Practice Relating to Rule 75. Riot Control Agents
The Military Manual (1993) of the Netherlands states:
Opinion is divided over whether or not the prohibition applies to tear gas, defoliants and other non deadly means. It is said, with regard to tear gas, that it should be prohibited in armed conflicts. It can be used to control order. This should be distinguished from the use in armed conflict because there it runs the danger of provoking the use of other more dangerous chemicals.
The Military Manual (2005) of the Netherlands states:
Riot control agents such as tear gas may not be used as a method of warfare (Chemical Weapons Convention Article 1). Use as a means of maintaining order, including the control of internal unrest, is not prohibited. Military use must be distinguished from this. This conceals the danger that the use of a relatively harmless chemical may unleash the use of some other, more lethal one by the adversary.
The manual further states:
[M]ilitary use of a non-lethal weapon may pose the danger that the adversary perceives it as a forbidden means, which may induce the adversary to use other, more lethal means. One example is the use of tear gas, mentioned above.
In its chapter on peace operations, the manual states:
Some countries consider that the use of tear gas (CS gas) conflicts with the 1925 [Geneva] Gas Protocol. This poses inevitable problems for the peace force when using means of riot control. In such a case, troops must be sought from a country which sees no conflict with the Gas Protocol, so that tear gas may still be used, e.g., to disperse a crowd if strictly necessary.