Proof of poison gas: Circumstantial evidence is key

Most traces of a poison gas attack will likely disappear two to three days after the attack itself.

Chlorine gas, for example, is highly oxidative and reacts with many elements, meaning it changes and becomes something else.

A further complication is the fact that, as an element, chlorine exists everywhere — in abundance. It can be found in various salts and in the soil and water. After some time, it is almost impossible to prove its origin as stemming from a military source.

That, of course, does not apply if investigators do manage to find grenades, bombs or other containers which held the substance — or unusually high concentrations in the vicinity of the crime scene.

With Sarin, finding proof of its use in an open environment is similarly difficult. The substance is soluble in water and acids within days or even hours.

Rainfall will make proving anything especially difficult. 

Read more: What traces of chemical weapons could OPCW investigators still find in Syria?

Taking samples from victims

In victims, however, the symptoms of a poison gas attack do constitute firm evidence — even weeks after the poisoning occurred.

Eyewitness accounts, videos and photographic materials can provide circumstantial evidence of a poison gas attack, allowing investigators to come in and take samples from victims — survivors and dead alike.

Blood, urine or tissue will then show characteristic patterns in terms of their medical values. Such measurements constitute hard evidence.

If, for example, a large number of young people have unusually bad liver values, it points to a mass poisoning. The same can be said if huge numbers of people show lung damage.

A missing enzyme

For a Sarin gas attack, low levels of an enzyme called acetylcholesterase constitute even clearer evidence.

Sarin blocks this enzyme. That results in the body building up extremely high levels of a neurotransmitter called acetylcholine. That neurotransmitter is responsible for transporting electric impulses, and if its levels are too high, muscles will be permanently stimulated, resulting in cramps and eventually the paralysis of the lungs and heart. Victims will suffocate or suffer a cardiovascular collapse.

This is also where the antidote atropine comes in to play. If the antidote helps relieve symptoms, it is a very clear indicator of exposure to poison gas.

Lab tests

Finally, investigators can take probes into a well-equipped forensic laboratory.

With the help of a gas chromatograph, they can look for additional traces of degradation products from Sarin.

The probes can stem from medical samples — blood, urine or tissue — as well as from the environment.

One of the best places to look for residues in the environment is in a place where there is higher-than-average humidity, but not enough to wash the traces away. One good spot, for example, would be the walls of humid basements.

All in all, it is possible to prove the use of poison gas, even if direct chemical proof turns out to be sparse.

It constitutes a jigsaw puzzle, and it includes some circumstantial evidence.