Using a CO2 Meter to Infer Covid Virus Risk

A significantly elevated CO2 reading at a local Trader Joe’s store.

I was at a restaurant last Saturday.  I was struck by the ridiculousness of their mask wearing requirement.  We all had to have masks on when entering the building.  But as soon as we were shown to our table, our masks came off again.  We spent the next 90 minutes eating and drinking, talking and laughing, maskless; the same as other people at other tables around us.  Then we put our masks on for the under one minute walk to the exit, and removed them again as we went back to our cars.

I was curious as to the degree of risk we were accepting as a result of our maskless meal.  Were we enveloped in a thick cloud of Covid particles from other diners?  Or were we safely isolated, with nothing to worry about (except each other)?

Regrettably, there’s no easy way of knowing the answer to that question.  If there were, there’d be monitoring equipment everywhere.  But, just possibly, there might be an empirical measure that approximates the level of Covid risk – by measuring the CO2 concentration in the air.

Matching CO2 Levels With Covid Particle Levels

Covid particles are released by an infectious person every time they breathe out.  Some types of breathing – deep breathing – release more particles than other types, but even normal breathing causes the virus particles to spread into the air.

It now seems generally accepted that the virus particles hang suspended in the air, in an aerosol form, for some reasonable period of time – at least 30 minutes, maybe longer.  They’ll randomly disperse, and any air currents will pick them up and carry them along with the air flow.

There’s one other thing that people breathe out, too.  Carbon dioxide – CO2.

So, here’s the thinking.  If there’s a lot of CO2 in the air, we know that a lot of people have been breathing out and their breath is filling the room, and not then being flushed out and replaced by fresh air.

If the room has a good air flow, with fresh air coming in and the stale air being exhausted out, then the CO2 levels will drop to reflect this.

Is it fair to say that, all other things being equal, the higher the CO2 concentration, the higher the risk of there also being a significant number of Covid virus particles, too?  The same air flows (or lack thereof) which either dilute/remove the CO2 or allow it to pool and concentrate have a similar effect on virus particles, too.  So, yes, maybe it is fair to say that.  But…..  please keep reading.

The Problems With Equating CO2 Levels to Virus Levels

There are three flaws to the concept that the level of CO2 is directly and tightly linked to the risk of Covid virus particles also being present in the air.

The first flaw is because there is a way that Covid particles can be removed – if there’s a high quality HEPA filtration system (and with reasonably fresh filters) that the air is being regularly passed through.  This can remove many/most of the Covid particles, but – unfortunately for the purpose of trying to match CO2 levels to virus levels – it doesn’t make any difference to the CO2 levels.

Call us cynical, but we doubt many commercial buildings have upgraded their HVAC systems to now feature appropriate HEPA filtration units.  That drives the cost of the units considerably higher, and probably requires changes to the main HVAC units.  You can’t just swap over the removable filters if you’re going to do this properly.  We would guess that some buildings might have a new policy of changing their filters more often, and some might even upgrade the level of filter they have, but we do not think it common that commercial buildings have upgraded their HVAC over the last 18 months to the appropriate “hospital grade” type of air filtration.  We’ve all been expecting that Covid is about to disappear, and so it has never seemed to make sense to invest in costly capital improvements to reduce what has always been perceived as a short-term problem.

So this first flaw probably doesn’t often arise, and is weak rather than strong.  The good news is that if the air is being well treated, you might be getting a “false positive” reading, but better to get a false positive and be cautious, than to get a false negative and be carefree or careless.

There is another aspect to the removal of virus particles, which brings us to the second flaw.  Are people wearing masks or not?  Masks do not reduce CO2 at all, but they probably reduce the number of exhaled virus particles that get into the free air on the other side of the mask by about 50% or so.

In other words, you can feel more relaxed at any given CO2 level if the people around you are masked than if they are not.

The third flaw is there’s no way of knowing if any of the other people in an area might currently be infectious or not.  Just because there’s a certain level of CO2 concentration, you don’t know for sure how that translates to a matching level of virus concentration.  All the CO2 concentration tells you is whether you are breathing in air that other people has breathed out, not whether the air is safe or dangerous.

