air quality

Infection is not inevitable: how to stop the spread of infections like COVID-19, flu, RSV, colds, and more in your house

by Dana Lewis

I observe a number of people who seem to think it’s inevitable that once someone gets sick, the rest of the house is going to get sick with 100% certainty.

Nope.

First of all, household transmission rates are less than 100% for all of these conditions, even if you didn’t take any precautions or make any behavior changes.

Secondly, with knowledge about how these things spread and some mitigation measures, you can reduce this a lot – and in some cases to nearly zero.

I will caveat: that of course depending on the situation some of these precautions may not make sense or be possible. For example, if you have kids, your exposures may be different. We don’t have kids in our house, so we are dealing with adult to adult possible transmission. That being said, some of these things may still be worth doing to some degree, to cut down the risk of exposure and/or to limit the viral dose you are exposed to, even in a situation that is less straightforward like a parent taking care of sick kids.

PS – if you’re reading this in January 2025 and don’t read the rest, make sure you’ve gotten your flu shot (yes, it helps) for the 2024-2025 flu season. No, it’s not too late. If you’re >65, you should also check about the RSV vaccine (which like the flu shot is a seasonal vaccine). It’s not too late and given the current high rates of RSV and flu (and soon to be uptick in COVID), they can help prevent getting or limit the severity if you do get exposed.

Our experience preventing the spread of RSV and the common cold

I can speak with recent, practical experience on this.

Twice.

First, let’s talk about RSV.

Before Thanksgiving, Scott and I were exposed to a nibling (aka a niece or nephew – of which we have 10 plus several honorary ones!) who had what we thought was a lingering cough from a cold from a few weeks ago. Because I am avoiding infection, I wore a mask inside and did not get up close to the nibling, so as a result of all of this likely had minimal exposure. Scott did not mask and had spent a lot more time with this nibling hanging all over him and coughing near or on him. Within 48 hours, he started to get symptoms of something.

We activated our plan for household transmission avoidance. Well, with a rolling start: Scott recognized by Thanksgiving evening that he was starting to feel unwell and had a tiny bit of coughing. I thought I could hear something in his chest differently, in addition to the occasional cough, so I went into full precaution mode while Scott did a partial precaution mode. This meant we set up air purifiers by each of our beds, and a fan pointed in my direction so all air was blowing away from me. I also wore a mask to go to sleep in. (This was super annoying and I don’t like doing this, especially because I usually take a shower and go to bed with wet hair. Wearing an n95 with head straps on wet hair plus having a fan and purifier blowing on me is chilly and unfun.) I would’ve preferred Scott to mask, too, or go to the guest bedroom to sleep, but it was late in the evening; he wasn’t convinced he was really sick; and I was too exhausted to argue about it on top of the fact that we were leaving on a trip the next morning. So he did not mask that night.

The next morning, though, he was definitely sick. He tested negative for COVID, and the nibling and everyone else from that house had been testing for COVID and negative, so we were fairly confident based on serial testing that this was not COVID. At the time, the thought was this was a common cold.

Since we were planning to mask in indoor spaces, anyway, including in the airport and on the plane, we felt comfortable going on our trip as planned, because we would be unlikely to infect anyone else. (This includes no indoor dining: we don’t take off our masks and eat inside.)

Because Scott realized he was sick, he masked from that point forward (with a non-valved N95). We both masked in the car, in the airport, on the plane, and again when we arrived while driving in the rental car. Then a challenge: we needed to eat dinner (we got takeout), and we were sharing a hotel room overnight. We switched from a hotel room with a king bed to a room with two queen beds, which would give us some more space overnight. But we took turns eating dinner unmasked in the hotel room (it was too cold to be outside) with the far-UVC lights on and the purifiers around each of us when we ate. While we ate, the other person was masked. (And I went first, so there was no unmasked air from Scott while I was eating and he went second). We also took turns showering, again with me going first and him not having been in the bathroom unmasked until after I had gone in. Other than that, we stayed masked in the hotel room including overnight, again with purifiers between us and the far-UVC lights running.

(This hotel did not have windows that opened to outside, but if there had been windows I would’ve eaten in front of the open window and we would’ve likely kept it cracked open and the heat turned up, to improve the room’s ventilation).

