From wikipedia.
Schedule 1 chemicals have few, or no uses outside chemical weapons. These may be produced or used for research, medical, pharmaceutical or chemical weapon defence testing purposes but production at sites producing more than 100 grams per year must be declared to the OPCW. A country is limited to possessing a maximum of 1 tonne of these materials. Examples are sulfur mustard and nerve agents, and substances which are solely used as precursor chemicals in their manufacture. A few of these chemicals have very small scale non-military applications, for example milligram quantities of nitrogen mustard are used to treat certain cancers.
Schedule 2 chemicals have legitimate small-scale applications. Manufacture must be declared and there are restrictions on export to countries that are not CWC signatories. An example is thiodiglycol which can be used in the manufacture of mustard agents, but is also used as a solvent in inks.
Schedule 3 chemicals have large-scale uses apart from chemical weapons. Plants which manufacture more than 30 tonnes per year must be declared and can be inspected, and there are restrictions on export to countries which are not CWC signatories. Examples of these substances are phosgene (the most lethal chemical weapon employed in WWI), which has been used as a chemical weapon but which is also a precursor in the manufacture of many legitimate organic compounds (e.g. pharmaceutical agents and many common pesticides), and triethanolamine, used in the manufacture of nitrogen mustard but also commonly used in toiletries and detergents.
certain notable exceptions exist. Chlorine gas is highly toxic, but being a pure element and extremely widely used for peaceful purposes, is not officially listed as a chemical weapon.
I take that as saying that there is no complete ban on any chemical. Limitations are much more lax than I expected, which is excellent in times of world peace, like now. That would quickly change if there was a Bhopal type accident involving these chemicals. Permitting more than 30 tonnes per year per plant of phosgene, unexpected and somewhat shocking. 100 grams per year of a schedule 1 agent is a hell of a lot.
Another fact from the documentary is that mustard gas is rarely fatal. Extreme burns yes, but mostly non-lethal.
I seem to remember hearing about an irritant, nonlethal in itself but makes people take off their gas masks.
I also seem the remember that gas masks have to be tuned to different chemical agents, a mask that works against one chemical may not work against another, and vice versa.
Before watching the documentary I had the completely wrong impression that a serious chemical warfare attack involved hundreds of tons of chemicals, agent orange in Vietnam was 76,000 tons. I couldn’t have been more wrong. Dropping of a VX bomb on Hiroshima the same physical size as the atomic bomb could have killed more people.