Karen is one of our volunteers on the Covid-19 project, and takes a keen interest in the scientific side of the virus. Here, she writes about how CO2 links to Covid-19, and how she uses this information to manage her risk when out and about.
Disclaimer: this is a blog post by a BuDS member, and all views expressed within are entirely those of the writer. BuDS does not necessarily share the views or opinions of the writer, and any relevant parts of this blog can be used in other BuDS articles and posts.
“This post aims to describe my personal experiences of using CO2 monitors to manage my and my CV partner’s COVID risk as we go about our day-to-day activities. I bought two portable CO2 monitors, one “high-end” one purchased from a gas monitoring company and one “personal” one bought from Ebay, and took readings in various cafes, supermarkets, chemists, events, and other places we liked to go.
A Bit of Background
My partner is a Hepatitis B carrier, meaning COVID wouldn’t be fatal to him but that it might activate the dormant virus, potentially leading to a liver transplant and everything involved in being an organ donor. I have the usual non-life-threatening health problems of most post-menopausal desk workers. We’re both autistic – mine is more problematic in terms of anxiety, social phobia and sensory overload.
We both work in health research, and I’ve been involved in some COVID research as part of a government/academia collaboration, including some funded research into Long COVID. So, we have medical and general reasons for not wanting to catch it if we can! We also know where to find reliable information and how to interpret complex analyses. It’s also worth mentioning that we have the disposable income to buy two CO2 monitors for testing purposes.
We’ve been using a stash of around 20 FFP2 masks whenever we go out. We reuse them unless they’re dirty but make sure we give each time to air out between uses. We also have Cambridge reusable FFP2 masks and wore them over winter but find them a bit heavy for summer.
For us, going out for brunch or grabbing a coffee were things we missed most about lockdowns. Both our families live overseas, so we’re used to keeping in touch over the phone or digitally, and we’re not into going clubbing or to pubs (my partner doesn’t drink, and the crowds freak me out). But we always enjoyed chilling in a café or similar, and that was the thing we craved. So, it was the main thing we wanted to get back to and want to continue doing. But we live in Wales, so eating outside isn’t always possible and cafes can be crowded.
Disclaimer
This blog is based on managing MY AND MY PARTNER’S risk using the options we have available to us. It’s aimed as a guide to provide you with some information to help YOU manage YOUR risk more effectively.
It also should go without saying that these are real world tests and not conducted under clean, controlled conditions. But that’s the point. Lab experiments might not translate well to real world situations. Google “effectiveness versus efficacy” for more information on clinical evidence.
Ventilation
With COVID-19 being an airborne virus, ventilation has been held with masking as one of the two prevention standards. The theory being that, with enough airflow, any COVID-19 molecules in the air would be quickly diluted and dispersed so that anyone who does inhale them would only be inhaling small quantities. This might still lead to infection, especially for CV/CEV whose immune systems are weaker, but might lead to “milder” infections (i.e., not leading to hospitalisation). Since CO2 is an indicator of the potential amount of exhalations you are inhaling, and in the absence of any other readily-available measure of COVID prevalence in a space, CO2 monitors are being used as a proxy measure for the amount of air you inhale having come from someone’s lungs.
Some public health experts believe that the combination of masking, ventilation, vaccinations and paid options to isolate while positive would be enough to manage not just COVID-19 but all airborne diseases, such as Monkeypox (Ather, 2022; Tang, 2020; gov.uk, 2022). However, it’s largely up to individuals to manage their own risk.
There’s actually a standard formula to measure your “inhaled exhalations” based on the proportion of CO2 present in the space, the breathing rate of the individual and the amount of time the individual spends in that space (Peng, 2021). Roughly, at 800ppm or 0.08% CO2, 25% of the air you inhale is exhalation. The higher the CO2 reading, the more exhalation you’re inhaling and the greater your chance of inhaling COVID-19 or other airborne viruses. Masks, especially FFP2 and higher, will filter out some of the larger exhaled particles before they enter your system. But knowing the rate of CO2 in an environment can help you decide how to behave in that space. Table 1 shows our general behaviour based on three CO2 ranges and what these ranges might look like if you don’t have a CO2 monitor.