But, all things being equal, clearly the more the air you’re breathing in has already been breathed in and out by other people, the more likely it is that one of those people might be infectious and breathing out both CO2 and virus particles.

What You Can Infer From CO2 Levels

So, in general terms and to an approximate degree, the higher the CO2 concentration, the higher your Covid infection risk grows.  If you have a choice between being in a place with a high or a low CO2 concentration, clearly, you’re better advised to be in the place with the lower concentration.

If you’re in a place with a high CO2 reading, you should limit your time there and hurry out of that area as quickly as possible.

For example, if you have a choice between two supermarkets to shop at, or two different coffee shops, and one consistently scores much higher CO2 levels than the other, your choice of supermarket seems obvious.  If you’re at a store or location you normally like to linger and browse in, but it has a high CO2 level, reduce the time you spend there.  If you’re at a restaurant that has eat-in and to-go options, if their CO2 score is high, take the to-go option; eat it outside or in your car or wherever else that is safer.

Weather Issues and Variable Scores

Just because a place scores high or low today doesn’t mean they’ll score the same tomorrow.  It is obvious that CO2 scores can be expected to vary depending on how crowded a location is.  There might also be different scores at different times of day – perhaps lower in the morning, and then trending gradually higher during the day with people coming in, and the air exchanging not keeping up, but then the air exchanging “catches up” over night and the air is freshest/cleanest in the morning.

There are also longer-term trends.  In particular, as we transition, currently, from summer to fall and on to winter, places that were using fresh air ventilation might now be closing their doors and windows to keep the heat in as it gets colder outside.  Or, vice versa, places that were shutting out the hot air and using a/c might now be switching to natural cooling with more fresh air coming in.

For these reasons, we recommend you should be sampling the air quality all the time, rather than just once and assuming it will always be the same in the future.

Choosing a CO2 Meter

You’ll be unsurprised to learn that Amazon is full of CO2 meter choices.  Some are reasonably priced and convenient to use.  Most work with an NDIR sensor – Non Dispersion of Infra Red.  It measures the absorption of IR light by CO2.  There are other types of sensors too, but the NDIR type is usually the best at the budget end of the market.

Some meters are “multi-function” and report on other air-quality measurements (TVOC, HCHO) and particulate matter as well.  These usually use a different type of sensor and tend to be less accurate.

Just about all meters include temperature and humidity readings, which may or may not be accurate.  Ignore them, you’re buying a CO2 meter, not a thermometer/hygrometer.

Lower-priced (ie under $100) NDIR-based meters/monitors tend to have an accuracy in the range of about +/- 3% – 5% and a further imprecision of +/- 25 – 50 ppm on the display.  So, if the exact reading was, say, 500ppm, a worst case scenario might have a meter showing anywhere between 425 and 575, and a best case scenario would be in the range of perhaps 460 – 540.  High humidity levels can cause less accurate readings.

That might seem like too large an imprecision, but it is generally good enough for our purposes.  My feeling is that the +/- 3% – 5% is more a constant – that is, a device will regularly under-read or over-read, whereas the other part of the imprecision, the +/- 25 – 50 ppm, is a true variable.  You’ll get a sense for what it normally shows as normal, and you’re looking for deviations from that.

Although this generally interesting and helpful article suggests the units are generally reliably accurate, you really can not trust the accuracy claims of the different units offered for sale – see page 12 of this 2019 report.  Fortunately, you don’t really need much accuracy, just a general and comparative sense of “higher” or “lower” rates of CO2, and of vaguely “high” and “very high” levels.

Some units have a calibration feature, and others have an auto-calibration feature which is nonsensical and more prone to be misleading than helpful.  The calibration feature usually requires you to take the unit outside into the fresh air, let it settle, and the unit will “tell itself” that whatever it senses is the normal outside air CO2 level (which is generally stated as being around the 410 – 415 parts per million level).  If you can manually calibrate your unit, it is an idea to do so every few weeks, because some units tend to have their measurements drift away from the set point, and benefit from regular recalibration.

