What is a chemical weapon? Are chemical weapons considered weapons of mass destruction?

  • A chemical weapon is a toxic chemical agent (i.e, substance) deliberately used to cause death or severe harm to humans, animals, and/or plants. [1]
  • Chemical weapons, like biological and nuclear weapons, are considered weapons of mass destruction.
  • The chemical agents used in chemical weapons come in many forms. They can be gaseous, liquid, or solid.
  • There are a variety of potential delivery systems for toxic chemical agents. Which delivery systems are most likely to be employed varies based on resources and access.
    • States typically use special munitions such as grenades, rockets, artillery rounds, or missile warheads.
    • Non-state actors might successfully steal or develop these kinds of explosive dispersal mechanisms, but are more likely to use less traditional delivery systems. These could include non-explosive dispersal mechanisms, such as crop dusters or handheld spray canisters.

What are the different types of chemical weapons and their effects?

  • Chemical weapons agents are usually categorized by their effects on the human body. They include choking agents, blister agents, blood agents, and nerve agents. [2]
  • The effects of chemical weapons use vary widely and depend on the type of agent used, the amount an individual is exposed to, and the rate of exposure. In addition to killing or disabling victims, CW can also deny an adversary the use of a particular space, an effect known as area denial. Some chemical agents may persist on surfaces for hours or days, limiting movement in an area to individuals with appropriate protective gear.

Choking Agents

Choke Agents

Examples: chlorine and phosgene.

  • Effects: Choking agents attack lung tissue when inhaled, leading to respiratory failure.
  • Dispersion method: Vapor.
  • Lethality: Cause severe, often permanent, lung damage, and in some cases death.

Blister Agents

Blister Agent

Examples: mustard and lewisite.

  • Effects: Blister agents cause burns or severe blisters on the skin, eyes, and lungs, as well as possible airway irritation.
  • Dispersion method: Liquid or vapor.
  • Lethality: Not typically fatal, but neurological effects or a blocked airway can cause death.

Blood Agents

Blood Agent

Example: Hydrogen cyanide.

  • Effects: Blood agents disrupt the body’s ability to absorb oxygen through the bloodstream. Some types cause red blood cells to burst, while others disrupt cells’ ability to process oxygen.
  • Dispersion method: Vapor.
  • Lethality: Often fatal, especially without countermeasures.

Nerve Agents

Nerve Agents

Examples: Sarin and VX.

  • Effects: Nerve agents disrupt the mechanisms used by nerve cells to send signals to organs and muscles, causing convulsions, paralysis, and respiratory failure.
  • Dispersion method: Vapor or liquid.
  • Lethality: Highly lethal, especially without rapid administration of an antidote such as atropine.

Are riot control, incapacitating, or herbicidal agents considered chemical weapons?

  • Riot control, incapacitating, and herbicidal agents are commonly used by law enforcement or in the agricultural industry. However, they are only considered chemical weapons according to the Chemical Weapons Convention if they are used as a method of warfare. [3]



Examples: Lachrymators (cause eye irritation and tearing), such as tear gas.

  • Effects: A toxic chemical agent of relatively low-toxicity that produces highly irritating effects upon contact
  • Lethality: Non-lethal in low concentrations
  • Controlled under CWC? The Convention allows the use of RCAs for domenstic law enforcement. But under the CWC, the use of RCAs as a method of warfare is prohibited.


Incapacitating Agent

Examples: Fentanyl and BZ.

  • Effects: Cause psychological or mental effects that prevent victims from performing their duties.
  • Lethality: Effects dissipate over time.
  • Controlled under CWC? The Convention is not clear on the status of ICAs, often referred to as potential chemical weapons. Their status is actively discussed by CWC member states.



Examples: Praquat, Agent White, Agent Orange, and Agent Blue.

  • Effects: Toxic to plants. Also known as defoliants.
  • Lethality: Lethal to plants.
  • Controlled under CWC? There are no specific reporting or destruction requirements for herbicides, but the general purpose criterion under the CWC applies: if the intended use of an herbicide is to cause death or severe harm, then the herbicide is considered a chemical weapon.

Have chemical weapons ever been used?

  • Yes, both state and non-state actors have used chemical weapons. This storymap explores state use of chemical weapons.

Click for story map

Which countries have or at one time had chemical weapons?

  • Countries in the process of destroying chemical weapons

  • Countries that used to have chemical weapons

  • Countries suspected of possessing chemical weapons and not members of the CWC

  • Countries that are not members of the CWC

When and why did most countries renounce chemical weapons programs?

