The tear gas that police fired ahead of Trump’s visit to St. John’s as well as other similar chemicals fall within a substantial exception to the global prohibition against chemical weapons enshrined in the Chemical Weapons Convention. Just as it’s not legal to shell an opposing army with sarin or mustard gas, it’s also illegal to fire off commonly used riot control agents such as chloroacetophenone (CN) and chlorobenzylidenemalononitrile (CS) in war. Against protestors in one’s own country, however? Perfectly legal. Law enforcement use of so-called riot control agents isn’t prohibited by the treaty.RELATED:“We’re Number… 24.” US near back of pack among developed nations on environment

Some weapons researchers think the law enforcement exception in the Chemical Weapons Convention was a mistake.

By the 1990s, when the Chemical Weapons Convention was being finalized, most chemicals used against people were employed in law enforcement contexts, University of Massachusetts Lowell professor Nicholas Evans said. Evans, who researches national security and emerging technology and has taught courses on weapons of mass destruction, said that a lot of scholarship suggests that without an exception for riot control agents, countries would never have agreed to the Chemical Weapons Convention. “This was the catch, which is that countries reserve the right to effectively use chemical weapons on their own people,” he said. “I think we missed the forest for the trees when we thought through what it was to use a chemical weapon.”

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