What’s the best way to detect chemical weapons?

If you’re invested in the question of whether Syria has used chemical weapons on its own citizens, you’ve been exposed to a range of terms like "soil samples," "witness interviews," "dilated pupils," and "urine samples." For most laymen, it’s not clear which piece of evidence is more reliable than the other. But the question couldn’t be more important given allegations by France, Britain, and now Israel that Syrian forces have deployed chemical weapons — an act that would constitute a "red line" for the United States if true. Since all evidence is not created equal, we asked international chemical weapons consultant Ralf Trapp about the various investigative techniques cited in the case of Syria — and their value. 

Pictures of victims whose mouths are frothing and pupils are dilated

This evidence was cited today by Brig. Gen. Itai Brun, the chief of research and analysis for the Israeli army’s military intelligence division. "The dilated pupils, the frothing at the mouth and other signs testify, in our view, to the use of liquid chemical weapons, some kind of liquid chemical weapons, apparently sarin," said Brun. The quality of this evidence depends on a range of factors: Who took the pictures, how many pictures there are, how old the victims are, and whether the images are authentic. At the moment, it’s not clear how many images the Israelis have seen, and it’s very difficult to determine anything conclusively with a small number of images, said Trapp. "A small number of pictures from victims is an important lead, but isn’t in itself evidence," he explained.

Soil samples

France and Britain say soil samples support charges that Syria used nerve agents in Aleppo, Homs, and possibly Damascus. According to Trapp, collecting soil samples is a reliable technique because chemical agents can remain in soil for a long time. The question in this case is: Was the sample protected from interference? The ideal scenario is if a soil sample is collected, packaged, and protected from outsiders, but little is publicly known about how the soil was collected in Syria.

Urine samples

No one has cited urine samples as evidence of chemical weapons use in Syria yet, but the CIA has requested urine samples from Syrian rebels, according to the New York Times. While this technique can be useful because certain biomarkers will appear in the urine of victims, it’s recommended that the test be conducted within two days of the incident or a test could result in a false negative. Blood tests can also be useful, but are more difficult because they require a physician to draw the blood and the consent of the victim.

Witness interviews

The French and British have also cited "witness interviews," which can be valuable but rely crucially on how the interview is conducted. It’s important to query a range of witnesses and compare the testimonies to each other for corroboration or contradiction. It’s also important to avoid leading questions. If Trapp was assigned to Syria, he’d ask the witnesses simples questions: What happened? When did it happen? What did you see, hear, and smell?

Hair samples

The CIA is also seeking out hair samples, which can be useful because evidence can remain in the hair for a long time. Obviously, it’s important to authenticate the individual and the context in which they were exposed before using the sample as evidence.

While each of the above pieces of evidence has value, Trapp adds that an optimal piece of evidence would be a chemical artillery shell discovered in the field. Thus far, that hasn’t happened, which explains the reliance on other indicators.