The use of chlorine gas against civilians in the Syrian city of Aleppo should be investigated as a “war crime,” a top United Nations diplomat said Thursday. It was a deadly reminder of the persistence of makeshift chemical weapons in Syria despite an international effort to destroy the country’s chemical weapons caches.

Four people were killed when at least four barrels containing the gas were dropped Wednesday over Zubdiya, a rebel-held neighborhood in eastern Aleppo, witnesses said. The bombing was the latest in a series of chlorine gas attacks that have killed or wounded scores during Syria’s five-year civil war.

What Is Chlorine?

Chlorine is one of the most common naturally occurring elements on earth and has a variety of beneficial uses. It is used to make pesticides and the bleach that disinfects hospitals, and it is injected into municipal drinking water to make it clean and potable.

It is a legal and necessary chemical, freely traded across international borders. But chlorine was among the first chemical gases to be turned into a weapon during World War I. Given its accessibility and the ease with which it can be weaponized, it has been commonly used in homemade bombs.