Toxic mechanism of heavy metals
Mercury, lead and other heavy metals, even if the content in the body is very low, will still have toxic effects. There are three ways for heavy metals to enter human body: esophagus, respiratory tract and skin. Heavy metals entering the human body no longer exist in the form of ions, but combine with organic components in the body to form metal complexes or metal chelates, which are harmful to the human body. Protein and nucleic acid in the body can react with heavy metals, and trace active substances such as vitamins and hormones, phosphoric acid and sugar can also react with heavy metals. Due to chemical reaction, the above substances lose or change their original physiological and chemical functions, resulting in pathological changes. In addition, heavy metals may change the conformation of the active site by binding with the inactive site of the enzyme, or exchange with the metal acting as coenzyme, so that the activity of the enzyme is weakened or even lost, thus showing toxicity.
Take lead as an example: lead mainly enters the human body through the respiratory tract or digestive tract in the form of dust or smoke, and 5- 10% of lead entering the digestive tract is absorbed, while the absorption and deposition rate of lead entering the lungs through the respiratory tract is 30-50%. 90-5% of lead invading the body forms lead phosphate and is deposited in bones, and other lead is excreted through the excretory system. When the lead accumulated in bones is damaged, infected with infectious diseases or ingested with acid-base drugs, which changes the blood acid-base balance, lead can be transformed into soluble lead hydrogen phosphate and enter the blood, causing endogenous lead poisoning.
Factors affecting the toxicity of heavy metals
The degree of toxicity of heavy metals to human body mainly depends on dose, action time, combined action of various factors, chemical state and individual sensitivity. The dose of trace elements entering the human body reaches a certain level, which can cause abnormal reactions and develop into diseases. This dose can be used as the maximum allowable dose of human body, also known as the poisoning threshold. For example, in people who are most sensitive to methylmercury, when the accumulation of methylmercury in the body reaches 25mg, there will be abnormal perception, walking disorder at 55mg, dysphonia at 90mg, and hearing loss at 170mg. Different forms of heavy metals have different effects on human body. When mercury vapor invades respiratory tract, it can be completely absorbed by alveoli and transported to the whole body through blood. Metal mercury in blood can enter the brain tissue through the blood-brain barrier, and then it is oxidized into mercury ions in the brain tissue, which accumulates in the brain tissue and damages the brain tissue. Inorganic mercury (HgCl2) entering human body often causes kidney injury and mild liver injury.
The toxicity of heavy metals varies in different ways. When metallic mercury is ingested orally, it is rarely absorbed by the digestive tract. However, when mercury vapor is inhaled through the respiratory tract, the lungs can absorb a considerable amount of mercury vapor, resulting in strong toxicity.
Heavy metal pollutants accumulate in the body. The amount of savings is affected by factors such as intake and biological half-life (the time required for pollutants to be reduced by half in organisms). The half-life stays in the body for a long time, and the concentration is easy to rise, reaching the threshold concentration, resulting in toxicity.
Manifestations of heavy metal poisoning
The toxicity of different heavy metals to human body and the clinical manifestations of poisoning forms are different.
Chromium-containing compounds have local effects on skin and mucosa, which can cause dermatitis and perforation of eardrum. People who are often exposed to low-concentration lead will have symptoms such as headache, dizziness, fatigue, memory loss, insomnia, constipation and abdominal pain when the blood lead reaches 60 ~ 80ug/ 100ml. The clinical manifestations of chronic metal mercury poisoning are mainly nervous system symptoms, including headache, dizziness, numbness and pain in limbs. When chromium-containing gas passes through the respiratory tract, pulmonary edema and dyspnea will occur; When entering from the respiratory tract, symptoms such as vomiting, abdominal pain and diarrhea appear. Heavy metals are an important carcinogen, and the main cause of carcinogenesis is the presence of atypical carcinogens on the radioactive surface of metals. For example, chromium and nickel can induce lung cancer.
The hazards of several heavy metals are listed as follows:
Harm of cadmium pollution to human health
Cadmium is an essential element for human body. It often exists in the form of compounds in nature, and its content is generally very low, which will not affect human health under normal environmental conditions. Cadmium and zinc are homologous elements. In nature, cadmium often coexists with zinc and lead. When the environment is polluted by cadmium, cadmium can be enriched in organisms and enter the human body through the food chain, causing chronic poisoning. After cadmium is absorbed by the human body, cadmium protein is formed in the body. It selectively accumulates in the kidney and liver, where the kidney can absorb nearly 1/3 of cadmium entering the body, which is the "target organ" of cadmium poisoning. Other organs such as spleen, pancreas, thyroid, hair, etc. also have a certain amount of accumulation. Cadmium can combine with protein molecules containing hydroxyl, amino and sulfur in the body, inhibit various enzyme systems, and affect the normal function of enzyme systems in liver and kidney organs. Because cadmium damages renal tubules, patients have diabetes, proteinuria and amino acid urine. In particular, bone metabolism is blocked, resulting in a series of symptoms such as osteoporosis, atrophy and deformation. In cadmium polluted areas, the main points of diagnosis of cadmium poisoning are as follows: the high cadmium concentration in urine and blood of patients reflects the high cadmium load of human body; The patient has conscious symptoms and other symptoms of cadmium poisoning. Its main symptoms are systemic pain and multiple pathological fractures, which cause bone deformation and significant body atrophy. At the same time, there are symptoms such as headache, dizziness, salivation, nausea, vomiting, limited breathing and restless sleep. The high concentration of amino acids and low molecular weight proteins in patients' urine suggest renal tubular nephropathy.
