Explainer
Chemical Weapons Explained: What the Chemical Weapons Convention Bans
The Chemical Weapons Convention uses a broad purpose-based definition: toxic chemicals used to cause harm through their toxic properties can qualify as chemical weapons.
Reviewed 18 September 2026. This is an evergreen explainer; case-specific legal findings should be checked against the newest authoritative source.
In short: The Chemical Weapons Convention uses a broad purpose-based definition: toxic chemicals used to cause harm through their toxic properties can qualify as chemical weapons.
Chemical weapons are not defined only by a list of famous agents such as sarin or mustard gas. The Chemical Weapons Convention uses a broad “general purpose” approach: toxic chemicals and their precursors are treated according to their intended purpose, alongside munitions and equipment designed to employ them.
What is a chemical weapon?
The OPCW describes a chemical weapon as a toxic chemical used to cause intentional death or harm through its toxic properties, together with relevant precursors, munitions, devices and associated equipment under the Convention’s definition. This design prevents a new toxic chemical from escaping the treaty simply because it is not on an old list.
Peaceful chemical activity is still allowed
The Convention does not ban chemistry. Toxic chemicals and precursors can be produced and used for industrial, agricultural, research, medical, pharmaceutical, protective and other purposes not prohibited by the treaty, provided the type and quantity are consistent with those purposes.
Riot-control agents and warfare
States Parties may possess riot-control agents for domestic law-enforcement purposes, but the Chemical Weapons Convention prohibits their use as a method of warfare. That distinction is one reason “tear gas is legal for police, therefore legal in war” is incorrect.
The OPCW’s role
The Organisation for the Prohibition of Chemical Weapons implements the Convention through declarations, inspections, verification, technical assistance and other treaty mechanisms. It can conduct technical work related to allegations, but findings about responsibility may depend on the mandate and mechanism involved.
Allegation, detection and attribution are separate
Symptoms or social-media footage rarely prove a particular chemical agent by themselves. Reliable investigation may require environmental and biomedical samples, chain of custody, laboratory analysis, munition examination and contextual evidence. Establishing what chemical was present is different from establishing who used it.
Chemical incidents require careful language
Until evidence is available, terms such as “alleged use of a toxic chemical” may be more accurate than declaring a chemical-weapons attack. The precision protects both public understanding and later accountability.
Frequently asked questions
Is tear gas a chemical weapon?
Under the CWC, riot-control agents may be used for domestic law enforcement but must not be used as a method of warfare by States Parties.
Are all toxic chemicals banned?
No. Purpose is central; many toxic chemicals have legitimate industrial, medical or research uses.
Does the OPCW have its own military force?
No. It is a treaty organisation focused on verification, implementation and technical work.