Joseph Allen is director of the Healthy Buildings program at Harvard T.H. Chan School of Public Health and co-author of “Healthy Buildings: How Indoor Spaces Drive Performance and Productivity.” If you’ve been following advice about covid-19 from the Centers for Disease Control and Prevention or the World Health Organization, chances are you’ve heard a lot about how the coronavirus can travel through large droplets via coughing and sneezes. You’ve also probably heard about the virus being transmitted through surfaces. But you probably haven’t heard anything about airborne transmission, which many organizations have largely ignored. That’s a mistake. Airborne transmission — caused by small particles that can linger in the air for extended periods of time, unlike droplets from coughs, which settle quickly — is key to understanding why this disease spreads so rapidly in certain circumstances. It’s also key to figuring out how best to reopen our country.