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One of the (many) unsettling things about COVID-19 and other contagious diseases is that they’re invisible. You walk into a room and have no idea whether it’s safe or teeming with SARS-CoV2 virus particles floating on invisible aerosol droplets.

One way to make an intelligent guess is with a CO2 monitor. It’s won’t tell you about the prevalence of the virus indoors but it will tell you how much other people’s breath is lingering – that is, how well ventilation works to protect you in case some of those other people are contagious.

There’s an excellent piece about it in Sciencedaily.com. You can read the whole thing here, with links to a research paper modeling risk. Here’s an excerpt:

There is no single CO2 level at which a person can assume a shared indoor space is “safe,” Peng emphasized. That’s partly because activity matters: Are people in the room singing and talking loudly or exercising, or are they sitting quietly and reading or resting? A CO2 level of 1,000 ppm, which is well above outside levels of about 400 ppm, could be relatively safe in a quiet library with masks but not in an active gym without masks.

But in each indoor space, the model can illuminate “relative” risk: If CO2 levels in a gym drop from 2,800 to 1,000 ppm (~2,400 above background levels to 600), the risk of COVID-19 transmission drops, too, to one-quarter of the original risk. In the library, if an influx of people makes CO2 jump from 800 to 1,600 (400 to 1,200 above background), COVID transmission risk triples.

In the new paper, Peng and Jimenez also shared a set of mathematical formulae and tools that experts in building systems and public health can use to pin down actual, not just relative, risk. But the most important conclusion is that to minimize risk, keep the CO2 levels in all the spaces where we share air as low as practically possible.