The next day, we had more of a drive, and again we masked. We also slightly rolled down the windows in the backseat to improve ventilation. Scott sometimes took his mask off for comfort stretches, because he was driving, but put it back on fully and sealed it before coughing. I kept my mask on without ceasing. We did a 4.5 hour drive this way.

Luckily, once we arrived at our destination, there was a spare bedroom, so that became Scott’s headquarters. He stayed masked in the living room/shared areas. He sat downwind outside and masked up when coughing if anyone was outside. We left the sliding door to the outside cracked open, in order to keep the air in the common areas well-ventilated. This worked, because we were able to keep CO2 levels (a proxy for ventilation) down below 700 ppm most of the time.

Because we had separate bedrooms, we did not mask while sleeping the rest of the week, because we each had our own rooms (and own airflow). I did keep a purifier running in my room all week, but that’s my habit regardless because I’m so allergic to dust.

And guess what? It all worked. We masked again on the drive back to the airport and in the airport and on the plane and again once we got home.

I never got RSV. The four other adults we spent time with and shared a house with….also did not get RSV. So we are pretty confident that the transmission chain stopped completely at Scott.

In summary, what worked:

  • Masking in shared spaces, and two-way masking when it wasn’t possible to ventilate
  • When we had to sleep in the same room, two-way masking even for sleeping overnight
  • Scott masking in shared spaces that were well ventilated, and often left the room to go cough even when masked (or coughing outside). This often meant he masked, but the rest of us did not mask inside the whole time.
  • Generally keeping distance. Droplets were managed by the N95 mask, and we were ventilating to reduce aerosol transmission risk, but still keeping physical distance to further reduce the risk.

RSV is *very* transmissible especially with aerosols, and Scott was coughing a lot all day and night. (At one point, his Sleep Cycle app was estimating 18 coughs per hour). It took a long time for that to get down to normal, so he continued to sleep in our guest room when we got back and we continued to ventilate well even when we gradually reduced masking once he stopped coughing. It took about 10 or so days for all of his biometrics to normalize, and about 14 days for his cough to completely go away. It probably was closer to three weeks before he finally felt all recovered.

So with that timing in mind, you know what happens 4 weeks after Thanksgiving? Christmas/other end of year holiday gatherings.

We had plans to see 8 kids and 8 adults (plus us) for Christmas. And at Christmas, it seemed like everyone had a cold already. So again, I went in and mostly masked except for when I was in front of an open window and the room was well ventilated, without anyone coughing actively in the room. (If anyone was in the room with me and coughing, especially the kids, I would mask even with the window open).

I did not get the cold that 8-10 (out of 16) people eventually got.

But…Scott did. And this time, he was mostly masked, but he still spent more time up close with kids who were coughing quite a bit. And this is where some of the dynamics of knowing WHAT people have is helpful. You can’t always know, but you can sometimes use the symptoms to figure out what people have.

For example, based on symptoms of the nibling who passed on germs to Scott around Thanksgiving, and Scott’s symptoms (instant, incredible chest cough but no runny nose, sore throat, fever, or aches) we had ultimately guessed that Scott had RSV. We then knew that the biggest risk was either droplets from coughing (especially because the volume of coughing), which could be reduced drastically by masking, or aerosols, which again would be helped by his masking and also ventilation, and in closed spaces, two-way masking (me masking).

For the Christmas germs, everyone seemed to have mild symptoms with congestion, runny noses, some coughs. But no fevers or aches and it seemed less severe. Given our recent experience with RSV, we narrowed it down to likely being a cold (rhinovirus), given again everyone testing repeatedly negative for COVID.

Given that, we knew the risk was going to be highest for us from droplets and fomites. So we again masked in shared spaces; Scott went to sleep in the guest bedroom as soon as he started getting symptoms; and we both did a lot of hand washing. Scott washed his hands before touching any of my things and regularly wiped down the kitchen. I tried not to go in the kitchen much (our main overlapping shared space), but also wash my hands after any time that I did. He didn’t have much of a cough and it was more controlled, so he would hold his cough until he could cover it with a mask or be in the room by himself. We also did our usual running of purifiers and opened windows and ran fans to increase ventilation to keep CO2 low.

