How to be (and make others) sick less often

No one enjoys being sick, not even with a common cold -- let alone with a serious flu, stomach bug, or COVID. No one wants to make others sick either. Fortunately, we have tools to fight viruses.

Feb 13, 2025

What do COVID, flu, RSV, measles, and various viruses that cause colds have in common? They are all transmitted through the air that we breath and share. If you’ve noticed that rates of infection seem to be higher than usual right now, you are not imagining things. COVID levels remain much higher than they should be, and we’ve had recent surges of both RSV and flu. Fortunately, we are not doomed to suffer through infection after infection (or to contribute to others suffering so) because we have a list of simple tools that we can use to reduce the amount of virus in our shared air. Below is a list that I have compiled to share with friends and family and anyone else who wants to experience less illness and avoid contributing to illness in others. This has something of a Canadian focus in terms of the availability of different items mentioned, um, sorry not sorry about that. The table of contents is there so you can skip to specific sections that you may want to consult again, and there is a short summary at the end as well.

It should go without saying that none of this is about living in fear and it does not involve lockdowns or mandates. It’s about being aware and proactive if you care about public health. The way to think about these is as a series of layers, each helping to reduce the amount of virus that we are all exposed to in shared spaces.

1. Ventilation and CO2 monitors

Ventilation involves exchanging the air in an indoor space with fresh air from outside. Without adequate ventilation, more and more of the air in an enclosed space will consist of air that has been breathed out by the people sharing that space. Carbon dioxide (CO2) monitors don’t measure the amount of virus in the air, but they do give a good indication of how much exhaled air is accumulating in a space — that is, how much of other people’s potentially virus-laden breath you are breathing in. CO2 monitors come in different sizes and with various bells and whistles (add-ons such as wifi, data storing, small particle counts, etc.), but as long as they have a proper CO2 sensor, even the cheaper models are very useful.

Outdoor air has a CO2 level around 400ppm. The closer to that an indoor space gets, the better the ventilation is. In general, below 800ppm is pretty good (better yet, below 600ppm). Above 1,000ppm becomes more concerning.

Here’s a useful summary of CO2 levels: https://itsairborne.com/intro-to-monitoring-co2-20f191dd8f60

How much of other people’s breath you’re re-breathing as indicated by CO2 levels. Source: https://medium.com/its-airborne/using-co2-and-particulate-matter-to-assess-airborne-disease-risk-f4e28eebdfe2

CO2 levels as an indication of indoor air quality. Below 600 is excellent, below 800 is good, below 1000 is fair, up to 1500 is moderate, and above 1500 is poor quality.

Indoor air quality as indicated by CO2 levels. Source: https://www.iqhome.org/index.php?route=extension/d_blog_module/post&post_id=17

Here is a very useful resource with reviews of different models: https://breathesafeair.com/carbon-dioxide-monitors/

I have tested quite a few myself, from cheap to higher-end. Any of them would be a fine option in my opinion. These include (in order of increasing price):

Vitalight (~$50 CDN)
Inkbird Plus PTH-9C (~$70 CDN)
Inkbird INK-CO2W (~$100 CDN)
Inkbird IAM-T1 (~$130 CDN)
Qinping Air Monitor (~$200 CDN)
Airspot (~$200 CDN)
Temtop M2000 2nd gen (~$230 CDN)
Aranet4 Home (~$340 CDN; bonus: I designed and 3D printed a case for it)

Aside from providing a rough measure of ventilation and by extension potential infection risk, CO2 itself is not a good thing to have at high levels in the air we breathe. Too much can reduce cognitive function, cause headaches, or worse. This is another reason that having good ventilation in schools is especially important. CO2 can also build up to quite high levels in a car as well — so remember to open the window now and then to exchange for some fresh air. Using a CO2 monitor, you’ll be amazed at which spaces actually have quite good ventilation and which don’t.

If you run a small business, you can help to keep your staff and patrons safe by monitoring and displaying CO2 levels, ensuring adequate ventilation, and adding air filtration (see below). In some places, displaying CO2 levels is becoming more common (here’s an example from Japan). For an excellent example of a Canadian small business that takes indoor air quality seriously, have a look at the Apricot Tree Café in Mississauga.

