Two Deadly Gases

In the early decades of the 20th century, farmers had few pesticides at their disposal more effective than arsenates to protect their crops from insect depredations. During World War II, German and Allied laboratories produced complex and lethal chemical compounds, including DDT and lindane. Since then new generations of pesticides such as carbamates (Bux Ten, Furadan and Mobam) have proved to be efficient at curbing insects and microscopic pests without producing the strong toxic effects of DDT on the environment. At the same time, however, these new silent killers still pose a potential threat to other forms of life, including human.

Last week's disaster in Bhopal focused world attention on two highly volatile and toxic gases, methyl isocyanate and phosgene. They, along with many other chemicals, are used in the production of pesticides. Methyl isocyanate (MIC) helps produce Union Carbide's Temik, a product marketed under Robert Gordon Haines, the company's manager for new pesticides. It is one of a group of chemicals called isocyanates that are used to make polyurethane, which, in turn, is used to make paint and varnish. The MIC compound also has been made in the U.S. at Union Carbide's plant in Institute, W. Va., as well as by other companies in West Germany, Japan and South Korea.

MIC reacts quickly with water, and can easily be absorbed through the skin or inhaled. It causes moist human tissues like lung interiors to swell and the eyes to develop cataracts. Victims can suffocate because MIC causes the lungs to fill with fluid, and they can suffer liver damage and burning of the nasal passages, throat and trachea.

Phosgene, synthesized early in the 19th century and also known as carbonyl chloride, is a colorless, highly toxic gas. It is used to make chemicals like MIC, as well as dyes and resins. Phosgene first gained infamy during World War I, when the Germans used it alone or with chlorine in deadly gas attacks. Later, the gas came to be widely used in the manufacture of pesticides.

Because of its long history, phosgene's effects on humans are well known. Inhalation causes severe lung injury, but since the gas has no effect on the upper respiratory tract, victims have no immediate warning, other than a musty odor, that they have breathed in a poisonous gas. Choking is usually the first symptom. Then the lungs eventually fill with fluid and asphyxiation occurs. In most cases of moderate exposure to MIC, the effects are treatable. Even small doses of phosgene, however, can be lethal.