And again? It worked. I did not get the cold, either from any of the ~8+ folks who did across the holiday period, or from Scott. Scott’s vitals all returned to normal at the five day mark, although we continued to mask in the car through day 7, to be more cautious (due to my personal situation).

So, infection is not inevitable, even in small houses and apartments.

Here’s what we’ve taken away from these experiences with more aerosol-based (RSV) transmission diseases and more droplet and fomite-based (cold) experiences:

  1. Two-way N95 masking works. Mask in the car, run the fan, keep the windows cracked, run purifiers at home, and ventilate spaces, but you still want two-way masking when something is aerosolized and you’re in the same spaces. This can prevent transmission.
  2. Keep distance when someone is coughing and sneezing (and if they have a cough or sneeze type illness, you want 6 foot distance even when they’re not actively coughing or sneezing, because they make droplets just from breathing and talking). The person who’s coughing and sneezing should mask, even inside, unless they are in their own room in private (and it’s not a shared room).Keep your air ventilated (if you haven’t, read my post about ventilation and using a Co2 monitor)Depending on the illness, to fully protect yourself you’ll need to commit to wearing a mask at all times indoors to protect yourself if the person who is sick is not masking. (Eg, Scott got a cold while mostly masked around heavily coughing niblings, but not throughout the whole house the whole day). With adults, the adults who are sick should definitely mask if they’re in shared spaces with other adults. (It’s harder with kids, and it should be a conversation depending on the age of the kids about them masking in shared spaces, such as if they want to play with Uncle Scott, or help them understand that someone may not want to play up close if they’re sick and coughing and not willing to mask. That’s fine, but that’s a choice they can make when kids are old enough to understand.)
  3. Have the infected person sleep in a different space (on the couch or in another room if you have a spare bedroom). If you have to share a room, both should mask.
  4. Use cleaning wipes to wipe down shared surfaces (e.g. fridge handles, microwave, counters, bathroom surfaces like the flush on the toilet or sink faucet, etc) and wash your hands after using these shared spaces every time. Fomites can last longer than you’d expect.
  5. Use metrics from your wearable devices (eg Apple Watch or Oura ring or similar) to track when your temperature, respiratory rate, heart rate, cough rate, etc. return to normal. That tied with symptom elimination can help you determine how long you’re likely most infectious for. The general estimates of contagiousness for each condition generally seem to be right (e.g., two weeks for an adult with RSV and 5-7 days for a cold) in our recent experiences. I would continue precautions for at least those minimum time frames, if you can.
  6. Yes, there’s a cost to these precautions, in terms of human contact. There was no hugging or hand holding or kissing or any touch contact during these time periods. I felt pretty lonely, especially because it was me we were trying to protect (because I am at high risk for bad outcomes due to immunosuppression right now), and I’m sure Scott also felt lonely and isolated. That part sucked, but we at least knew it was a fixed period of time, which helped.

What we’d do differently next time

Infection is not inevitable -how to reduce transmission of illness in your household (including COVID-19, RSV, flu, and the common cold), written by Dana M. Lewis from DIYPS.orgThis basically has been our plan for if either of us were to get COVID-19 (or the flu), and it’s good to know this plan works for a variety of conditions including RSV and the common cold. The main thing we would do differently in the future is that Scott should have masked the very first night he had symptoms of RSV, and he has decided that he’ll be masking any time he’s in the same room as someone who’s been coughing, as that’s considerably less annoying than being sick. (He really did not like the experience of having RSV.) I obviously did not get it from that first night when he first had the most minor symptoms of RSV, but that was probably the period of highest risk of transmission of either week, given the subsequent precautions we took after that.

Combined, everything we did worked, and we’ll do it again when we need to in the future, which should not be very often. We went five full years without either of us getting any type of infection (yay), and hopefully that continues from here on out. We’ll also continue to get regular COVID-19 boosters; annual flu shots; and other annual shots if/when they become available (e.g. when we reach the age, getting the RSV vaccine).

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Remember, if you’re reading this in January 2025, RSV and flu levels are very high in the US right now, with COVID-19 expected to pick up again soon. It’s not too late to get your boosters and given the rates of respiratory illness, consider situational masking even if you don’t typically mask.

