What is a chemical weapon?

Chemical weapons are toxic chemical compounds in gas, liquid, or solid form that harm humans, animals, or plants. They are delivered to targets via artillery shells, land mines, aerial bombs, or other methods. These weapons are categorized based on lethality, persistency, and how they affect the body. Types of chemical weapons include choking agents, blister agents, blood agents, nerve agents, incapacitants, riot-control agents, and herbicides. The Chemical Weapons Convention (CWC) of 1993 prohibits the use, development, production, acquisition, stockpiling, and transfer of chemical weapons.

How do chemical weapons work?

Chemical weapons are toxic chemicals in liquid, solid, or gaseous form that harm humans, animals, and plants. They can be inhaled, absorbed, or ingested. These weapons are delivered through various means, such as artillery shells, land mines, or aerial bombs. Once dispersed, the effects vary. For instance, nerve agents like sarin can cause paralysis and death within minutes, while others, like tear gas, are irritants. Some, such as VX, persist for weeks, while others like sarin dissipate rapidly. Choking agents damage the lungs, causing fluids to build up and potentially leading to asphyxiation. Blood agents prevent oxygen transfer in the body, leading to asphyxia. Mustard gas blisters the skin and mucous membranes upon contact, causing burns and potential long-term health issues.

What are some examples of chemical weapons?

Chemical weapons are toxic agents that can kill, injure, or incapacitate people, animals, or plants. They can be delivered as a gas, liquid, or solid and cause damage when inhaled, absorbed through the skin, or ingested. Examples of chemical weapons include: * Choking agents such as chlorine, phosgene, and chloropicrin, which cause fluids to build up in the lungs, leading to asphyxiation. * Blister agents such as sulfur mustard and lewisite, which burn the skin, eyes, windpipe, and lungs. * Blood agents such as hydrogen cyanide and cyanogen chloride, which prevent the transfer of oxygen to the cells, causing the body to asphyxiate. * Nerve agents such as sarin, tabun, soman and VX, which affect the transmission of impulses through the nervous system and can cause paralysis and respiratory failure.

What effects do chemical weapons have on people and the environment?

Chemical weapons are toxic chemicals in liquid, solid, or gaseous form that directly harm people, animals, and plants by being inhaled, absorbed, or ingested. They can be delivered through artillery shells, land mines, bombs and other methods. These weapons can have varied effects, from causing irritation to death. For example, nerve agents can kill quickly, while others, like tear gas, are irritants. Some blister agents like mustard gas, result in chemical burns, irritation and potential blindness, and can cause long-term respiratory issues and cancers. The effects of chemical weapons depend on factors such as persistency and mode of action. Some dissipate quickly, while others can remain lethal for weeks, and they can enter the body through the lungs or skin.

How have chemical weapons been used in history?

Chemical weapons have been used in numerous conflicts throughout history. Germany first used them in World War I in 1915, and by the war's end, both sides had used them extensively, causing over a million casualties and 91,000 fatalities. Despite the 1925 Geneva Protocol banning their use, chemical weapons were employed in conflicts such as the Russian Civil War, the Sino-Japanese War, and the Iran-Iraq War. More recently, they have been used in the Syrian Civil War. Chemical weapons are delivered through various means, including artillery shells, aerial bombs, and spray tanks.

What international laws exist to ban or control chemical weapons?

The primary international law banning chemical weapons is the Chemical Weapons Convention (CWC) . It prohibits the use, development, production, acquisition, stockpiling, or transfer of chemical weapons. Adopted in 1992 and effective from 1997, the CWC aims for total chemical weapons disarmament. Signatories must destroy chemical arms and production facilities. The Organisation for the Prohibition of Chemical Weapons (OPCW) implements and enforces the CWC, verifying compliance through inspections and promoting cooperation in the peaceful uses of chemistry. The OPCW can inspect sites and investigate suspected attacks, reporting to the UN. The Nobel Committee lauded the OPCW and the CWC for making chemical weapon use "taboo under international law".

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Images & Videos

American soldiers in World War I uniforms, some wearing gas masks, advance with rifles during a simulated gas attack exercise on a rocky battlefield.

Two German soldiers wearing helmets and gas masks operate a machine gun during World War I.

A protester stands in the street with arms raised, facing a cloud of tear gas and a line of police during nighttime unrest in Baltimore, with orange smoke filling the scene.

U.S. wartime use of defoliant in Vietnam

Two American soldiers in World War I uniforms use large paddles called gas flappers to fan poison gas inside a dirt trench.

Person in chemical protective gear and mask attends to a mock patient on a stretcher inside a yellow portable decontamination chamber during a hazardous material team drill.

At a Glance

chemical warfare summary

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Written by

Barry R. Schneider

Fact-checked by

Britannica Editors

Last updated

Nov. 4, 2026 •History

Top Questions

What is a chemical weapon?
Chemical weapons are toxic chemical compounds in gas, liquid, or solid form that harm humans, animals, or plants. They are delivered to targets via artillery shells, land mines, aerial bombs, or other methods.

