TL,DR : CO2 concentration in air is easily measured and has been used as a proxy to monitor the level of potentially infectious particle people would release in a room while breathing. The idea is the more people breathe, the more they release CO2 and also possibly infectious particles.

It turns out that CO2 also plays an important role in buffering the pH of the aerosolized particles in which viruses like SARS-CoV2 travel from one person to the next. Dissolved CO2 is slightly acidic and prevents the particles from becoming too basic, which would destroy the virions. In doing so, higher CO2 concentrations in ambient air significantly extend the survival of the airborne virus and thus the average time these particles remain infectious.

A CO2 concentration of just 800ppm (parts per million), while usually considered a value consistent with a well ventilated room, is nevertheless enough to significantly extend the lifespan of viruses. This means we should strive to lower CO2 concentrations in rooms as much as we can during epidemics.