A chemical or biological attack usually involves dispersing agents into the air. This can be done in various ways, such as firing artillery shells that burst in mid-air, or using airplanes to spray the agents over an area. If released outdoors, these types of weapons can be affected by weather conditions. Rain would reduce the effectiveness of the agents, and wind might spread them in unexpected directions. Because chemical and biological agents are seen as random, dangerous, and particularly cruel weapons, they have rarely been used. In the 20th century, chemicals were used extensively as battlefield weapons only in World War I (1914-1918) and the Iran-Iraq War (1980-1988). The release of the nerve agent sarin in a Tokyo subway in 1995 was a rare terrorist chemical attack.

Chemical warfare involves the use of chemical compounds to kill or seriously injure an enemy. Several countries began eliminating their chemical weapons stockpiles in the 1990s, but the threat of their use still exists.

Large-scale use of chemical weapons first occurred in 1915 during World War I, when German troops released chlorine gas from cylinders as the wind blew toward French lines a few hundred yards away. The yellow-green cloud enveloped the French soldiers, who choked and panicked. As the war continued, phosgene and other chemical weapons were used, culminating with Germany's introduction of mustard gas in 1917. By the end of the war in 1918, all the major powers had used chemical weapons.

The suffering caused by the gas attacks led to the 1925 Geneva Protocol, which banned the use of chemical or bacteriological agents in war. Although most major countries became parties to the agreement, the United States declined to until 1975. Nevertheless, the Protocol encouraged an international norm that helped deter the use of these weapons.

Biological weapons are a unique class of weapons, a living organism. These biological agents represent a dangerous military threat because they are alive, and are therefore unpredictable and uncontrollable once released. This is one important reason that biological weapons have rarely been used.

Biological warfare agents include bacteria, viruses, fungi, and other living microorganisms that can kill or incapacitate. Since they can reproduce, biological agents have the unique potential to make an environment more dangerous over time. If used for hostile purposes, any disease-causing microorganism could be considered a weapon. For the purposes of warfare, specific characteristics of certain agents make them more likely to be used than others.

Some potential warfare agents can make their victims very sick without necessarily killing them. Examples include the microorganisms that cause tularemia, Q fever, and yellow fever. After suffering debilitating illness, victims of these diseases often recover, although not always. Other agents are more likely to be lethal. The bacteria that cause bubonic plague and the virus that causes smallpox can kill large numbers of untreated people. Early antibiotic treatment usually cures plague victims, and smallpox vaccinations before exposure to the virus can prevent the disease.

The use of biological weapons has been more rare than the use of chemical weapons. In the 14th century, plague-infected cadavers purportedly were catapulted into an enemy camp in the Russian Crimea. In colonial America, the British delivered blankets from their smallpox infirmary to Native Americans, hoping to infect them with the disease. In the 20th century, the only extensive military biological attacks were by Japan against China in the late 1930s and 1940s. The Japanese dropped plague and other bacteria from airplanes over several towns, causing outbreaks of disease. The only large-scale terrorist attack with a biological weapon occurred in 1984 in the United States. Members of the Rajneesh cult in Oregon placed salmonella bacteria in the salad bars of several restaurants. Although 750 people became ill, none died.

Chemical and biological agents are most effective when dispersed into the air. These agents are often fitted into bombs or artillery shells that are designed to explode in the air and spread their contents over an enemy. In the 1980s, the United States began to deploy binary chemical weapons. Older chemical shells and bombs housed a single blistering or nerve agent. As they aged, these weapons could leak their poisons. A binary weapon is safer because it contains two relatively harmless chemicals. Only after firing do the chemicals combine to form a potent mix.

In some warfare or terrorist scenarios, an explosive release is not necessary. Members of Aum Shinrikyo attacked the Tokyo subway by packing sarin in plastic containers. To release the nerve agent, they pierced the containers with sharp umbrella tips. The leaking liquid and vapor affected thousands of passengers.

Microorganisms are generally more fragile than chemicals, and some might not survive an explosion. But several, like anthrax spores, do remain potent after an explosive release. In any case, United States Army tests have shown that biological agents can be broadly dispersed in a variety of non-explosive ways. In the 1950s and 1960s the Army released bacteria and chemical particles in hundreds of tests in populated areas throughout the country. Agents were sprayed at San Francisco from a boat offshore, dispensed from slow-moving cars in Minneapolis and St. Louis, and released from light bulbs dropped in the New York subway. The bacteria and chemicals in the tests were not as dangerous as actual warfare agents, although they posed some risks to the exposed populations. They demonstrated that an enemy or terrorist could expose millions of people to disease-causing organisms by a variety of simple techniques.

Total protection from chemical or biological attack is difficult, but steps can be taken to reduce the effects. Four approaches to protection commonly prescribed include early detection of chemical or biological agents, physical shelter from the agents, decontamination of exposed materials and clothing, and appropriate medical treatments.