Key things to consider are the size of the meter (for portable and discreet use) and its battery life (and, of course, price).  Most meters tend to have short battery life (8 – 12 hours), and the meter I purchased, claiming 12 hours of life, actually delivers little more than 8 hours.  Make sure it has a rechargeable battery, otherwise you’ll be going through AA (or whatever) batteries at an alarming rate, and ideally you can run it with an external power source (probably via a USB port).

Important – Carbon Dioxide not Carbon Monoxide

Be sure you’re getting a carbon dioxide (CO2 or CO2) meter, not a carbon monoxide (CO) meter.  There are carbon monoxide meters and monitors, of course, but they are for a totally different purpose, and work on a different basis.  We’ve seen some articles and even advertisements that use the two terms semi-randomly and interchangeably.

You want to be sure your meter is for carbon dioxide.

Using Your Meter

The key thing to appreciate is that the meter probably takes a while to adjust to a new CO2 concentration level, and even more time if first switched on.  The meter I have slowly adjusts to a new number when I move it to a new environment, and takes three minutes, sometimes more, to settle at the new level.  So you can’t walk around a room looking for higher and lower concentration levels – you’d need to place the meter in one spot, wait perhaps five minutes, see the reading, then place it somewhere else, get a new reading, then go back to the first place to see if the reading has changed there in between times, and so on.

Remember also the inherent inaccuracy of the readings.  Anything less than about 75 ppm difference might be random rather than significant.  My meter is continually moving up and down from one second to the next, swinging between about 444 and 467 in its normal location – that is, one second it could show 444, the next 467, the next 444 again, the next 467 again, and so on.

If I’m in a supermarket, I simply place the meter in the cart where I can see the display as I move around.  If I’m at a restaurant, I place it on the table.  It helps to have a unit that doesn’t look like a scientific test instrument and is therefore less “threatening” to the store/restaurant owner!

You’ll get the best readings if the meter is no higher than you are – that is, it should be sampling the air at a level you breathe at, more or less.  You want to keep the meter out of drafts, because by their nature, they may be “cleaner” due to the air flowing.  Similarly, keep away from doors and windows if you want to understand the air quality inside a room, but if you’re sitting at or by a door/window, certainly measure – remember that air going through a door or window might be in either direction – maybe fresh air is coming in through that opening, but maybe stale air is going out.  Having fresh air coming in is great, but having stale air going out is not so great if you are seated close to that opening – you’re getting all the air from everyone passing by you.

One other thing.  I went to a supermarket, and as I always do, used some hand sanitizer gel to clean the handle of the carry basket, into which I then placed the meter.  A minute later, it was sounding an alarm and reading off the scale (max reading for this particular meter is 5,000 parts per million).  I was astonished, never having seen such a high reading before.  But, after some testing, I realized, it was a false positive.  The evaporating alcohol (mainly C2H5OH, perhaps some C3H7OH as well, ie ethanol and possibly propanol) seemed to be triggering the CO2 alarm.  It probably has a similar characteristic to the CO2 in the meter’s detection circuitry.

Analyzing Results

I’ve been using my meter for several days now.

Around the house, it shows CO2 levels indistinguishable from outside – in both cases, it is reading a bit higher than reality, around the 450 ppm level.  With a fairly large house, plenty of fresh air, and just me and a dog breathing inside, that number was much as expected.

Out of curiosity I also checked to see what happens when the gas stove is on.  Happily, the CO2 level didn’t rise too much at all.

Another thing I was interested in is what happens if I sleep in a small bedroom with the door closed overnight.  Rather to my surprise, after eight hours or so in a smallish room, just me and the dog, with no obvious airflow in or out, there was little more than an almost imperceptible rise in CO2 levels.

I did one more at-home test.  I had some friends around on Saturday evening for several hours.  After a couple of hours, I tested the CO2 level.  It was reading about 1600, with four adults, two children, and two dogs in a large open plan area with high ceilings.  I didn’t have any open windows and neither the heaters nor a/c was on, so there wasn’t any air flow, but it was surprising to see the CO2 rate go so high.