| CO2 Concentration | What This Means | Risk and Mask Wearing Recommendation |
| 400-600ppm | Well ventilated. Might be open air with few people around. Might also be an enclosed space with people but where a door is left constantly open. Premises with a constantly open door at either end earn extra brownie points | Okay to not be masked for extended periods. |
| 700-900ppm | Enclosed with some ventilation but might not be consistently or well ventilated, might be very crowded or you might be situated away from the open door. One café was busy and had a single window open. Even though we were sitting in the open window, the number of talking people around us gave a consistent reading of 800-900ppm. Big but sparsely-populated supermarkets also might have these readings – you’re too far from the open door to get the benefit | Keep masked unless eating or drinking. Wear mask when the server arrives. Be aware of people talking animatedly close to you. Depending on your individual risk, you might want to opt for take-away instead. |
| 1000ppm+ | Enclosed spaces where the door is kept closed. Doesn’t have to be busy – if one person is in there with no ventilation, their exhalations will eventually drive up the CO2 levels. Might also occur in gyms or other places where people are breathing heavily or talking/singing loudly | Opt for take-away and stay masked at all times. |
Here are some photos to illustrate what I mean
This was taken at an outdoor cafe. People were walking past relatively close to our table (it’s in an open-air arcade) but it was a slightly windy day and ventilation was good. We felt comfortable leaving our masks off. The cafe is small and was very crowded inside but had doors at both ends so, even when we went inside to pay, the CO2 levels stayed below 0.08% or 800ppm.
Different cafe, inside by an open door. The cafe was less busy but there were people sitting at the next table. Also note that this reading includes our own exhalations! We kept our masks on unless we were eating.
This one really surprised me! It was taken at Costco around 7pm on a Monday evening. As you can see, there weren’t a lot of people around me at the time (there were a couple behind me, but I didn’t want to accidentally include them in the photo). To me, this shows the importance of constant airflow – Costco warehouses aren’t exactly cramped and there are two big doors at the front. But these doors weren’t enough to refresh the atmosphere deeper in the store. We’d always felt safe shopping in Costco because of its excellent masking and distancing policies. It’s not that we don’t feel safe now, but we make sure our masks are well-fitted at all times.
No big surprises, really!
In the waiting room at the vet. We were next to an open door and there was good airflow through the open front and back doors but quite a few people in a small space. Inside the surgery consulting room was even worse – 1300+ppm. My partner and I wore FFP2 masks but the vet didn’t. I appreciate that, for safety and privacy reasons, doors and windows can’t be left open. But when the surgery called a few days later to reschedule our next appointment because the vet had COVID, I did mention our readings and suggested they invest in HEPA air purifiers for their consultation rooms.
Another cafe where I was sitting at the open window. But the cafe was crowded (the next table was less than a foot from my chair) and, while the window was open, it’s situated in one of Cardiff’s covered arcades. So, ventilation was present but not as good as outdoors. Again, we kept our masks on unless eating and didn’t linger after lunch.
CO2 Monitors: Should You Get One?
Look, I’m never going to recommend people spend money. We’ve found the results in Table 1 above to be pretty consistent and we now don’t always bring our CO2 monitor with us if we’re going to regular haunts. But we’ll bring them if we’re going somewhere new or if we think there’s a risk we might find ourselves in a high CO2 environment. We’re away this weekend, for example, and I used it in our hotel room before taking my mask off. Standard Operating Procedure for us in hotel rooms is to open any openable windows for a minute or two before we take our masks off anyway, but the CO2 monitor gives me a slightly more accurate or empirical idea of when it’s safe to do so.
I Want One – How Much Do I Have To Spend?
There are loads of air quality monitors on the market and some reviews suggest that some aren’t very accurate. Obviously, if you’re relying on a device to measure your safety and it’s not telling you the “truth” then you might as well not use it. For the purposes of these tests, I bought (with my own money) two from different ends of the pricing spectrum: The Analox CO2 Buddy (£300+) bought from a specialist gas safety equipment supplier, and the CEM DT-326 Air Quality CO2 Monitor (£8) bought from Ebay.
The Analox CO2 Buddy is designed for those working in industries where they need to monitor gas levels of different sorts, either for their own safety or to check for the safety of others. As you can see from the photos, it’s small and pretty unobtrusive. It’s also really light and can be clipped to my bag strap for easy reading. It only reads CO2 but, when I’m out and about, that’s all I want it to do.
The CEM DT-326 is designed, really, to sit on a shelf. It’s small enough to be held with one hand, even for someone with tiny hands like me, but is a bit bigger and clunkier than the CO2 Buddy. But would easily fit into a larger bag. It also displays more information – humidity and temperature as well as CO2. Plus, it’s £8 rather than >£300!
In terms of accuracy, I measured the CEM DT-326 against the CO2 Buddy outside then inside. The CO2 Buddy had been professionally calibrated when I bought it but I checked it against outside air readings anyway and it seems fine.
Initially, I had concerns about the CEM DT-326. Outside (left above), it was consistent with the CO2 Buddy. But inside (right, above), it was way off! However, I left it inside overnight and found that it had calibrated itself by morning.
I took the photo below in my favourite bakery (yes, I have a favourite bakery. You would, too, if you knew this place) with their permission. Yes, the readings are super high – they keep the door closed to prevent flies. No, we don’t eat in there. We mask up, go in, buy their delicious cakes, and leave. No, I haven’t suggested they buy a HEPA filter yet because they’re lovely and I don’t want to upset them by implying their premises are unsafe. But I will because they’re lovely and they deserve to work in a safe environment.
