One amazing thing about the COVID-19 pandemic, compared with previous epidemics, has been the availability of high-quality open data, keeping medical professionals, epidemiologists and even the general public aware of trends and risks.  Ideally we wouldn’t have needed these, but too many politicians, doctors and public health professionals have chosen to play games with the data instead of making straightforward recommendations.

And yet, one area where there wasn’t enough information was indoor air quality.  We knew fairly early that COVID is transmitted in aerosol particles that can remain in the air for hours.  So we knew that enclosed indoor spaces were some of the most dangerous places to be, and uncrowded outdoor spaces were among the safest places.  We knew that masks helped prevent transmission, but we have to take our masks off to eat and drink.  But we didn’t have good guidance about which indoor spaces were the most risky, and which were the safest.

In 2022 I had friends who dismissed all the risks from COVID and tried to go back to their pre-COVID lives as quickly as they could.  I decided I wanted to continue taking precautions, partly because I don’t like getting sick, and partly because I realized it’s one of the easiest ways to save lives.  Our politicians were telling us just to go to concerts, eat indoors, fly on airplanes and work in offices, but I checked the statistics and saw that people were still dying – hundreds of people a day in some waves, just in New York City.  I wanted more clarity, so I bought a carbon dioxide monitor.

Why carbon dioxide?  Because everyone who exhales COVID (or flu, or RSV) aerosols also exhales carbon dioxide, and good ventilation removes both disease aerosols and CO₂. Under many conditions, the concentration of CO₂ particles in a space can give us an idea of how much risk there is of catching or passing on a respiratory disease.

Unfortunately, the relationship between the numbers on a carbon dioxide monitor and the disease risk is complicated, so there are some things to know if you want to do your own monitoring.  On the plus side, after over three years of checking the CO₂ levels in a variety of places, I’ve learned a lot about how are moves around, to the point where I don’t feel the need to carry the monitor with me all the time.  In a future post I’m going to talk about some of the things I’ve learned through monitoring CO₂, and in this post I’m going to discuss four things I learned about CO₂ monitoring itself.

There are no absolutes

I would love it if we could just look at a readout and know how likely we are to catch a respiratory disease in any given room, but our bodies are complex and the air is complex.  The likelihood of transmitting COVID, the flu or RSV is affected by many factors including the temperature, humidity, wind, indoor ventilation, how many infected people are in the space, how infected they are, how heavily they are breathing, whether they are trying to project their voices, how robust the other person’s immune system is and whether they’ve been vaccinated against the strain that’s in the air.

There have been studies suggesting that a person can catch COVID from another person who’s simply jogging past them in a park.  When I contracted COVID in the winter of 2023, one of the ways I might have gotten it was on the open, upper deck of a ferry crowded with tourists.

That said, during an outbreak, a general rule is that if the carbon dioxide levels are below 800 parts per million, it’s generally safe to not wear a mask, and if they’re over 1200 parts per million, an N95-type mask may not be enough to protect against these viruses.

Find a CO₂ monitor that fits your budget

I started off with one of the cheapest carbon dioxide monitors available on Amazon, but I soon discovered that it didn’t directly measure CO₂.  It measured the concentration of formaldehydes, and used a mathematical formula to estimate CO₂, but that wasn’t always accurate.  In particular, it was affected by other sources of formaldehydes, like gas stoves, alcohol wipes and gasoline-powered cars.  These are not great for your health, but do not transmit these diseases.

I then bought another one on Amazon for around $80.  I was satisfied with it, so when the battery stopped charging, I bought a similar model.  They are listed under different brand names, but the one I have now is called INKBIRDPLUS.

The gold standard for CO₂ monitoring is the Aranet4, which currently sells for a little under $200.  Another public health advocate lent me one for a couple of weeks, and I compared the output of the Aranet4 to the other two monitors, and found that the INKBIRDPLUS-type monitor tracked the Aranet4 fairly well.

Plants can affect CO₂ levels without affecting respiratory disease levels

I know of two combination florist/cafes, one near my apartment and one near my office.  They’re both very pleasant, but I wouldn’t trust any carbon dioxide readings from there.  Plants consume carbon dioxide, and thus might cause the CO₂ levels to be lower than those in a similar room without as many plants, but they don’t take virus aerosols out of the air.

Good air purifiers can reduce disease transmission, but won’t remove CO₂

Air purifiers are the converse of plants: they can filter out disease aerosols, but carbon dioxide particles just pass right through them.  So we may get high CO₂ readings, which can be bad in themselves, but they aren’t evidence that there’s a high risk of catching or passing on COVID, the flu or RSV.

Those are some things to keep in mind if you want to do your own carbon dioxide monitoring. In an upcoming post I’ll share some things I’ve observed from my CO₂ monitoring.