Mercury is present in nature in trace amount which has adverse negative impact on environment and living creatures.Mercury and its compounds are highly toxic in nature.

The toxicity of mercury compounds depends their solubility.Mercury compounds which are volatile or soluble are potential hazards for human body. But fortunately most of the compounds of mercury are insoluble so do not cause toxicity. Inorganic salts of mercury are not very toxic if swallowed. Most of the salts are excreted unchanged from the body. But organic salts of mercury are more toxic than inorganic salts. Mercury reacts by replacing the two hydrogen atoms in a (-SH) sulfhydry group. The free -SH groups are essential for enzyme activity. When the bond with mercury is formed enzyme becomes inactive. As a result, biological process in cells are disturbed and symptoms of mercury poisoning are seen.

Naturally during volcanic eruption and burning of coal also dispose of mercury in atmosphere takes place. Combustion of coal releases about 5000 tons of mercury in atmosphere every year.

Mercury vapor is very toxic ,if inhaled it passes into blood stream, the digestive system, lungs and the transported to brain. Exposure to methyl mercury compounds can cause brain damage,loss of vision, madness and even death. The symptoms of mercury poisoning are depression, irritability, shaking of hands and psychotic behavior.

In the 19th century, mercuric nitrate and mercuric chloride were used in the production of felt hats. The dust from this factory was a serious occupational hazard to the workers of this factory.The mercury salt affected the central nervous system of workers. The workers suffered from tremors and a physical disturbance due to the mercury poisoning . This gave rise to the phrase "as mad as s hatter" and consequently it was banned.Besides these, industries producing sodium hydroxide and chlorine by electrolysis of brine, some mercury is lost through waste products into river water causing mercury poisoning.

Two major epidemics due to mercury poisoning took place in Japan due to the industrial release of mercury compounds in the Minamata Bay.The industries which manufactured acetaldehyde and PVC in Minamata were using mercury compounds as catalyst and they released their effluents into Minamata Bay. The mercury compounds present in the effluent were converted to organomercury compounds by anaerobic bacteria in the mud in the seabed. Planktons took it up which was eaten up by fish and mercury was accumulated in the fish. The people of Minamata consumed such fish and suffered from fatal diseases known as "Minamata Epidemic" which killed thousands of people, several were affected and many new babies were born with severe birth defects including brain damage.