Mercury and all of its compounds are extremely poisonous, and mercury is one of the few substances known to have no natural function in the human body. Classified as a heavy metal, mercury is difficult for the body to eliminate. This means that even small amounts can act as a cumulative poison, collecting over a long period of time until they reach dangerous levels. Humans can absorb mercury through any mucous membrane and through the skin. Its vapor can be inhaled, and mercury can be ingested in foods such as fish, eggs, meat, and grain. In the body, mercury primarily affects the nervous system, liver, and kidneys. Mercury poisoning symptoms include tremors, tunnel vision, loss of balance, slurred speech, and unpredictable emotions. The phrase "mad as a hatter" owes its origin to symptoms of mercury poisoning that afflicted hatmakers in the 1800s, when a mercury compound was used to prepare beaver fur and felt materials.
The toxic qualities of mercury have been known for hundreds of years. In the seventeenth century, Swiss toxicologist Johann-Jakob Wepfer studied the characteristics of mercury poisoning. In the early 1920s, German chemist Alfred Stock discovered that he had been suffering from undiagnosed mercury poisoning for most of his adult life. His case was probably caused by years of exposure to mercury vapors in poorly ventilated laboratories. Stock analyzed the pathology of mercury poisoning and devised techniques for detecting very small amounts of mercury. Often using himself as an experimental subject, Stock traced mercury's path through the body and its accumulation in various organs. He published numerous articles warning of mercury's dangers and suggesting safety precautions.
Until recently, scientists thought that inorganic mercury was relatively harmless, so industrial wastes containing it were routinely discharged into large bodies of water. Then in the 1950s, more than 100 people in Japan were poisoned by fish containing mercury; 43 people died, dozens more were horribly crippled, and babies born after the outbreak developed irreversible damage. It was found that inorganic mercury in industrial wastes had been converted to a much more harmful organic form—methyl mercury. As this substance works its way up the food chain, its quantities accumulate to dangerous levels in larger fish. Today, the dumping of mercury-containing wastes has been largely banned, and many of its industrial uses have been halted. However, mercury is still used in electrical switches and relays, fluorescent lamps, and electrolytic cells for manufacturing chlorine. Tiny amounts are also present in dental fillings.