Ever since 5,000 Allied soldiers died from exposure to poisonous chlorine gas on a spring day in 1915 near Ypres, Belgium, modern medicine has been struggling to combat one of the deadliest and most frightening aspects of warfare.

Now, nearly a century later in an era replete with sophisticated military technology that includes satellite photos and cruise missiles, often invisible and sometimes odorless poison gases remain capable of severely hampering the ability of troops to fight and of battlefield doctors to treat them.

Dubbed "the poor man's atomic bomb," chemical weapons are among the deadliest agents known. They include poison gases such as chlorine, cyanide and phosgene, as well as nerve and mustard gases. Two nerve gases -- tabun and sarin, both known to be produced by Iraq -- are so toxic that a pinhead-sized droplet can kill within minutes.

These two compounds, along with another nerve gas called soman, were developed by the Germans during World War II. They contain phosphorus, hydrogen and carbon and are chemically related to pesticides. They kill by blocking a key protein or enzyme in the body. That, in turn, leads to a build-up of acetylcholine, a chemical messenger that normally crosses the gap between nerve and muscle cells and causes debilitating disturbances of the central nervous system. Almost immediately, those exposed to a large dose of nerve gas experience excessive sweating convulsions and loss of consciousness. Death occurs within minutes.

Mustard gases, known as blistering agents, also can be lethal. Sulfur mustards produce yellowish clouds that smell like garlic and primarily affect the skin, eyes and lungs. They attack the body's key genetic material, DNA, and kill cells. Sulfur mustard gases inflict severe burns, sear the lining of the lungs and can cause blindness. They kill about 1 to 5 percent of those exposed.

Surviving chemical warfare largely depends on complete isolation from the environment. Soldiers wear lifesaving garb that resembles the suits worn by astronauts on space walks. The suits are heavy, cumbersome and hot and limit a soldier's vision. During training exercises, the chief problem with the suits was heat stress, according to Carol Fullerton, a psychologist with the Uniformed Services University of the Health Sciences in Bethesda who has studied combat exercises.

Protective clothing and masks make diagnosing and treating injured soldiers on the battlefield difficult. "You can't take a pulse or blood pressure through the gear," said Robert Ursano, acting chairman of the department of psychiatry at USUHS who also studied soldiers' reactions to the protective suits during simulated combat. "If you're lucky and the person is still conscious, you may get him to talk to you or see his eyes, but that's about it."

The garb also creates psychological problems for some soldiers and adds to the intense stress of combat. Three studies of military exercises that simulated battlefield response to chemical warfare found that as many as 20 percent of GIs who wore the protective garb "experienced moderate to severe psychological symptoms, including anxiety, claustrophobia and panic," Ursano and Fullerton reported in an article published earlier this year in the journal Military Medicine.

In one training exercise involving 100 troops, 7 percent either felt severely claustrophic or had so much difficulty with their equipment that they simply removed it, an action that would have been fatal during an actual chemical attack.

"With adequate training and experience, people can learn to overcome that," said Col. Michael A. Dunn, commander of the U.S. Army Medical Research Institute of Chemical Defense at the Aberdeen Proving Ground, Md.

Yet even when well-trained troops are able to don their gear rapidly, chemical weapons attacks may occur so fast that they are still exposed to lethal doses.

"Some soldiers on a chemical battlefield may be at risk for absorbing up to five times the lethal dose of {soman} during an intense chemical attack," according to Dunn and Frederick Sidell, both physicians at Aberdeen.

As an additional measure, U.S. troops who are bound for battlefields where they are likely to face chemical weapons take tablets containing pyridostigmine, a drug that acts as a preventive treatment against nerve gas. Pyridostigmine alone can't protect against exposure to nerve gas. But the drug greatly boosts the ability of two antidotes -- oximes and atropine citrate -- which counteract the toxic effects of nerve gas exposure and must be injected soon after an attack.

Pyridostigmine prevents the poison chemicals from attaching to one type of protein that regulates the nervous system signals. Taken at the correct dosage, pyridostigmine causes no side effects, according to Dunn.

