As students jostle into school buildings and Broadway theaters shudder back to life, the most obvious signs of a new hygienic alertness will be masks and vaccination cards. But we also have an invisible vector of anxiety. Outdoors, the air dissipates pathogens; indoors, it can pass them around. Fortunately, we have the technology to cleanse the air as it goes swirling through cafeterias, lobbies, and other crowded spaces. The problem is that the public can only guess how well the machinery is doing its job. Is that a teacher’s monotone making you drowsy or the buildup of your classmates’ carbon dioxide? Is a frosty room the sign of an efficient HVAC system or a hint that it’s recycling already-cooled air? Do we need to worry that the products of an intermission coughing fit may linger through the finale?