CO₂ sensors are magic COVID finders

Michael Herf
2 min readDec 18, 2020

Recently, I wrote about practical ways to ventilate for COVID-19, and why this is important. The idea from medical facilities and building scientists is that you want to optimize for “ACH” (or “air changes per hour”). This is a good start, because more air movement is better. But it’s really only a measure of how much fresh air you’re bringing in, not how much “bad” air is being produced.

Occupancy matters

There is a difference between a large space with two people in it, and a crowded coffee shop or classroom, or even more seriously, a choir practice. The amount of air produced by people depends on density (how much air per person), but also depends on what it is they are doing.

You could try to model all these things (and some people might want to or be able to), but I didn’t have a ready reference for how much air volume is produced in all these situations, or how buildings breathe throughout the day. And you’d really like a way to know if you got it right.

CO₂ meters to the rescue

It turns out that HVAC engineers use CO₂ meters to estimate air changes, because it’s a simple mixture of “outdoor” air (around 400 ppm) combined with exhaled air (about 38,000 ppm). There are other factors that contribute, of course, but in many buildings, CO₂ is mainly produced by people.

We can see what the rough “mixture” of fresh air and human air is, just by measuring CO₂, and we can see how it changes throughout the day.

As a starting point, keeping CO₂ levels below about 800 ppm seems to produce a level of “aerosol” safety similar to hospital designs. This is not an exact comparison, and it may require tuning for your building.

I spent about $100 for a simple CO₂ meter (not an eCO2 meter, those only detect VOCs) from (no affiliation).

Other benefits (and confounders)

There are non-COVID benefits to fresh air, also.

For instance, excessive CO₂ can make you sleepy and perform worse at work, and the effects are pretty substantial. Too much CO₂ in the bedroom seems to make sleep quality lower in a few studies, too.

After buying a meter, I started leaving the windows open more during the day, when it wasn’t too cold. I found that our gas range contributed a lot of (non-human) CO₂ to the kitchen, so this method isn’t a perfect COVID detector in spaces like kitchens, but venting is still beneficial.

The highest level for me was in the bedroom, which was too well sealed at night, so I turned on the air conditioner fan to get some fresher air at night. Maybe I’m sleeping better, but not sure yet.

Overall, this approach is cheap and very easy. More buildings could keep track of data like these, or you could take your portable meter around to figure out what’s going on in the spaces you inhabit.



Michael Herf

co-founder of f.lux, finding the connections between circadian rhythms, sleep, healthy buildings, and light. (Previously made Picasa.)