These weapons are categorized based on lethality, persistency, and how they affect the body. Types of chemical weapons include choking agents, blister agents, blood agents, nerve agents, incapacitants, riot-control agents, and herbicides. The Chemical Weapons Convention (CWC) of 1993 prohibits the use, development, production, acquisition, stockpiling, and transfer of chemical weapons.
2 Britannica Sources
1. Chemical Weapons Convention: Negotiating a treaty
2. chemical weapon: Blood agents
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
How do chemical weapons work?
Chemical weapons are toxic chemicals in liquid, solid, or gaseous form that harm humans, animals, and plants. They can be inhaled, absorbed, or ingested.

These weapons are delivered through various means, such as artillery shells, land mines, or aerial bombs. Once dispersed, the effects vary. For instance, nerve agents like sarin can cause paralysis and death within minutes, while others, like tear gas, are irritants. Some, such as VX, persist for weeks, while others like sarin dissipate rapidly.

Choking agents damage the lungs, causing fluids to build up and potentially leading to asphyxiation. Blood agents prevent oxygen transfer in the body, leading to asphyxia. Mustard gas blisters the skin and mucous membranes upon contact, causing burns and potential long-term health issues.
2 Britannica Sources
1. mustard gas
2. chemical weapon: Properties of chemical weapons
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
What are some examples of chemical weapons?
Chemical weapons are toxic agents that can kill, injure, or incapacitate people, animals, or plants. They can be delivered as a gas, liquid, or solid and cause damage when inhaled, absorbed through the skin, or ingested.

Examples of chemical weapons include:
* Choking agents such as chlorine, phosgene, and chloropicrin, which cause fluids to build up in the lungs, leading to asphyxiation.
* Blister agents such as sulfur mustard and lewisite, which burn the skin, eyes, windpipe, and lungs.
* Blood agents such as hydrogen cyanide and cyanogen chloride, which prevent the transfer of oxygen to the cells, causing the body to asphyxiate.
* Nerve agents such as sarin, tabun, soman and VX, which affect the transmission of impulses through the nervous system and can cause paralysis and respiratory failure.
1 Britannica Source
1. chemical weapon: Blood agents
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
What effects do chemical weapons have on people and the environment?
Chemical weapons are toxic chemicals in liquid, solid, or gaseous form that directly harm people, animals, and plants by being inhaled, absorbed, or ingested. They can be delivered through artillery shells, land mines, bombs and other methods.

These weapons can have varied effects, from causing irritation to death. For example, nerve agents can kill quickly, while others, like tear gas, are irritants. Some blister agents like mustard gas, result in chemical burns, irritation and potential blindness, and can cause long-term respiratory issues and cancers.

The effects of chemical weapons depend on factors such as persistency and mode of action. Some dissipate quickly, while others can remain lethal for weeks, and they can enter the body through the lungs or skin.
2 Britannica Sources
1. mustard gas: Effects of mustard gas
2. chemical weapon: Blister agents
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
How have chemical weapons been used in history?
Chemical weapons have been used in numerous conflicts throughout history. Germany first used them in World War I in 1915, and by the war's end, both sides had used them extensively, causing over a million casualties and 91,000 fatalities. Despite the 1925 Geneva Protocol banning their use, chemical weapons were employed in conflicts such as the Russian Civil War, the Sino-Japanese War, and the Iran-Iraq War. More recently, they have been used in the Syrian Civil War. Chemical weapons are delivered through various means, including artillery shells, aerial bombs, and spray tanks.
2 Britannica Sources
1. Chemical Weapons Convention: Negotiating a treaty
2. chemical weapon: Blister agents
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
What international laws exist to ban or control chemical weapons?
The primary international law banning chemical weapons is the Chemical Weapons Convention (CWC). It prohibits the use, development, production, acquisition, stockpiling, or transfer of chemical weapons. Adopted in 1992 and effective from 1997, the CWC aims for total chemical weapons disarmament. Signatories must destroy chemical arms and production facilities.

The Organisation for the Prohibition of Chemical Weapons (OPCW) implements and enforces the CWC, verifying compliance through inspections and promoting cooperation in the peaceful uses of chemistry. The OPCW can inspect sites and investigate suspected attacks, reporting to the UN. The Nobel Committee lauded the OPCW and the CWC for making chemical weapon use "taboo under international law".
3 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

chemical weapon, any of several chemical compounds, usually toxic agents, that are intended to kill, injure, or incapacitate enemy personnel. In modern warfare, chemical weapons were first used in World War I (1914–18), during which gas warfare inflicted more than one million of the casualties suffered by combatants in that conflict and killed an estimated 90,000. In the years since then, chemical arms have been employed numerous times, most notably in the Iran-Iraq War (1980–88) and the Syrian Civil War. The United States and the Soviet Union, during their decades of confrontation in the Cold War (1945–91), built up enormous stockpiles of chemical weapons. The end of the Cold War enabled those former adversaries to agree to ban all chemical weapons of the types that had been developed during World War I (first generation), World War II (second generation), and the Cold War (third generation).

