An arms control treaty - the Chemical Weapons Convention - was signed more than 20 years ago to outlaw the production, stockpiling and use of chemical weapons.

One hundred ninety-two countries signed and agreed to the terms in 1997.

Toxic chemical substances are subject to different regulations. Even though they can be used to develop chemical weapons, they are not prohibited outright by the convention.

Several human rights groups have found evidence of the use of chemical weapons, such as in Syria, but they say the response hasn't been a strong enough deterrent.

Al Jazeera's Mike Hanna reports.