Air Quality, CO2 monitoring, and Situational Masking

by Dana Lewis

I do a lot of things most people don’t want to do themselves – and I get that. (For example, recording macronutrients while running? Running for up to 16 or 25 hours? Let alone other choices like building DIY and making open source automated insulin delivery systems not only for myself but more widely available for other people.) I’ve also talked before about functional self-tracking and how I don’t track things for the sake of tracking, I track when the data/information is actionable either retrospectively or in real-time.

I’ve spent enough time now collecting real-time data on air quality (via a proxy of CO2 levels) that I think it would be useful to share for other people to consider the retrospective data for THEIR decision making.

You may not want (or be able to afford) a CO2 monitor, and you may not want to mask inside all the time, but the below outlines the general scenarios in which air quality tends to be better or worse and when you would get the most benefit from situational masking in response to those situations.

(Think about situational masking indoors like you think about situational masking for smoke and poor air quality outside. Most of the time, you likely don’t mask outside. But if you’re on the east coast right now or have lived through a previous west coast US summer with a “smoke season”, you’ve probably experienced multi-day air quality outside that was so poor that you considered or did wear a high-quality (N95/K95) mask outside or limit your time exposed to that outdoor air.)

Air quality assessment via CO2 monitoring

In the last few years, Scott and I acquired two different CO2 monitors. The first was cheap, required to be plugged into a battery pack to run it, and was simply viewable on the device display. It was useful to start to get a sense for what the CO2 levels were in indoor spaces as compared to outdoor air.

Later, we decided to invest in an Aranet CO2 monitor, which runs on two AA batteries and lasts months on a single pair of batteries. You can view the data on the device display AND see a retrospective and realtime graph of the data in your phone, because it connects via Bluetooth. You can see not only CO2 but also temperature, humidity, and air pressure.

We have found this useful because CO2 is something that we all produce when we breathe out. The more we breathe out, and the more people that are breathing out, the higher the CO2 levels. The more of that air that is replaced with low-CO2 outside air, the lower the CO2 levels. Measuring the CO2 then helps us understand the ventilation (how much air is flowing through the space and how often it is being cleared out) and the risks of being in that space. A higher CO2 level means more people and/or less air being cleared out of the space, meaning  you are more likely to be breathing in what someone else is breathing out.

How we evaluate CO2 levels

An outdoor CO2 level would be around ~450 ppm in urban areas, or as low as 400 ppm out in nature. Since a perfectly-ventilated space would be 100% outside air, we want to compare any indoor air CO2 reading to outdoor air.

For example, at home in our enclosed apartment with 2 people (and 2 cats), we typically run around 700 ppm, which means ~250 ppm above outdoor air levels. When we open our door or a window, it drops to ~500 ppm, or only ~50 ppm above outdoor air levels. Given that we have confirmed our air intake into our HVAC system for our apartment is outdoor air, this means the ~250 ppm we are sharing between the two of us is just our (and the cats) exhalations, rather than anyone outside our household. So those levels are acceptable to us, but our choice of interventions would change if we were sharing air with other people, especially random strangers. (Stranger danger is one way to think about air, further contextualized below with data.)

In a shared space with random strangers, your risk of COVID aerosol-based transmission is proportional to how elevated the CO2 level is above that of outside air, and the amount of time spent in that space. So a CO2 reading of 650 ppm, which is ~200 ppm over outside air, would be half as risky as a reading of 850 ppm, or ~400 ppm over outside air. And timing matters, so a 1 hour bus ride or the hour you spend boarding and waiting for takeoff on your plane when CO2 levels are highest and the air filtration (see below) is off will be of greater risk than short exposure to the same levels.

Now, we’ve also used our CO2 monitors in many other places, such as in airports and on planes and other public transportation, and other indoor shared spaces like grocery stores etc.

Here’s what we’ve learned about where CO2 levels trend (based on our repeated but n=1 testing).

Trains, buses, and rideshare (e.g. Uber, Lyft, etc) = BAD NEWS BEAR AIR

Public transportation, in every location and every country we have been in, has much higher CO2 levels.

What do I mean by much higher? Often 1000-1500 ppm easily (and sometimes >2000 ppm), which is anywhere from 500-1500 ppm above outdoor air quality.

Trains/metros/light rail where the doors are constantly opening and closing to outdoor air would seem like they would be better, but sometimes they still have (due to the density of riders) >1500 ppm.