2. Air filtration

If the air in a space can’t be exchanged for fresh outside air (for example, if there are no windows, or it’s too cold to open a window, or the building lacks a working ventilation system),

Air filters for home / work:

There are various options for air filters, ranging quite a bit in the size of room they can handle and in price. These are the ones I use at home:

BLUEAIR Blue Pure 511 (~$140 CDN) — one in each bedroom
BLUEAIR Blue Pure 211i Max (~$400 CDN) — one upstairs, one downstairs

Note that you don’t need to spend a lot of money to have an effective air filter. One of the best and most economical options is to make your own Corsi-Rosenthal Box, which involves putting together a box fan and four furnace filters. Note that these need to be at least MERV-13 filters. MERV-8 and MERV-11 are not high enough filtration. MERV-17 or HEPA filters give even more filtration than MERV-13, but most instructions for making CR Boxes suggest MERV-13 to balance effectiveness and cost. Having more than one may be needed depending on the size of the space.

Here are some links to instructions on making your own CR Box:

And here is a video showing how easy this is to make:

These diagrams shared by Amanda Hu on Xitter are also very helpful:

I have made two CR Boxes, which are now in use in my lab at the university. I used BOOGUAN 20X20X1 MERV 13 filters available on Amazon and box fans from Wal-Mart. And the essential ingredient of duct tape too, of course. Easy peasy.

Corsi-Rosenthal box and a CO2 meter in my lab. Ventilation in the building is actually very good.

Air filters for travelling:

Large air filters are obviously not practical to bring on trips, but there are portable options that one can use on airplanes and in hotel rooms. Airplanes generally have good air filtration systems, but these are usually not operating before take-off or after landing. Along with masking up (see below), I use these portable air filters when travelling:

Peakseries Blackcomb (~$50 CDN) — great for hotel rooms
Westinghouse 1804 (~$60 CDN) — easy to bring on airplanes

3. Masks / respirators

Respirators are like portable filters for your face. The right ones, worn properly, reduce the amount of virus you breathe in and, if you’re infected, the amount of virus that you breathe out. Not all masks are created equal, however, and in general “baggy blue” surgical masks are not sufficient. N95s, on the other hand, use electrostatic charge to trap viral particles and should be your go-to. (You may also see KN95 — N95 and KN95 are very similar in terms of standards). Here’s how they work:

The better the seal, the better the respirator will perform. In general, elastomeric respirators (basically, ones with a rubber facepiece) seal the best and earloop masks seal the least well, but a good mask will still reduce the amount of virus that you breathe in or out. There are various options including different styles and colours. For the most part, my family uses Breatheteq KN95s, which we get from Canada Strong Masks. The 3M Aura is also a very popular option.

I also have both of these elastomeric respirators, which do provide a better seal than the earloop KN95s:

What if you have a beard? Should you still mask up? Well, I do have a beard and I do mask up — keeping it shorter than 2.5mm is fine for getting a good seal with a mask, and even 10mm still gives 80% effectiveness according to this study.

In my opinion, masking should not be seen as all-or-nothing. Certainly, the more people cover up, the less virus will be spread around. However, even if you aren’t going to wear one in all situations you should at least consider the following circumstances for putting one on:

In you are in a high-risk setting. Crowded indoor spaces with poor (or unknown) ventilation are good places to use your on-face filter.
If you are symptomatic. Covering your breathing holes with an N95 is much more effective than covering your cough with your elbow. It is simply good social practice to avoid exposing others if you are sick.
If you are around vulnerable people. Going to visit someone in the hospital? Will be around older folks? Excellent time to mask up.
If someone else is wearing a mask. They probably have a good reason (perhaps they or a loved one are medically vulnerable, for example) and you can think of it as a courtesy and show of solidarity to mask up too.
If you have tested positive for an infection or know that you’ve recently been exposed.

4. Nasal sprays

Nasal sprays may provide a physical barrier and/or anti-viral properties to help reduce infection risk. There are several options, though some have to be ordered internationally. Here are some of the options, the main ingredients, how well they work according to recent research, and where to get them.

Ingredient: iota caraggeenen. Betadine Cold Defence was my go-to spray, but I believe this has been discontinued.
Ingredient: astodrimer sodium. Viraleze contains this ingredient. It isn’t available in Canada but it can be ordered from overseas.
Ingredient: hydroxypropyl methylcellulose. Vicks Early Defence contains this ingredient. It is available in Canada, but a) studies suggest this formulation is not as effective as the two listed above, and b) it smells like weird roses to me, which I don’t really want in my nose all the time.
Ingredient: nitric oxide. Enovid and SaNOtize use this ingredient, but they are not available in Canada and are very expensive to purchase internationally.