Like nuclear weapons and biological weapons, chemical weapons are often classified as weapons of mass destruction. Under the Chemical Weapons Convention (CWC) of 1993, the use of chemical weapons in war is prohibited, as is all development, production, acquisition, stockpiling, and transfer of such weapons. Nevertheless, while the aim of the CWC is complete elimination of most types of chemical weapons, not all countries have abandoned their chemical warfare capabilities. In particular, some weaker states have pursued chemical weapons programs as deterrents to being attacked by enemies that have either stronger conventional forces or their own weapons of mass destruction, and some regimes have used chemical weapons to threaten especially vulnerable foes outside and even within their own borders.

Furthermore, some individuals and militant organizations have acquired or have sought to acquire chemical weapons in order to attack their enemies or to secure their own ends through terror. The continued threat from chemical weapons has led many states to prepare defenses against them and to exert diplomatic pressure on dissenting or noncompliant states to abide by the CWC.

Types of chemical weapons

Chemical weapons are chemical agents, whether gaseous, liquid, or solid, that are employed because of their direct toxic effects on humans, animals, and plants. They inflict damage when inhaled, absorbed through the skin, or ingested in food or drink. Chemical agents become weapons when they are placed into artillery shells, land mines, aerial bombs, missile warheads, mortar shells, grenades, spray tanks, or any other means of delivering the agents to designated targets.

Not all poisonous substances are considered suitable for weaponization, or use as chemical weapons. Thousands of such chemical compounds exist, but only a few dozen have been used as chemical warfare agents since 1900. The compounds of most utility must be highly toxic but not too difficult to handle. Furthermore, the chemical must be able to withstand the heat developed when delivered in a bursting shell, bomb, mine, or warhead. Finally, it must be resistant to water and oxygen in the atmosphere in order to be effective when dispersed.

Chemical agents

Since World War I, several types of chemical agents have been developed into weapons. These include choking agents, blister agents, blood agents, nerve agents, incapacitants, riot-control agents, and herbicides.

Choking agents

Choking agents were employed first by the German army and later by the Allied forces in World War I. The first massive use of chemical weapons in that conflict came when the Germans released chlorine gas from thousands of cylinders along a 6-km (4-mile) front at Ypres, Belgium, on April 22, 1915, creating a wind-borne chemical cloud that opened a major breach in the lines of the unprepared French and Algerian units. The Germans were not prepared to exploit the opening, which gave the French and Algerians time to rush reinforcements into the line. Eventually both sides mastered the new techniques of using choking agents such as chlorine, phosgene, diphosgene, chloropicrin, ethyldichlorasine, and perfluoroisoboxylene and launched numerous attacks—though without any militarily significant breakthroughs once each side had introduced the first crude gas masks and other protective measures. Phosgene was responsible for roughly 80 percent of all deaths caused by chemical arms in World War I.

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Trench warfareFrench troops wearing gas masks await attack in a trench on the Western Front during World War I.

Choking agents are delivered as gas clouds to the target area, where individuals become casualties through inhalation of the vapor. The toxic agent triggers the immune system, causing fluids to build up in the lungs, which can cause death through asphyxiation or oxygen deficiency if the lungs are badly damaged. The effect of the chemical agent, once an individual is exposed to the vapor, may be immediate or can take up to three hours. A good protective gas mask is the best defense against choking agents.

Blister agents

Blister agents were also developed and deployed in World War I. The primary form of blister agent used in that conflict was sulfur mustard, popularly known as mustard gas. Casualties were inflicted when personnel were attacked and exposed to blister agents like sulfur mustard or lewisite. Delivered in liquid or vapor form, such weapons burn the skin, eyes, windpipe, and lungs. The physical results, depending on level of exposure, might be immediate or might appear after several hours. Although lethal in high concentrations, blister agents seldom kill. Modern blister agents include sulfur mustard, nitrogen mustard, phosgene oxime, phenyldichlorarsine, and lewisite. Protection against blister agents requires an effective gas mask and protective overgarments.

Blood agents

Blood agents, such as hydrogen cyanide or cyanogen chloride, are designed to be delivered to the targeted area in the form of a vapor. When inhaled, these agents prevent the transfer of oxygen to the cells, causing the body to asphyxiate. Such chemicals block the enzyme that is necessary for aerobic metabolism, thereby denying oxygen to the red blood cells, which has an immediate effect similar to that of carbon monoxide. Cyanogen inhibits the proper utilization of oxygen within the blood cells, thereby “starving” and damaging the heart. The best defense against blood agents is an effective gas mask.

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Types of chemical weapons

Chemical agents

Choking agents

Blister agents

Blood agents