Buses where you can’t open the window can be as high of CO2 levels as planes, without the benefit of air exchange or HEPA filtration of the air. Our recent 20 minute bus ride was up to >2500 ppm on a full bus.

Watch out for rideshares, too. Often times we get in a rideshare and the driver intentionally or accidentally has “recirc” or “recirculating air” on, meaning the air isn’t exchanged outside and the driver and riders are re-breathing each other’s air over and over and over and over again..yikes. Specifically looking at the console when you get in the car is useful: if you see the recirc button lit up, ask the driver to turn it off. If they don’t understand or refuse, or you don’t want to try to explain it, opening a window helps immensely to reduce the CO2 levels and the amount of re-breathing air. (The recirc icon usually looks like a car with a U-shaped arrow on it).

Planes (including airports, during boarding, in flight, and during landing/deplaning) = ALSO BAD NEWS BEAR AIR

Airports sometimes have better-ventilated spaces: you can often find less crowded corners of a terminal and see CO2 readings of <900 ppm. However, it’s still pretty common to be in the airport and see >1000 ppm, meaning that the CO2 is >500 ppm above outdoor air quality, and it is air from a whole assortment of random strangers coming and going, so it’s less safe than the air you’d be breathing in at home or in private spaces.

When boarding, both standing close in line with other people but also on the jet bridge and while you are on the plane, is usually even HIGHER CO2 levels than the airport. The typical air for a plane (that they tout with HEPA filters and high air exchange rates) is not turned on until you start to take off, and then it takes some time to exchange all of the air. This means there is a MUCH higher rate of re-breathing other people’s air while boarding and until you are in the air.

Now, we have measured CO2 levels during all of these times. If indoor airport air is around 900 ppm, it usually jumps to 1100-1300 on the jetbridge (if you’ve got a backed up line) and when you’re sitting on the plane watching other people board, it can go up to 2500+. And then it continues to go up as you have a full flight of people breathing in this enclosed space. During flight, we’ve seen CO2 levels hover between 1700-3000 ppm, and in some cases have gone up to ~4000 ppm. This is a lot of CO2! However, there are HEPA filters cleaning the ~half of the air that is recirculated instead of replaced. So, it’s harder to say when the airplane air systems ARE running (during most of the flight) whether the risk is as high (for infectious disease transmission) as it is in other environments that aren’t studiously exchanging and HEPA-filtering any recirculated air.

Note that when they spin down the engines after landing and all the way through taxiing, deplaning, and getting back into the airport – the CO2 level again tends to rise because they again change the air flow when they’re on the ground. So like standing in line to get on or waiting for other people to board, standing in line to get off/waiting for everyone to get off produces high CO2 levels *without the benefit of in-flight air exchange*, so it’s likely higher risk during those times than in the air during the middle of the flight, even if CO2 levels are equally high during flight.

Indoor spaces like grocery stores or conference rooms/meeting halls

Indoor spaces can vary quite a bit, and often by country or venue.

For example, most indoor spaces in the US we’ve found to often have a fairly high (e.g. 900+ ppm) indoor CO2 level, even without a huge density of people. For example, we quickly went into a grocery store the other day and the CO2 was high-800s without being around many people in the aisles, across the entire store. For not having people actively occupying the space, this is fairly high and less optimal.

In contrast, we recently were in Sweden for a conference and were honestly gobsmacked when we got off the plane and found the CO2 levels to be <600 ppm in the airport! And in the hotel lobby! And in the hotel elevator! And at the local grocery store!

(Seriously, it shocked us, because we’ve also recently been in the UK with our CO2 monitor and found US-like CO2 levels typically around 900-1000 ppm or higher, and also in Spain last year where we similarly found it to be >900 ppm even when not densely occupied. The exception to optimal air quality in Sweden was our ~20 minute bus ride where CO2 levels were >2500 ppm).

So, the CO2 levels may vary quite a bit and this is why measuring is helpful. Because you can’t assume that one country/one room means that all of the rooms in that country or even that venue will be the same.

Case in point? Conference rooms/halls or meeting rooms.