Some studies have also shown that saline nasal irrigation, such as with a Neti pot, may also be useful as another layer of protection.

5. Mouthwash

Rinsing with the right mouthwash may help to prevent infections or reduce the amount of virus present after an infection. Mouthwash containing an ingredient called cetylpyridinium chloride, abbreviated CPC, is one readily available option that research suggests can be effective against SARS-CoV-2. Some (but not all) mouthwashes with different ingredients may work too, but it’s easy enough to find CPC options as Colgate, Crest, and Scope all make mouthwashes that contain CPC and can be found on store shelves in Canada. Check the label, as some mouthwashes are based on alcohol, hydrogen peroxide, or other ingredients and don’t contain CPC.

6. Testing and isolating

Rapid antigen tests are harder to come by than they were previously, but if you have some they should still work against the newer variants of the virus, and even if they are past their expiry date. Current best practice for rapid antigen tests include swabbing the cheeks and throat as well as the nostrils. In general, a positive test means an infection but a negative test does not rule out infection — current advice is to test several days in a row. If you have been exposed recently, and especially if you test positive, it is best to stay home if you can so as not to infect others. You can still transmit even if you are not experiencing symptoms.

I have not tested at-home PCR test systems like PlusLife MiniDock, but I know others who have and they report much greater sensitivity than rapid antigen tests (see also here). It can also test for other viruses such as flu. Unfortunately, PlusLife is not sold in Canada or the USA but can be ordered from Germany. It’s not cheap, though (~$500 CDN).

7. Vaccines

There are vaccines available for several of the most notable airborne viruses that we are dealing with, including seasonal flu, RSV, measles, and COVID-19. A different strain of seasonal flu circulates each year, so it’s important to get an annual flu shot to stay up to date. RSV vaccines are now available in Canada, mostly for elderly folks. The measles vaccine is highly effective and is usually given to children, but vaccination rates have been falling and measles is making an unwelcome comeback.

The COVID-19 vaccines reduce the risk of severe acute infection and (to some extent) long COVID, but they don’t prevent infection or transmission. It’s still important to remain up to date on the most recent version, but I am listing this at the end because it’s not enough to “vax and relax”. Instead, think of vaccines like seatbelts: you’ll want that protection if things go wrong, but you should also practice defensive driving to reduce the chances of getting into a wreck in the first place. At the time of this writing, the most recent formulations of the mRNA vaccines (Pfizer and Moderna) target the KP.2 variant, whereas the non-mRNA Novavax vaccine (which isn’t available in Canada) targets JN.1. Both KP.2 and JN.1 are descendants of BA.2.86 (“Pirola”). Just about everything circulating right now (Winter 2025) is part of that lineage of variants, and those vaccines still work (in terms of reducing severe acute illness) against more recent variants. Once a new major lineage emerges, it will be time to update vaccines again. Also, note that protection from either vaccination or past infection wanes over time. Again, prevention is your best strategy and vaccination is just one important layer of protection.

8. Summary: what to have in your anti-virus toolkit

Bringing it all together, here is a quick and easy list of what you can add to your anti-virus toolkit if you want to be sick less often and avoid infecting others. It’s all about using layers of protection to reduce the amount of virus you breath in (or out).

CO2 monitor — Use a CO2 meter to assess how much used air you’re re-breathing. Open a window if possible, make sure your building’s ventilation system is working properly, and avoid crowded spaces with poor air exchange.
Air filters — Some very good options exist for home and work as well as for travel. The cheapest solution is to make your own Corsi-Rosenthal boxes.
N95/KN95 masks — Mask up, at least under the conditions listed above. Be sure to use an N95/KN95 or better, not a “baggy blue” surgical mask, and aim to have as good a seal as possible.
Nasal spray — Good to have with you, especially when you’ll be around other people who are not masking.
Mouthwash — Check the label and look for mouthwash containing cetylpyridinium chloride (CPC).
Testing and isolating — Rapid antigen tests still work for COVID, but be sure to swab cheeks and throat as well as nostrils, and test for a few days in a row. A positive is a positive, but a negative does not necessarily rule out infection. If you’re sick, avoid exposing others (stay home if you can, mask up if you can’t).
Vaccines — Stay up to date. Just don’t rely on them alone to keep yourself and others safe.

T. Ryan Gregory’s Substack

Discussion about this post