The CEM DT-326 displays PPM instead of percentages and so can give more detailed readings than the Analox. In my experience, the Analox doesn’t display below 0.04% or 400PPM and, because the percentage readings are already in the second decimal point, you don’t know how much between 0.04 and 0.05 you might be. To be honest, though, that’s not that big a deal for my purposes.
Neither actually store the data and neither have options for displaying changes in levels over time. Analox do an air quality monitor, the Guardian, the has longitudinal monitoring and costs ~£70. If I didn’t have the CEM DT-326, I’d seriously consider that one. But both the Analox Guardian and the CEM DT-326 record CO2, temperature and humidity and I’ve spent enough money on air quality gadgets recently (more below) so I can’t justify getting it.
Caveats On Readings
CO2 monitors measure CO2, not COVID-19 molecules. Just because you get a high reading doesn’t mean you’re likely to get COVID. I keep the CEM DT-326 on my desk at home and have recorded CO2 measurements of >800PPM while alone if I’m stretching at my desk. My “office” is a small box room with painted-shut windows and a door behind me onto the landing, i.e., no ventilation.
How loudly or forcefully you and those around you are speaking matters, too. A loud pub would require shouting in someone’s ear to be heard and that close proximity and forceful speaking would raise the CO2 dramatically. My partner and I recorded readings of 0.10% during a heated discussion about this blog post – his view is that uncontrolled readings have too many variables to provide any meaningful impact, mine is that laboratory environments aren’t applicable to real-world scenarios and would be equally meaningless. Again, high readings are not a problem if you know everyone around you doesn’t have COVID-19. But there have been some well-known instances of distanced choir practices turning into super-spreader events, so, unless you’re absolutely sure, err on the side of caution.
Where you place your monitor while out will make a difference. Again, you breathe out CO2, so placing it too close to you and your companions is likely to mean you’re measuring your own exhalations rather than those of the wider environment. Also be aware of where the sensors are on your monitor – the Analox CO2 Buddy has them at the front (the white square) while the CEM DT-326 has them at the back. So side-by-side comparisons above aren’t entirely accurate. But I did try to raise the CEM DT-326 up to ensure there was decent airflow.
So, with that in mind, if I were out, I’d face the Analox CO2 Buddy away from me but the CEM DT-326 towards me. Both would have the sensor directed at the biggest group of people. the CEM DT-326 needs up to a minute to get an accurate reading, so I’d take it out as soon as I entered a place and wait for the readings to stabilise before deciding on whether or not to remove my mask. The Analox CO2 Buddy also needs a few seconds to adapt but I tend to keep it clipped to my bag strap or a belt loop, so it’s adapting more quickly anyway.
A final note on readings – HEPA air purifiers don’t filter CO2. So it’s very possible to have extremely high CO2 readings seated next to an operating HEPA purifier. Trust me – I’m testing mini-HEPA purifiers at the moment and ran one for 15 minutes in an enclosed space (shopping centre toilet stall) with unchanged readings of 0.12%. I can’t say yet whether the purifier I was using is a dud as I’m waiting for the particle sensor to arrive. More on that in another blog post.
Care And Maintenance
Both the Analox CO2 Buddy and the CEM DT-326 are charged using micro-USB cables. I find the CO2 Buddy needs charging every day. I haven’t tried the CEM DT-326 long enough to know but suspect it’s the same.
As I said, the Analox CO2 Buddy was calibrated before I received it by the gas safety firm I bought it from. They offer 6-monthly servicing and calibration for approximately £45 if that’s something you’d be interested in. But the instructions include how to calibrate it at home – leave it outside for 15 minutes then do a sequence of button holds and presses and you’re good to go. I did notice that it was recording 0.07% in a room that generally recorded 0.05% so I recalibrated it and it seems fine. But something to watch out for. You’ll get a feel for what’s “normal”. If something seems consistently abnormal, your monitor might need recalibrating.
The CEM DT-326 didn’t mention calibration and doesn’t include instructions on how to recalibrate it yourself. But my experience has been that it tends to adjust itself given enough time. Leave it somewhere stable for a few hours and it should be okay.
Conclusions
While I don’t know if our CO2 monitors have prevented us from catching COVID so far, they have helped us to adapt our behaviour, to feel safer in environments where we might be unsure and to know when it’s better to be more vigilant. But CO2 predictable – constant airflow = low CO2, inconsistent airflow = high CO2. So, yes, it’s helped but it’s not necessary. We don’t always carry ours with us anymore. But it’s a reassuring thing to have if we want it and the cheap ones seem to be pretty good. Until more public spaces can be appropriately ventilated, this is one way we’ve found to “live with COVID”. Stay safe.”