In Barcelona, Spain in April 2022, I spoke at a conference. The CO2 levels in the hallways and in the meeting room before the session started were around 800-900 ppm when not occupied. Again, a little high for not having people actively in the spaces. Then, when the conference started, Scott watched the CO2 monitor and saw it rise..and rise…and rise. Within 45 minutes, the CO2 levels were around 2000 ppm (>1500 ppm over outdoor air quality)! He went to the back of the room and opened the doors to try to get some air circulating in the room, although it didn’t make a big difference. That room did not have a high number of air exchanges per hour and was not successfully clearing out the air people were breathing out.

In Sweden (May 2023, where the CO2 was <600 in a lot of public indoor spaces), we found the same challenge in a high ceiling, large meeting hall. With 300 people, the start of the session had about 950 ppm (as opposed to the <600 ppm of less occupied hallways). Not too bad given 300 people in the space. However, by the end of the session, the CO2 level had risen to ~1350! And it continued to rise even as people had exited the room; we didn’t see a drop in CO2 levels until we went out in the hallway to continue talking to people, and it took another ~25 minutes before CO2 levels in the hallway were back down <600 ppm.

Again, we were surprised, because this venue (the hallways, lobby, elevator, etc) all had really great otherwise indoor air quality with CO2 <600 ppm!

But the challenge is the space (and the infrastructure for filtration and air exchanges); the number of people filling the space; and the amount of time, in terms of what happens to the CO2 levels.

The takeaway from this? Conference halls, meeting rooms, and anywhere where you are sitting with a group of people over a period of time is going to have a much higher CO2 level and it will increase in proportion to the time that you are occupying that space (e.g. a 30 minute or 1 hour session is going to have a much higher CO2 buildup than a 10 minute talk where the audience is turning over and leaving the room and it clears out some before the next session).

So what should you do about this information? Consider situational masking.

I really have found a CO2 monitor helpful, because even my best guesses about air quality (e.g. thinking Sweden’s conference hall would have good air quality given the size of the room and ceilings) aren’t always accurate. But if you don’t want to invest in a CO2 monitor, here’s where you can get the biggest bang for your buck with situational masking.

What do I mean by situational masking? Maybe you don’t think you’re at very high risk for COVID or other infectious illnesses, but you are interested in reducing the likelihood that you spread anything you get to other people (thanks!). But you don’t want to have to think about it, and maybe you’ve chosen previously to drop masking so you don’t have to think about it. Here’s a set of easy rules/situations in which, like learning to dump your liquids out before going through airport security, you can get into a habit of doing and not have to think about it much.

  • Public/shared transportation.

    Riding a bus, train, metro, or a car with a stranger and especially with multiple strangers – these have high CO2 levels.

  • Airports, boarding a plane and during takeoff, and during descent/landing/deboarding the plane.

    This is when the CO2 levels are highest and the air exchanges/HEPA filtration is not running.

    Think of it like the seatbelt sign. You board the plane and put your seatbelt on, then eventually once you’ve reached cruising altitude the seatbelt sign goes off. If you’re standing in a line of people (to board or deplane) OR if the seatbelt sign is ON, that’s a huge ROI for wearing a high-quality (N95 or KN95) mask. When the seatbelt sign first turns off during the flight (or you hear the 10k-feet chime) and you want to take and leave it off, or take it off a while to eat or drink – that’s less risky during those times due to the HEPA filtration and air exchanges during flight. But when the seatbelt sign goes on for the plane’s final descent? The air quality is going down, too, so putting your seatbelt AND your mask back on is a higher ROI thing to do.

    (You do you inside the airport, too, but see below about density of people and temperature as a guide for whether you might want to consider situational masking in airports when you’re not eating/drinking.)

 

  • Conferences or meetings where you are sitting for more than a few minutes and there are many people in the room.

    Even with super big rooms and super high ceilings, so far every conference space I’ve presented in during the last several years has high CO2 levels even before the talk starts, and is even higher (>500-1000 ppm added) by the end of the session). If you’re not presenting or eating and drinking and are just sitting there listening and engaging in the session…it’s a low hassle opportunity to pop a high-quality mask on so you’re not breathing so much of the air around you from everyone else. When you’re done with the session and head out and want to socialize? Like leaving the plane, you’ll be around fewer people, and the CO2 levels (and risk) goes down. But sitting there quietly is a great time to wear a good mask and reduce your intake of other people’s exhalations.

