Tear gas has been controlling crowds for a century. How does it work? Can it cause permanent harm?

Christopher Rapuano, chief of the cornea service at Wills Eye Hospital, got a small dose of tear gas a few years ago as he stood on the fringes of a demonstration in Hong Kong. His eyes started to burn. Then they teared up and his vision blurred. He ran to fresher air. Chris Cramer, a University of Minnesota chemist, got much bigger doses while in the Army, where he was a chemical weapons specialist. To underscore the value of gas masks, soldiers in training would don the masks, wait until tear gas had been fired, then take them off. “It feels as though bees are stinging you in yo...

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