Carbon dioxide monitors have been around for decades. But in 2020, they became, almost overnight, a hot commodity. All of a sudden, people wanted them to help assess the safety of indoor spaces — to gauge the likelihood of breathing in coronavirus-laced particles that until very recently had been in someone else’s lungs. No sensor can monitor how many infectious aerosols are swirling around us in real time. But carbon dioxide, or CO2, can act as a convenient proxy. People exhale it when they bre...