Typically, soldiers take one tablet every eight hours. However, taking too high a dose of the drug or taking it too often can cause visual or intestinal disturbances. The chief drawback of pyridostigmine is that it cannot reach the brain, where nerve gases inflict some of the greatest harm. This is why it must be coupled with the two antidotes.

American soldiers carry with them two types of automatic syringes loaded with atropine citrate and oximes. Both antidotes can counteract nerve gas but are power- less against mustard gas. The syringes can pierce protective garb and clothing with a tiny hole, and soldiers are

trained to inject themselves in the thigh at the first sign of exposure.

Oximes are organic compounds that attach to the nerve gases and remove them from the key proteins, thereby allowing the nervous system to function normally. The second antidote, atropine citrate, counteracts the effects of nerve gases throughout the body. It works by interfering with the build-up of acetylcholine, a key chemical messenger, and blocks disturbances in the central nervous system that the gases are designed to create.

Like pyridostigmine, the antidotes can cause side effects if they are used improperly.

The high temperatures and arid conditions of desert warfare further complicate the problems of exposure to chemical weapons. The chemicals are often invisible and odorless, and, as a result, symptoms of heat stress, such as sweating, may be mistaken for exposure to nerve gas.

The soldier who mistakes heat stroke for gas exposure and injects himself with atropine will suffer the side effects of that drug, which are similar to an overdose of cold medication. "You feel like your heart is speeding up," Ursano said. "With very high doses, you will feel frightened and have psychotic hallucinations that mimic the kinds of things you might have from a nerve gas exposure." More atropine may then be administered, which only worsens the problem.

Emergency medical facilities in gas-tight mobile units or underground shelters can provide care, but first the injured must be decontaminated, a time-consuming process similar to going through a series of air locks on a spacecraft.

Health care workers wearing protective suits cut away the injured soldier's gear and lift him out of it. But regular clothing worn underneath, as well as wounds, are also likely to be contaminated with poison chemicals after a bullet, shrapnel or a grenade rips through the protective gear.

To prevent the chemicals from spreading into the gas-tight medical unit, the injured must be processed in stages until they are sufficiently decontaminated to enter the main medical facility. "It's a very difficult operational and medical environment," Ursano said.

Those who survive chemical warfare attacks may face long periods of recovery. Mustard gas burns are similar to those from fire and take a long time to repair. Nerve gas exposure results in increased irritability, memory loss and difficulties with more subtle mental functions, such as the ability to solve problems.

WHAT TEAR GAS DOES

Despite concerns in the international community about some of the uses of tear gas, it remains a common means of controlling and subduing crowds throughout the world.

It was frequently employed by police during the anti-war demonstrations of the 1960s and early '70s, including at the 1968 Democratic convention in Chicago.

More recently, tear gas was used on the West Bank and Gaza strip.

Considered by medical authorities to be a riot control agent rather than a deadly chemical weapon, tear gas is a common term for a family of chemical compounds that cause temporary disability. More than a dozen chemicals are used as tear gases worldwide.

All produce intense burning of the eyes, which causes tearing and sometimes temporary blindness. Other effects include irritation of the nose, trachea and lungs, coughing, vomiting, diarrhea and skin sensitivity.

Proponents claim that when tear gas is properly used, its noxious effects are transient and pose no long-term health risk. But with enough exposure, toxicology studies show, tear gas can cause pneumonitis -- a chemical inflammation of the lungs similar to pneumonia -- and can lead to a fatal condition called pulmonary edema or fluid in the lungs. A 1972 report in the American Journal of Diseases in Children described the treatment of an infant who was exposed to tear gas when police fired canisters into a house to subdue a mentally disturbed adult. The baby developed pneumonitis and was hospitalized for 29 days.

Most people who encounter tear gas recover by rinsing their eyes and skin with water. Persistent eye irritation can be relieved with local anesthetics, according to a 1988 report in the Journal of the American Medical Association, while skin irritations can be treated with topical steroid creams.

But lingering questions about the use of tear gas remain. Some anecdotal reports have linked miscarriages and stillbirths to exposure, but no studies have been done. A 1988 report by Harvard researchers published in the Journal of the American Medical Association called for "an investigation into the full toxicological potential of tear gas chemicals and renewed debate on whether their use can be condoned under any circumstances."