Since many chemical and biological agents are colorless, odorless, and tasteless, an attack could take place without the victims realizing it. But if chemical detection devices were situated in an area of attack, they could signal that a dangerous chemical was present. The military can use chemical detection equipment in combat areas and is developing detection kits for soldiers. Detection kits might contain treated paper or liquids that change color when certain chemical agents are present.

Confirming the presence of a biological agent is more difficult. Even if a biological attack were known to be occurring, quick identification of the offending organism might be difficult. The U.S. military has developed a field apparatus that can test an air sample for the presence of specific biological agents. Called a Biological Integrated Detection System (BIDS), it can confirm the presence of a handful of microorganisms, including anthrax and plague bacteria. However, there are scores of possible biological agents that cannot be easily detected.

Several efforts are being made to develop a generic detector of dangerous organisms, using techniques like laser technology and mass spectrometry. Despite such efforts, the ability to rapidly identify all possible warfare agents in the field remains elusive.

Once chemical or biological agents are detected, a sealed, ventilated shelter can provide protection. Gas masks prevent agents from entering the lungs, and protective outer garments keep toxic agents from touching bare skin. From a military standpoint, each of these methods has drawbacks. Remaining in an enclosure constrains a soldier's ability to fight, while cumbersome outerwear restricts mobility, and a mask imposes limits on vision.

Quick decontamination of exposed surfaces is possible for most chemical and biological agents. Bleach, special powders, or just soap and water can neutralize some chemical and most biological agents. A few agents, like anthrax spores, may not be easily decontaminated if located in deep cracks or other inaccessible places.

Vaccinations can protect against some biological agents, such as anthrax, and some are susceptible to antibiotics. For many agents, there is no protection. In diseases that can be treated by antibiotics, therapy must begin promptly. To be effective against plague bacteria, antibiotics must be given within 24 hours after exposure. Against anthrax, antibiotic therapy must start within 12 hours or it will probably be useless.

Several international treaties and conventions have tried to regulate the conduct of warfare, with varying results. The Hague Conferences in 1899 and 1907 started the modern age of international arms control. More recent arms control agreements seek to regulate, and in some cases eliminate, certain classes of weapons.

Three international agreements deal explicitly with chemical and biological weapons. The first, the 1925 Geneva Protocol, prohibits the use in war of poison gas and bacteriological weapons. The second, the 1972 Biological Weapons Convention, bans not only the use, but the development, production, and stockpiling of biological and toxin weapons. The 1993 Chemical Weapons Convention does the same for chemical weapons. Unlike the earlier treaties, however, the chemical pact provides for methods to verify that countries are complying with the agreement and penalties for countries that do not comply.

The Biological Weapons Convention was the first international agreement to ban an entire category of weapons. It was established three years after a unilateral decision in 1969 by the United States to eliminate its own biological arsenal. Most major powers, including the Union of Soviet Socialist Republics (USSR) and the United States, had become parties to the biological treaty when it went into force in 1975. Later, more countries joined in the agreement and the world appeared about to be rid of germ weapons. However, in the early 1980s United States officials alleged that the Soviets continued an illegal biological weapons program. At the same time, Iraq was launching chemical attacks against Iran. The taboo on the use of these weapons seemed to be weakening despite the intentions of the Geneva Protocol and the Biological Weapons Convention.

In the 1991 Persian Gulf War, the United States and other coalition leaders worried that Iraqi President Saddam Hussein might unleash chemical and biological arms against them. Although he did not, the experience again prompted efforts to strengthen international agreements against these weapons. One result was the 1993 Chemical Weapons Convention, which contains an intrusive inspection system. Parties to the treaty would now have to allow outside monitors to visit suspected sites. By 1998, about 170 nations had signed the chemical treaty. Most countries must then ratify, or approve, the treaty within their own governments. As of early 1998, about 100 of the signing countries had ratified the treaty and become binding parties to the agreement.

Parties to the chemical convention have agreed to destroy their chemical weapons within the next decade. This is an expensive and complicated process for countries with large arsenals. Estimated costs to destroy the U.S. stockpiles are about $10 billion; experts estimate that Russia will have to spend more than $5 billion to eliminate the stockpiles of old Soviet weapons. The methods of destruction include chemical neutralization (adding other chemicals to form a nonlethal substance) and incineration. Projected expenses also reflect the need for safety and environmental protection during the process.

Biological agents can usually be more easily destroyed than chemical agents. Ultraviolet light or disinfectants like formaldehyde are effective against most microorganisms. But some, like anthrax bacteria, are harder to kill and can persist in an environment indefinitely.

Partly inspired by the verification provisions in the Chemical Weapons Convention, parties to the biological treaty are now working to strengthen the ban against germ weapons. As experts recognize, however, detecting illegal biological activities can be more difficult than chemical detection. The technology needed to build a large biological arsenal is simpler and requires less space than for a chemical arsenal, so biological weapons can be more easily hidden.