 

You might find yourself in situations where the room feels hot and stuffy, or in the case of conferences and meetings, the air feels FREEZING cold. It runs freezing cold because the room gets hot and stuffy with so many people, indicating this space is not well ventilated, so they pump the AC to change the temperature. But that is a compensation for a too-low rate of air exchanges, and pre-cooling doesn’t prevent CO2 and aerosol buildup, so a room that either gets freezing cold or hot and stuffy should be a signal that the air quality likely isn’t ideal.

So a good rule of thumb is, if you’re in a space that feels hot and stuffy OR freezing cold, that’s an indicator that the air quality might be non-optimal. Consider masking in those situations even if you don’t have a CO2 monitor to evaluate the air.

It would be great if we could get 10x people to consider situational masking like this. Avoid the worst of the bad-news-bear-air of public and shared transportation and indoor spaces, which would cut down on a lot of transmission, even if people otherwise are still socializing and eating in indoor spaces and doing whatever it is they want to do. The choice to situationally mask might occasionally protect them but would also protect everyone around them in those situations when their exhalations have the greatest risk of doing the most damage.

A good way to think about it is at a conference. You might be willing to go to bars and socialize, but someone who is higher risk may be choosing not to attend those indoor dining scenarios. That’s fine: you each get to make your own choices! But when you go and sit down next to that person in a conference session, your choices then influence that person by every breath you take in that conference session.

That’s why situational masking – knowing that a situation is low-hassle to wear a high-quality mask (sitting quietly in a session) but high-risk (due to the poor air quality) means you have a high ROI to pull a mask out of your pocket/bag and throw it on – can help the people around you very effectively with little hassle and thought on your part.

You can get in the habit of masking in the bad-news-bear-air situations/locations, and you don’t have to think much about it. You’ll make things a bit safer for yourself and for the people around you, for far less hassle than avoiding buying a drink before you go through airport security because you know you need to dump liquids out.

Data-driven situational masking based on indoor air quality

More Thoughts And Strategies For Managing Wildfire Smoke And Problematic Air Quality

by Dana Lewis

In 2020 we had a bad wildfire smoke year with days of record-high heat and poor air quality. It was especially problematic in the greater Seattle area and Pacific Northwest (PNW) where most people don’t have air conditioning. I previously wrote about some of our strategies here, such as box fans with furnace filters; additional air purifiers; and n95 masks. All of those are strategies we have continued to use in the following years, and while our big HEPA air purifier felt expensive at the time, it was a good investment and has definitely done what it needs to do.

This year, we got to September 2022 before we had bad wildfire smoke. I had been crossing my fingers and hoped we’d skip it entirely, but nope. Thankfully, we didn’t have the record heat and the smoke at the same time, but we did end up having smoke blowing in from other states, and then a local wildfire 30-40 miles away that has been making things tricky for several days on and off…several different times.

I’ve been training for an ultramarathon, so it’s been frustrating to have to look not only at the weather but also the air quality to determine how/when to run. I don’t necessarily have a medical condition that makes me higher risk to poor air quality (that I know of), but I think there’s some correlation with being allergic to a lot of environmental things (like dust, mold, trees, grass, etc) that makes it so that I also am more sensitive to most people I know to poor air quality.

Tired of wildfire smoke making it hard to exercise easily outdoors

Everyone’s sensitivity is different, but I’ve been figuring out thanks to multiple stretches of up and down AQI that my threshold for masking outside is about 50 AQI. If it gets to be around 100 or above, I don’t want to be walking or running outside, even with a mask. And as it gets above 150 outside, it becomes yucky inside for me, too, even with the doors and windows closed, the vents on our windows taped shut, and air purifiers and box fans etc running. My throat was scratchy and my eyes hurt, and my chest started to feel yucky, too.

It got so bad last week that I took a small, portable mini air purifier that I had bought to help mitigate COVID-19 exposure on planes, and stuck it in front of my face. It noticeably made my throat stop feeling scratchy, so it was clearly cleaning the air to a degree. On the worst days, I’ve been sitting at my desk working with the stream of air blowing in my face, and I’ve also been leaving it turned on and pointed at my face overnight.

