Public spaces such as subways, suburban trains, schools, or waiting rooms are breading grounds for germs that - especially in the fall and winter - contribute enormously to the spread of influenza and other types of viruses. Like many other illnesses, the flu is transmitted via droplet infections. That is, when talking, coughing, and sneezing infected individuals spray tiny droplets into common airspace, putting those around them at risk. For a long time, scientists have been looking for ways to keep the air in public spaces "germ-free" - without endangering people with harmful chemicals. Researchers at the University of Columbia...