisoning is consumption of contaminated food, such as fish. Several uses of mercury pose risk to pollution of the environment. Medical wastes, mining operations and emissions from plants using coal as energy source are some possible sources. Metallic mercury is used in devices such as thermometers, blood pressure measuring instruments, barometers, and electrochemical process of chlorine manufacturing that uses mercury as an electrode . Contamination of animals involved in human food chain ultimately leads to mercury poisoning in humans. Use of mercuric compounds in dental surgeries also poses a risk to human health.
Cadmium toxicity can result from exposure due to ingestion, contamination of environment and smoking. In nature cadmium is found in combination with zinc. Wastes generated from household and industries contain cadmium, which contaminate soil and water resources. In last several decades human exposure to cadmium has increased substantially  . Cigarette smoking also causes 4 to 5 times higher blood cadmium levels compared to non-smokers . Presence of cadmium in most food stuffs leads to its varying levels in different individuals, due to individual differences in food habits.
Epidemiology of heavy metal poisoning varies among countries, depending on the socioeconomic status of the given population and the age group. In countries like UK, even industries considered high risk have rare instances of heavy metal poisoning . In China and South East Asia, heavy metal toxicity has assumed the status of a serious occupational hazard associated with the informal electronic recycling industry. In this industry, protective measures are not adequately regulated.
Health risks due to lead poisoning following the use of paints, cosmetics, leaded gasoline and piped water supply in lower income countries continue as major health problem. Cases of lead poisoning have been reported in over 2000 children living near smelting plants  .
Humans are exposed to arsenic from the air, water and food. Contamination of drinking water by arsenic is caused by arsenical pesticides, mineral deposits containing arsenic and improperly disposed arsenical compounds. Water contaminated with arsenic is the major cause of arsenic poisoning in over 30 countries in the world . Chronic arsenic toxicity has assumed endemicity in some areas of Bangladesh and India due to the natural occurrence of arsenic in their native bed-rock.
In both acute and chronic cases of heavy metal toxicity, encephalopathy is the most common cause of mortality. In the United States, lead toxicity is reported among African Americans due to inadequate removal of lead bearing sources from the environment.
Children are the most affected class among the vulnerable populations to suffer from heavy metal toxicity, largely due to accidental exposure. Lead poisoning in children can cause cognitive and behavioral changes .
By the end of 20th century, production of heavy metals has declined in developed countries leading to a lowering of emissions. In the UK, for example, heavy metal emissions between 1990 and 2000 were reduced by 50%. However, in developing world, the contamination of environment by heavy metal remains problematic due to their continued usage. Mercury is still used in gold mining in parts of Latin America. Arsenic is used in wood preservatives and tetraethyl lead is used as an additive in petrol.
Pathophysiological effects of high concentration of different heavy metal in the body, often referred to as toxidromes, are relatively similar. Following their entry into the body by ingestion, inhalation or absorption through the skin and mucus membranes, heavy metals compete with other ions and bind to sulfhydryl groups of proteins such as enzymes and alter their functions.
The transport of most heavy metals throughout the body is carried out by the formation of ligands with protein, but some metals compete with ionized calcium and zinc and move through membrane channels in ionic form. Impaired enzymes lead to damage of organs and systems throughout the body and this damage manifests as toxic effects. The most commonly involved systems are central nervous system, gastrointestinal system, renal system, cardiovascular system, musculoskeletal system and reproductive system.
Binding to sulfhydryl groups also plays a critical role in the homeostasis of heavy metals and serves a protective role. An increase in the synthesis of metal binding proteins, reported after exposure to heavy metals, is primarily a part of the body's defense against metal poisoning.
The overall severity of pathophysiological changes in various systems, however, depends upon the metal involved, the extent of exposure and the age of person exposed.
Clinicians need to play a role in early detection of lead in patients subject to occupational exposure and advise necessary measures to prevent further exposure. Whenever cases of serious exposure are found, measures are required to reduce further risks and damage done by poisoning.
In case of children exposed to lead it should be considered as a public health emergency because a long-term approach is necessary for removal of contamination and its source. Protection of children against lead exposure continues to be the only most reliable measure because cognitive and behavioral changes following lead exposure are not reversible.
In the Unites States, healthcare providers are required to ensure that blood levels of lead in pregnant women and newborn are screened and documented in medical records.
To avoid exposure to heavy metals, their possible sources should be abandoned or handled with protective measures. Certain items should be abandoned such as lead-based paints, lead containing glazed food containers, and collapsible metallic tubes for toothpastes which may cause lead contamination.
There is a difference of opinion as to which elements fall within the category of 'heavy metals'. Some opinion favors a definition based on atomic weight while the other suggests specific gravity as the basis for consideration. According to the most commonly accepted view, heavy metals are elements that have an atomic weight and density five times greater than water. Heavy metals also include metalloids, like arsenic, which are toxic at low concentrations . In recent years multiple uses of heavy metals for industrial, agricultural, medical and domestic purposes have led to their widespread distribution in environment, raising serious health concerns .
There are 23 heavy elements in all: arsenic, antimony, bismuth, cadmium, cerium, chromium, cobalt, copper, callium, gold, iron, lead, manganese, mercury, nickel, platinum, silver, tellurium, thallium, tin, uranium, vanadium, zinc . While some of these elements such as zinc, copper, chromium, iron and manganese are essential for human physiological processes, many of these elements have no known benefits to bodily functions.
Heavy metal poisoning develops when a person is exposed to heavy elements over time leading to their accumulation at toxic levels in the tissues of the body. The common sources of heavy metal poisoning are contamination of food or water or during accidents in industrial settings. The nature of toxicity of heavy metals depends upon their chemical nature, route of administration and dose. Other factors that determine the effects of overall poisoning are age, gender, genetics and nutritional status of affected individuals.
From the public health point of view, heavy metals responsible for causing serious poisoning to humans are arsenic, cadmium, chromium, lead, and mercury. These metallic elements cause systemic toxic effects leading to multiple organ damage, even at low levels of exposure.
Heavy metal poisoning is the consequence of toxicity due to accidental or industrial exposure, for short-term or long-term, to heavy or semi-metals. The most commonly occurring cases of heavy metal poisoning are reported with mercury, lead, arsenic and cadmium. Arsenic poisoning is common in some parts of the world due to contaminated ground water. Poisoning with heavy metals is reported in children and adults following accidental exposure or in adults working in industries using heavy metals. Though use of heavy metals is regulated in advanced countries, cases of poisoning are still increasing in some parts of the world.
Heavy metal poisoning occurs by all possible routes, ingestion, inhalation, and skin absorption. Once absorbed in the body, heavy metals compete with ions in the body and get bound to proteins and accumulate in body tissues. As their toxic levels are built in tissues, body tissues are damaged leading to symptoms of toxicity or poisoning. Exposure over time can lead to chronic exposure causing nonspecific symptoms. If the symptoms are not recognized and condition is not diagnosed, it may lead to permanent damage to nervous tissue leading to neurological deficit, other organs' damage or even cancer. The symptoms of mild cases of poisoning are nonspecific such as fatigue, headache and inability to concentrate. In cases of severe toxicity, common symptoms are indigestion, constipation, anemia, dizziness, tremor, and muscle pain. Based on the patient's history, specific laboratory tests are carried out to determine the level of exposure and severity of poisoning.
Prevention against further exposure is the best approach to safeguard against serious conditions such as neurological damage. If radiological evidence suggests the presence of paint chips, coins or toys in the alimentary tract, treatment includes whole-bowel irrigation by using polyethylene glycol electrolyte. Chelation therapy is carried out for general toxicity caused by metal poisoning.