Mercury poisoning is a disorder caused by the ingestion of mercury compounds, the breathing of mercury vapor, or, occasionally, the absorption of mercury compounds through the skin. Fortunately, liquid mercury itself is only slightly absorbed from the digestive tract, so that if a clinical thermometer breaks while in the mouth, the broken glass is a greater danger than the mercury. The mercury used in amalgam dental fillings is also harmless.
Mercury Vapor Poisoning
Poisoning from mercury-vapor inhalation has long been an industrial hazard in electrical apparatus manufacturing and other industries using large amounts of mercury. In the body, inhaled mercury vapor is largely oxidized to inorganic mercury, which becomes concentrated in the kidneys and causes an increase in the output of urine (diuresis). Other early symptoms include blurred vision, bleeding gums, and a metallic taste in the mouth. With continued exposure there may be muscle tremors, loss of appetite, and emotional disturbances. Often, the symptoms clear up when the person is no longer exposed to the vapor.
For many years, large amounts of mercury were discharged as industrial wastes into bodies of water, where they sank to the bottom and were thought to be harmless. However, in 1967 it was discovered that certain bacteria in the bottom sediments could convert the mercury into organic methylmercury, which then entered the food chain and became more and more concentrated as it passed from smaller to larger organisms. The first major outbreaks of methylmercury poisoning occurred during the 1950's and 1960's in Japan, where a number of people died from eating fish containing 10 to 20 parts per million (ppm) mercury. The source of the mercury was the effluent of a local chemical plant. In the United States, large quantities of swordfish and canned tuna were taken off the market in 1970 and 1971 because they were found to contain more than the 0.5 ppm mercury level allowed by the FDA.
Methylmercury fungicides are another source of environmental mercury. These compounds, introduced around 1940 for treating crop seeds, have rapidly accumulated in the environment, poisoning seed-eating birds and their predators. In 1969 a New Mexico family was severely poisoned after eating pork from a hog that was fed grain treated with a methylmercury fungicide.
In the body, methylmercury accumulates in the liver, kidneys, and brain. Symptoms of methyl-mercury poisoning may not appear for months and are usually subtle neurological changes, such as disturbances in coordination, balance, and sensory perception. Loss of peripheral vision, along with slurring of speech and loss of memory, may follow.
The half-life of methylmercury in the body is about 75 days, and it is chiefly excreted in the feces. However, some irreversible damage to the nervous system usually occurs.
In pregnant women methylmercury also poses a threat to the fetus, which is especially vulnerable. Another danger of methylmercury poisoning is that it can cause chromosome breakage.