Harm of mercury pollution to human health
Mercury can exist in two forms: elemental mercury or mercury compounds under natural or artificial conditions. Elemental mercury is also called metallic mercury. Mercury compounds can be divided into inorganic mercury compounds and organic mercury compounds. Metal mercury poisoning is often caused by mercury vapor, which is highly diffusive and fat-soluble, enters alveoli through respiratory tract and is transported to the whole body through blood circulation. After entering the brain tissue, metallic mercury in blood is oxidized into mercury ions and gradually accumulates in the brain tissue, which will cause damage to the brain tissue when it reaches a certain amount. The other part of mercury ion is transferred to the kidney. Therefore, the clinical manifestations of chronic mercury poisoning are mainly nervous system symptoms such as headache, dizziness, numbness and pain in limbs, systemic tremor and dyskinesia. Methylmercury is easily absorbed in human intestine and distributed all over the body, and most of it accumulates in liver and kidney. Methylmercury distributed in brain tissue accounts for about 15%, but brain tissue is damaged before other tissues, and the main damaged parts are cerebral cortex, cerebellum and peripheral nerves. Therefore, methylmercury poisoning is mainly a symptom of nervous system. The order of symptoms is sensory disorder → motor disorder → language disorder → decreased vision → hearing disorder. Minamata disease is a famous public hazard in Japan, which is chronic methylmercury poisoning.
Harm of lead pollution to human health
Lead is an element harmful to human body, causing peripheral neuritis and causing motor and sensory disorders. In addition, lead flows into brain tissue with blood, damaging cerebellum and cerebral cortex cells. It interferes with metabolic activities, makes the supply of nutrients and oxygen insufficient, causes the swelling of small capillary endothelial cells in the brain, and then develops into diffuse brain injury. When the blood lead concentration reaches 60 ug/ml ~ 80 ug/ml, people exposed to low concentration of lead will have digestive system symptoms such as headache, dizziness, fatigue, memory loss, insomnia, loss of appetite, constipation and abdominal pain. When the lead content reaches a certain level, it will kill fish and destroy the self-purification ability of water.
Lead invading the body will also enter the brain tissue with the blood flow, damage cerebellum and cerebral cortex cells, interfere with metabolic activities, and lead to insufficient supply of nutrients and oxygen. Due to lack of energy, the endothelial cells of small capillaries in the brain swell, the lumen narrows, the blood flow stagnates, and vasospasm leads to cerebral anemia and brain edema, which develops into hypertensive encephalopathy. It is worth noting that tetraethyl lead has a great toxic effect on brain tissue. In the production environment, if the concentration of tetraethyl lead in the air reaches 100mg/m3, people can be poisoned by inhaling 1 hour. It is a strong fat-soluble nerve agent, which mainly invades the hypothalamus and the lower hypothalamus, making the metabolic process of the cerebral cortex disordered, so there are obvious obstacles in the sympathetic nervous system and parasympathetic nervous system during poisoning, and nervous system symptoms appear due to pathological hyperfunction of the cerebral cortex. Because tetraethyl lead has no affinity for sulfhydryl, EDTA, a commonly used inorganic lead antidote, has no alleviation effect on alkyl lead poisoning. At present, there is no reasonable treatment and treatment for organic lead poisoning.
Harm of arsenic pollution to human health
Arsenic and its compounds exist widely in the environment. Arsenic in elemental form is hardly toxic because it is insoluble in water. Toxic compounds are mainly arsenic compounds, among which arsenic trioxide (As2O3) is a highly toxic substance. In one case, soil, water, air, plants and human body all contain trace amounts of arsenic. The amount of arsenic compounds in the environment does not exceed the load of human body, which will not cause harm to health. If the amount of arsenic compounds ingested by human body exceeds its own excretion due to natural or human factors, if the arsenic content in drinking water is too high, long-term drinking will cause chronic poisoning. If the arsenic content in coal is too high, China has also reported cases of chronic poisoning caused by coal pollution.
Arsenic and its compounds enter the human body and are stored in liver, kidney, lung, bones and other parts, especially in hair and nails. The toxic effect of arsenic in vivo is mainly to combine with the enzyme system in cells, which inactivates and inhibits many enzymes and causes metabolic disorders. Long-term intake of low-dose arsenic will take more than ten years or even decades to get sick. Chronic arsenic poisoning is mainly manifested as peripheral neuritis and neurasthenia syndrome. It is characterized by hyperpigmentation and hyperkeratosis of the skin, and cracking ulcer is another feature of arsenic poisoning. Acute arsenic poisoning mostly comes from digestive tract, mainly manifested as severe abdominal pain, diarrhea, nausea and vomiting, and will die if not rescued in time.
Many factors such as the concentration, chemical forms, selective absorption capacity and enrichment capacity of heavy metals will affect human health, and preventive measures should be strengthened.
Lead pollution should be done: don't play on the road where heavy vehicles are driving: wash vegetables and fruits before eating, and peel those that can be peeled; Eat less canned and lead-free preserved eggs; Avoid using lead-containing materials, such as lead-containing paint, in home decoration. Eating more fungus, kelp and Chinese cabbage can also reduce the absorption of lead. In order to control automobile exhaust emissions, unleaded gasoline is used.
Attention should be paid to chromium pollution: low-toxic and non-toxic substances should be used instead of chromium as much as possible in electroplating. For example, many enterprises adopt low chromium passivation or trivalent chromium passivation process, which greatly reduces the emission of hexavalent chromium.
Prevention is the key to cadmium pollution. Because once cadmium is discharged into the environment, it is difficult to eliminate its influence, so patients with chronic cadmium poisoning are treated with vitamin D and calcium.