November 21, 2020

Furnace Filters and Air Purifiers: Powerful Tools for Reducing the Risk of COVID-19

 During the past week, I have gotten a number of emails asking about the weather on Thanksgiving Day. 

 Why?  Because a number of folks are planning on having their holiday meal outside in their garage, with the door open, to lessen the risk of COVID-19 transmission.

And it is true: a comprehensive search of the medical literature suggests little evidence of outdoor transmission.  COVID-19 spread is predominantly a problem in indoor spaces with poor ventilation, particularly when large number of people stay in such spaces for a sustained period of time.  

COVID-19 particle concentration levels are critical--lowering the concentrations can greatly reduce the risk and the severity of the disease if you get it.  Outside, there is a huge amount of dispersion of particles, so concentration are inevitably low.

So enjoying your meal in a garage with open doors greatly lessens the risk of COVID-19, no doubt about it.  And the latest forecasts suggest western Washington will have a dry Thanksgiving Day with no rain in the lowlands, light winds, and seasonal temperatures (upper 40s).  

Wear a sweater and cap, and add a heat lamp or radiant heat, and it might be ok.  Some warm cider or a hot toddy might make it better than that.

But for those folks who want to avoid moving to the garage, there is a powerful tool you can use to reduce the potential for COVID-19 transmission:  filtration of the air with top of the line furnace filters or a high quality air purifier system.

I know a bit about this approach because of the recent wildfires and their resultant smoke.  And the Centers for Disease Control (CDC) and the EPA acknowledge the value of air filtration for COVID as well.

The bottom line:  a good furnace filter or air purifier system should be able to greatly reduce the numbers of COVID-19 aerosols in the air you breath indoors (if there are any, of course).

COVID-19 Particle Sizes

The COVID-19 virus is about .1 micron in size (a micron is a millionth of a meter).  But the virus generally is enveloped in small aerosol droplet produced when we exhale or cough, ranging from .3-1 microns for the smallest to 3-10 microns for larger ones.

It turns out the high quality furnace filters can very effectively remove particles in these size ranges:  filters that are known as MERV 13 or FPR 10 (or better).

This summer, one of my UW colleagues and hiking buddy, Professor Dan Jaffe, tested the ability of a furnace filter to clean the air in a single room.   He created a very inexpensive filtration system, taping a MERV 13 furnace filter to a box fan (total cost about $40).   

There were a lot of particles in the air from the regional fires, so this was serious test!  Below are the results showing the how the concentrations of particles of .3  and 2.5 microns changed in time.  Just amazing... most the particles were taken out in 15 minutes.  It would do the same for COVID-19 aerosols.  A number of other groups have done similar tests with the same results.

Some of you might have constructed such a box fan filter during the wildfire smoke period, while others may have purchased a commercial air purifier.   If you have forced air heat in your home or apartment, make sure that it has a MERV 13/FPR 10 filter in place, and set the fan to run continuously.

This was proven to work very well for wildfire smoke (whose size is similar to small COVID-19 droplets)

Image courtesy of Dan Jaffe

If you have a fireplace in your home or apartment, use it, and open up a window.  The fireplace will suck in lots of outside air and warm you up to compensate.  More outside air will help reduce COVID-19 aerosol concentration.

This kind of air filtration will not necessarily eliminate any COVID risk, but is one approach to lessening airborne transmission. Enjoy your holiday meal.


The Wall Street Journal has put out a excellent video on ventilation and filtration for COVID-19:


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  1. Is it MERV 15 or 13, both are mentioned? Thanks.

  2. Indoor gatherings have been one of the biggest driving factors of COVID-19 spikes in Washington and nation-wide. Because of this, indoor gatherings with people outside the household will be prohibited unless they quarantine for the fourteen days (14) prior to the social gathering or quarantine for the seven (7) days prior to the gathering, and receive a negative COVID-19 test result no more than 48-hours prior to the gathering. Outdoor gatherings are limited to no more than five people.

  3. This is a good suggestion. Another is to give your guests a hot cup of elderberry tea. Then make use of the nice weather to take your guests to a stroll in order to develop vitamin D under the sun light.

  4. I'm predicting a sell out on furnace filters, now that we've finally got TP back in stock.

  5. I've been listening to "this week in virology" for a few months now, and they're saying there is no good evidence that there is much of a correlation between initial dose and severity of disease. If I understand right, when they talk about "infectious dose" its "how much is needed so that at least one viral particle actually gets in a cell". From that point on, the spread is essentially exponential. The adaptive immune system takes ballpark 3 days to respond, so unless the innate immune system keeps the initial infection in check, the initial dose isn't going to matter much. Its not even clear that the adaptive immune system really gets cranked up till there is enough antigen. It seems far more likely that the severity of disease is due largely to differing immune responses. At least that's what I got out of it. It still the case that if you can keep your exposure to less than the infectious dose, you're far more likely not to get infected. The number is an average though--its probabilistic (plus we don't know it yet--other than its pretty small, like maybe 200). The dose is also cumulative over the average "life" (viability) of the virus.

