Earlier this week, thousands of people joined a general strike in Puerto Rico’s capital, mobilizing on Mayday to protest the crippling austerity that’s been imposed on the island. In response, police fired rubber bullets and released choking clouds of teargas into the crowd.

What Puerto Rican protesters faced this week is, unfortunately, not unique. The use of tear gas has rapidly increased across the world in recent years. While banned in warfare, it is now internationally accepted as a humane form of riot control. This “humane” weapon, however, has left hundreds of people dead and thousands injured since it started to be used to suppress peaceful protests in the 1920s.

To learn about how tear gas became such an ubiquitous tool for dispelling peaceful protest, we spoke to Anna Feigenbaum, the author of Tear Gas: from the Battlefields of WWI to the Streets of Today. Feigenbaum astutely uncovers the history of the toxic gas, its role in quelling anticolonial movements, who benefits from the commercial tear gas market, and what accounts for its persistent use against civilians today.