Moving now to the car, that was surprising.  If I turned the vents off, the CO2 level rapidly rose.  The same thing happened if I had the air on recirculate rather than bringing in fresh air.  I was surprised at how quickly the CO2 level rose, and at first thought it to be an error, but a friend, who also has a CO2 meter, confirmed a similar effect in his car, too. That’s something to watch out for, especially on a long drive.  Maybe feeling drowsy, as might sometimes happen, is at least partially due to a lack of fresh air?

And then, on to a couple of supermarkets.  One was a large airy and pretty much empty Safeway store.  There were very few people in the large store, and unsurprisingly, the CO2 level barely lifted up from the 450 normal, and never exceeded 550.  Wonderful.

I then drove a couple of blocks up the road, to a Trader Joe’s.  After giving the meter a while to stabilize, I snapped the picture you see at the top of this article, with it reading in the high 700s.  A lot higher than Safeway, but not surprising – the store was much smaller, and there were easily five, maybe ten times as many people in the smaller store.

But, wait.  Or, at least, I should have waited.  After taking the picture, I put the meter in my shopping cart, covered it with groceries as I did some shopping, and went to check out.  I’d heard a bit of beeping from time to time, and wasn’t sure if it was one of the assorted strange noises that apps make on my phone from time to time, or something from someone else, and ignored them.  Only as I was pulling the stuff out of my cart at the checkout did I realize the beeping was an alarm on the meter.  It had continued to rise, up from the high 700s, and was now beeping an alarm, due to the over 1500 reading it was giving.  I didn’t feel able to photograph it in front of the checkout operator, so you’ll have to trust me on that, but it clearly had been alarming for a high reading for several minutes, and continued to do so until I left the store.

That’s actually the high side of recommended CO2 levels, ignoring the possible Covid connection entirely, and of course, it also indicates cause for caution in terms of potential Covid exposure.  Which leads to the last section.

How Much CO2 is Too Much

Please keep in mind in this section we’re only talking about CO2 proportions, without any consideration of a link to Covid presence.

In high proportions, CO2 can cause sickness and eventually death.  Happily, these proportions are never normally encountered, other than perhaps on a submarine or space craft with some severe problems.

But even reasonably low proportions of CO2 can cause discomfort, drowsiness, and headaches.  This webpage, from the Occupational Health & Safety magazine, describes rates of 1,000 – 2,000 ppm of CO2 as being associated with complaints of drowsiness and poor air.  On the other hand, it claims the background normal level of CO2 is 250 – 350 ppm, and that’s plain wrong (it is about 415 ppm).  Here’s another source, that suggests acceptable levels are under 600 ppm and refers to OSHA and ASHRAE (the American Society of Heating, Refrigerating and Air-Conditioning Engineers) standards to keep CO2 levels below 1,000 ppm.  Note that is not the maximum OSHA allows (which is 5,000 ppm over an 8 hour averaged period), but rather a recommended target.

This is nothing to do with Covid risks, of course.  But if you find yourself in a space that has CO2 levels over 1,000 ppm – especially if it is where you work – you can refer to articles such as those linked above to advocate for better air flow and more changes of air each hour.  The key point, if talking to an employer who is reluctant to spend more money, is that employees will be more positive and less drowsy with lower CO2 levels, so productivity and accuracy will improve.

It will also cut down the risk of people in that space to get any type of aerosolized infection, not just Covid.  So the extra cost of better air is probably going to be more than made up in better productivity and attendance levels.

And certainly, when it comes to Covid, the correlation between CO2 levels and aerosolized Covid virus particles is such you want to see that number as low as possible (down close to the minimum of around 415 ppm).


There’s no easy way to detect the presence of virus particles in the air, but there is a reasonable correlation between the level of CO2 in the air and the potential presence of Covid particles, too.

A CO2 monitor/meter can be had on Amazon for $50 – $100, and gives a moderately accurate sense of if the CO2 level is normal, slightly elevated, or significantly elevated.  All other things being equal (which of course they never are) your risk of catching Covid rises as the CO2 level rises.  Avoid places with high CO2 levels, or if you can’t, minimize your time there.



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