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What is a chemical weapon? When is chlorine a chemical weapon?

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A recurring question in the context of the investigation by the Organisation for the Prohibition of Chemical Weapons (OPCW) into the use of chlorine in the attack against Douma on 7 April is whether chlorine is actually a chemical weapon (CW).

The simple answer is ‘yes’ if the chemical element is released as method of warfare, an act of terrorism, or any other deliberate act intended to harm or kill a person or animal.
There are two elements in the Chemical Weapons Convention (CWC) to bear in mind, namely the definition of a CW and the three Schedules (or lists with chemicals), which are annexed to the convention.

Definition of a chemical weapon under the CWC

Most toxic chemical are potentially dual-use. Moreover, the CWC wishes to cover not only yesterday’s and today’s toxic chemicals but also the ones that may be developed in the future. To this end, the CWC uses the so-called General Purpose Criterion (GPC): not the toxic chemical as such is prohibited; however, the purposes to which it may be applied are.
In this context it is useful to know that the treaty’s default condition is prohibition. In other words, all usage of toxic chemicals is prohibited unless for purposes that are not-prohibited (note the negative formulation). The CWC considers only four non-prohibited purposes.

Thus Article II, 1 states:

1. “Chemical Weapons” means the following, together or separately:
(a) Toxic chemicals and their precursors, except where intended for purposes not prohibited under this Convention, as long as the types and quantities are consistent with such purposes; […]

Here one can see that any toxic chemical is defined as a CW, except where intended for purposes not prohibited …, in which case the toxic chemical is by definition not a CW and therefore does not fall under the CWC.

Article II, 2 defines ‘toxic chemical’ as:

‘Any chemical which through its chemical action on life processes can cause death, temporary incapacitation or permanent harm to humans or animals. This includes all such chemicals, regardless of their origin or of their method of production, and regardless of whether they are produced in facilities, in munitions or elsewhere’.

In other words, this definition clearly bears on chlorine.

Non-prohibited purposes

Article II, 9 lists the four non-prohibited purposes:

(a) Industrial, agricultural, research, medical, pharmaceutical or other peaceful purposes;
(b) Protective purposes, namely those purposes directly related to protection against toxic chemicals and to protection against chemical weapons;
(c) Military purposes not connected with the use of chemical weapons and not dependent on the use of the toxic properties of chemicals as a method of warfare;
(d) Law enforcement including domestic riot control purposes.

The highlighted non-prohibited purpose applies to industrial and commercial uses of chlorine, in which case the toxic chemical does not fall under the CWC definition of a CW.

Annex on Chemicals: The Schedules

Some confusion arises because chlorine is not listed in the Annex on Chemicals. The three Schedules are lists of specific compounds (warfare agents and their precursors) or families of chemicals, and are intended to help with the implementation of the verification requirements (reporting by states parties, inspections of industry, etc.).

Schedule 1 chemicals pose the greatest threat to the objectives and purpose of the CWC. As a consequence no production (except in small volumes in a single facility under strict OPCW supervision), trade, etc. is authorised. Almost by definition they are single purpose. They are subject to the most stringent controls. Schedule 2 includes chemicals that are key precursors to chemical weapons but which generally have greater commercial application. Besides the verification requirements, States Parties later decided that they are also subject to transfer controls. Schedule 3 chemicals pose some risks to the CWC, but have far greater commercial legitimacy and importance.

Not every toxic chemical is listed, such as certain discrete organic chemicals, which must nevertheless be reported to the OPCW if a firm produces specific quantities of them. Additional requirements apply for the manufacture of unscheduled discrete organic chemicals with the elements phosphorus, sulphur or fluorine above treaty-specified thresholds.


Chlorine is another unscheduled toxic chemical. It is produced, traded and consumed in the excess of 55 million metric tonnes worldwide each year. Besides the fact that in the late 1980s and early 1990s no negotiator of the CWC believed that chlorine would ever again be used as a weapon of choice (the last time was in World War 1), the volumes simply make any verification or inspection model impossible to emplement.

As noted above, that does not mean that chlorine is not prohibited if its purpose is to be used as a weapon. The Annex on Chemicals, section B states clearly:

‘Pursuant to Article II, subparagraph 1 (a), these Schedules do not constitute a definition of chemical weapons.’

Russia and Schedules

As I explained in my previous blog posting (in the context of the Skripal poisoning with Novichok in the UK), Russia tends to emphasise the Schedules over the GPC, which informs some of its positions, including with regard to chlorine.

1 Comment

  1. Links – April 27, 2018 |
    27 April 2018 at 5:01 pm

    […] read on Novichok, with photos from a visit to a Soviet chemical weapons facility in 1987. What is a chemical weapon? When is chlorine a chemical weapon?  Russia is pulling out all the stops to avoid […]

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