The First International Peace Conference

1. The Hague Conferences

The first attempt to control chemical weapons occurred at The Hague Conferences in 1899 and 1907, where participants agreed to prohibit the use of poisons or gases that would cause unnecessary suffering in war; projectiles from balloons; and projectiles to spread asphyxiating and deleterious gases. The Hague Conferences only had limited support. By WWI, chemical weapons had become integral to states’ military arsenals.
Source: Eric Croddy, Chemical and Biological Warfare: A Comprehensive Survey for the Concerned Citizen, (New York: Springer-Verlag, 2002).
Photo: The Hague Conferences, 1907 | Photo Credit: United Nations

World War I Gas Bomb

2. World War I

Chemical weapons caused approximately 90,000 deaths and one million casualties during WWI. More than 124,000 metric tons of 21 different agents were utilized on the battlefield. These chemicals and their effects ranged from irritating riot-control agents to the more lethal choking and blister agents, such as chlorine, phosgene, and mustard agents.

Sources: “Brief History of Chemical Weapons Use,” Organization for the Prohibition of Weapons,; Jonathan Tucker, War of Nerves, (New York: Anchor Books, 2007), p. 27.
Photo: A World War I-era British Gas Bomb | Photo Credit: WikiMedia Commons

2. World War I Cont.

Although not always deadly, CW resulted in high manpower attrition on the front lines due to casualties’ horrifying injuries and long convalescence periods. Post-traumatic stress symptoms, known then as “gas-shock,” were also prevalent among surviving troops. Over time, prophylactic measures such as gas masks reduced CW’s overall battlefield utility, but they provided inadequate protection against especially blister agents, such as mustard. This “King of Battle Gasses” caused more than 85% of WWI’s CW deaths.

Source: Gerald Fitzgerald, “Chemical Warfare and Medical Responses During WWI,” American Journal of Public Health, 2008,
Photo: Canadian solider with mustard gas burns in World War I. | Photo Credit: WikiMedia Commons

3. The 1925 Geneva Protocol

After widespread use of chemical weapons in WWI, countries again attempted to outlaw them. However, the 1925 Geneva Protocol banned use rather than possession of such materials, rendering it ineffective in preventing a CW arms race.

Source: Protocol for the Prohibition of the Use in War of Asphyxiating, Poisonous or Other Gases, and of Bacteriological Methods of Warfare.
Photo: British troops blinded by tear gas during the Battle of Estaires | Photo Credit: United Kingdom Government, via WikiMedia Commons

4. World War II

CW were not as widely used during WWII, except by Japan against its Asian adversaries. However, before, during and after World War II CW were widely stockpiled, leading to sizeable arsenals by the end of the Cold War. Countries also discovered increasingly deadly CW, including G-series, V-series, and Novichok nerve agents.
Photo: Japanese Special Naval Landing Force wearing gas masks and rubber gloves during a chemical attack | Photo Credit: WikiMedia Commons

Signing of the Chemical Weapons Convention

5. The Chemical Weapons Convention

After the Cold War ended, the international community finally negotiated a ban on both the use and possession of chemical weapons. Following the lead of the United States and the Russian Federation, the two countries with the largest CW arsenals, most countries signed the Chemical Weapons Convention (CWC) in 1993 and renounced CW. The CWC entered into force in 1997, includes robust verification provisions, and has few outliers, making it an unusually strong nonproliferation treaty.
Photo: Signing of the CWC, 1993 | Photo Credit: United Nations

What is being done to ensure that chemical weapons are never used again?

  • All members of the international community benefit from efforts to prevent chemical weapons from being used again. Such efforts include:
    • eliminating existing chemical weapons arsenals through CW disarmament and destruction;
    • preventing the further spread of chemical weapons through nonproliferation; and
    • preventing the misuse of chemistry through chemical security, while providing for its continued peaceful use.
  • Although significant progress has been made, several CWC states parties have yet to complete the destruction of their stockpiles (See Module 5 for details).
  • Today, only four countries are not states parties to the CWC – Egypt, Israel, North Korea and South Sudan. Nonproliferation efforts are vital to encourage these countries to join the regime, and subsequently fully to destroy any arsenals they may possess.
  • Chemical security efforts involve securing chemical research and industrial materials from non-state actors, such as terrorists or criminals.
  • Both chemical security and nonproliferation are essential in facilitating the use of chemistry for peaceful purposes, as chemicals are inherently dual-use and essential to modern life.


[1] Organization for the Prohibition of Chemical Weapons, Article II. Definitions and Criteria,
[2] Organization for the Prohibition of Chemical Weapons, Types of Chemical Agents,; Eric Croddy, Chemical and Biological Warfare: A Comprehensive Survey for the Concerned Citizen, (New York: Springer-Verlag, 2002).
[3] Organization for the Prohibition of Chemical Weapons, Potential Chemical Weapons,; Eric Croddy, Chemical and Biological Warfare: A Comprehensive Survey for the Concerned Citizen, (New York: Springer-Verlag, 2002).

Photo Credit
Header Image: Bomb with Sarin-filled bomblets. Source: WikiMedia Commons.