For months, we’ve known that the coronavirus spreads via airborne droplets, invisible molecules or globules of liquid that fall or hang suspended in the air whenever we exhale, talk, or sneeze. As more schools and businesses prepare to safely reopen, they’re looking for ways to purify indoor air—and the market has answered with a dizzying array of devices that go far beyond traditional HVAC (heating, ventilation, and air conditioning) systems and HEPA (high-efficiency particulate air) filters.