This is kind of a subjective, arbitrary “this helps”, but today we ended up being able to quantify how much it helps to have our big air purifier, box fans with furnace filters, smaller air purifier, and the mini air purifier. Scott ordered a small, portable PM2.5 / PM10 monitor to be able to see what the PM2.5 and PM10 levels are in that exact spot, as opposed to relying on IQAir or similar locally reported sensors that only tell us generally how bad things are in our area.

It also turned out to be useful for checking how effective each of our things are.

It turns out that our box fans with furnace filters taped to the back are most effective at fan speed “1” (they all go up to 3), probably because putting it up to 3 is prone to stirring up dust from the floor (despite robot vacuuming multiple times of day) and increasing PM10 levels. A box fan with 2” MERV 10 filter taped to the back doesn’t affect the already-low PM2.5 levels indoors; on fan level 1 the PM10 gets reduced to zero as long as it’s not pointed at the carpet and stirring up dust. So while it doesn’t help with smoke, these fans are good with increasing circulating air (so it feels cooler) and getting rid of the dust and cat hair that I’m allergic to.

The big HEPA air purifier we bought has a connected app that tells us the PM2.5 levels, and our portable PM2.5 monitor confirms that it’s putting out air with a PM2.5 level of 0. Yay! This sits in our kitchen by our front door, so it helps clean the smoky hallway air coming inside.

A cat sticking it's face toward the phone camera. Behind the cat, a portable PM 2.5 / PM 10 air monitor sits on the floor by a door to measure incoming air.

The hallway air is TERRIBLE. The hallway opens directly to the parking garage, and is usually about as smoky as the outdoor air: it only has a single A/C duct for the whole building, which isn’t always running. The stairwell leading outside is a little cleaner than the hallway and outside. (So I’m glad we have our best air purifier situated to take on the air coming in when we open the hallway door). So we won’t be spending time exercising in the hallways, either; with that level of air quality you might as well be outside anyway, because we need to be masked either way.

The other purifier we have is a smaller purifier. I have it sitting on the counter in our bathroom, because the air exchange to outside is really reduced compared to what it should be (and the building management doesn’t seem very interested in trying to figure out how to fix it). That purifier gets PM2.5 down from 4 to 1 ug/m^3, or about a 4x improvement! Which is pretty good, although not quite as good as the big purifier in our kitchen/entry. Since it’s small enough to sit on a desk or bedside table and blow clean air at me where I’m working or sleeping, we decided to order 2 more of these smaller purifiers for my office and our bedroom, since the box fans take care of PM10 but not the PM2.5.

PM2.5 and PM10 readings from the portable monitor, from on top of the air purifier; next to my office; next to a box fan with filter; in the hallway; in the stairwell; and outside. This is roughly in order of best (inside over the air purifier) to worst (hallway and outside; the stairwell is slightly better than the hallway).

Since the portable air quality monitor would be hard to fit inside his mask or his mouth, and impossible to read there, Scott also held up the PM2.5/10 monitor to the exhaust valve on his n95 mask (note: not all our n95 masks our valved but the valved ones are good for wildfire smoke and managing temperature levels inside your mask when exercising) while outside, and the average PM2.5 level there is about half that of the ambient air. Since about half the time he’s breathing in (and the meter is sucking in outside air) and the other half of the time he’s breathing out (so it’s getting the mask-filtered air he inhaled and then exhaled), this suggests that the mask is doing it’s job of reducing PM2.5 levels he’s breathing inside the mask to very low levels (probably about the same as our very clean indoor air).

He also held it over the small air purifier that I’ve been keeping my face over. It, too, reduces PM2.5 down to about 2 – so not as good as the bigger purifiers, but a ~2x improvement over the ~4 in the ambient air that I would otherwise be breathing.

TLDR:

  • Box fans with MERV 10 filters are great for allergens and PM10, but don’t noticeably reduce the PM2.5. Higher MERV filters might do better, but are very expensive, and probably less cost-effective than a purifier with a proper HEPA filter.
  • Small and big air purifiers work well for reducing PM2.5.
  • N95 masks are effective at drastically reducing the PM2.5 you’d be exposed to outside.
  • If you’re like me and are bothered inside when the air quality outside is bad, additional air purifiers (small or big) might help improve your quality of life during these smoky days that we are increasingly getting every year.