The use of chemical weapons is generally abhored in international law, and there are many rules to discourage or make difficult their acquisition and use. Of these the most important is the Chemical Weapons Convention.
Nuclear, Biological, and Chemical weapons were usually considered morally equivalent and referred to collectively by the phrase "NBC weapons", until this phrase was replaced by weapons of mass destruction, due to confusion about the line between chemical and biological weapons (e.g. prions which are not organisms but simple single-molecule proteins, and could thereby be considered either chemical or biological), concerns about genetic manipulation of biological entities, or nanotechnological methods to generate new molecules with lifelike characteristics, or to exude dangerous chemicals, and the danger of weapons using artificial intelligence and robotics, all of which could conceivably get beyond human control.
By comparison to these threats, the danger of chemical weapons is not considered to be extreme. Even such potential attacks as poisoning of an urban center's water supply (very common in the history of warfare) with a chemical agent, e.g. botulin, are assumed to be containable.