At the end of a recent 800-meter race in Oregon, a high-school runner named Maggie Williams got dizzy, passed out and landed face-first just beyond the finish line. She and her coach blamed her collapse on a deficit of oxygen due to the mask she’d been forced to wear, and state officials responded to the public outcry by easing their requirements for masks during athletic events. But long before the pandemic began, scientists had repeatedly found that wearing a mask could lead to oxygen deprivation. Why had this risk been ignored? One reason is that a new breed of censors has been stifling scientific debate about masks on social-media platforms. When Scott Atlas, a member of the Trump White House’s coronavirus task force, questioned the efficacy of masks last year, Twitter removed his tweet. When eminent scientists from Stanford and Harvard recently told Florida Gov. Ron DeSantis that children should not be forced to wear masks, YouTube removed their video discussion from its platform. These acts of censorship were widely denounced, but the social-media science police remain undeterred, as I discovered when I recently wrote about the harms to children from wearing masks.