    Your point is still valid: distance, air circulation, ventilation and filtration all reduce your chance of getting an infectious dose.

    There was a thing going around suggesting that people should get a low dose so they can get mild disease and be immune. This is probably not the case. In fact, it appears you can still die from getting only a low dose. Best not to get it at all.

  6. Bob.... there is a considerable literature that suggests that dosage is important. But in any case, using air filtration is a sensible step that will reduce exposure substantially if a member of your family or living group are asymptomatic and are shedding virus. ...cliff

  7. So while the COVID 19 germs are traveling through the air they will just miraculously avoid the humans in the way of that path to the air purifiers and furnace air filters?

    1. Marian... the point is that some folks WILL be having thanksgiving inside and filtration could help them lessen the risk. Lessened concentration is lessened risk

  8. Hosting a family gathering during a pandemic and relying on DIY virus control. What could go wrong?

  9. Or, you could just wake up and realize that Covid-19 is just another garden variety coronavirus and all of the overblown PCR testing is not grounded in real science but instead creating an irrational hypochondria. Start by watching Michael Yeadon's video and go from there:

    Look at the real data. The viral event ended in late April for most of the world. It was not a pandemic, but a typical epidemic. And look at worldwide Excess Death which tells the real story. 2020 has less death overall than 2018. And this year is no different than any other recent year in terms of total death. A lot of big money is behind vaccines. But that doesn't mean there's a real event that should cause you to panic.

    And even if there is a slightly more dangerous than usual virus out and about, there's nothing you can do to ultimately hide from the virus. You will eventually be exposed. Better to just get on with our socially evolved immune system as we always have for tens of thousands of years. Go out and live and realize that everything in life has a risk. The numbers show that your greatest risk in in driving, and a bad American diet and not a virus.

    Grow up.

    1. Yes, current treatments are reducing the mortality rate so the bump in mortality is not as great as it was earlier this year, but that is a long ways from claiming this is a garden variety coronavirus (e.g. common cold) or that it is no more deadly than the flu.

      We have a lot of experience with influenza every year, so years are better and some are worse, but
      1) The infection rate for COVID19 far exceeds the infection rate for influenza.
      2) The hospitalization rate COVID19 far exceeds the infection rate for influenza.
      3) The mortality rate for COVID19 far exceeds the mortality rate for influenza.
      4) COVID19 has some long-term effects not noted for other coronaviruses.

      These are easily seen in infection rates, hospitalization rates, and mortality rates. Even the link you provided showed a huge bump in mortality rates earlier in 2020. The fact that the rates do not show a significant bump now when many states have strict protocols in place and most testing is showing less than 10% positive test results can't be a huge surprise.

      The CDC figures show that overall in the US, an estimated 299,028 excess deaths occurred from late January through October 3, 2020, with 198,081 (66%) excess deaths attributed to COVID-19. The largest percentage increases were seen among adults aged 25–44 years and among Hispanic or Latino persons. This far exceeds a typical epidemic, especially with the shutdowns or restrictions that were implemented during most of that period.

  10. Just listen to Fauci in his own words:

    And since when has T-Cell immunity not played a roll in a virus? To believe that Covid-19 is still a threat, you have to believe that suddenly, worldwide, the human immune system is behaving differently than it ever has before, and that no one is developing natural T-cell immunity to this particular coronavirus, even though it looks remarkably similar to SARS and other previous coronaviruses.

    1. why are hospitals getting overwhelmed if this is run of the mill 'garden' variety coronavirus?

      were they overrun in 2018?

    2. They are not. Check the data. What’s reported are Covid beds and not total beds. And since Covid is treatable, most recover and leave those beds. It’s not a raging little understood virus. That ended in March.

  11. Think about it: We can't have a lethal pandemic sweeping the land AND not have it show up in mortality charts. Cliff Mass is a true scientist. Cliff, I hope will soon like other real scientists pick up on the data, and then he will probably start blogging about why the All Cause Mortality has not ticked up in 2020? And then the cancelation over his failure to adhere to Global Warming orthodoxy and Seattle Extreme Left Mob orthodoxy will look like small potatoes. Go Cliff!

    1. The very link you included shows a huge spike in early 2020. That does show up in the 2020 all cause mortality charts and it's clear -- how are you ignoring 299,028 excess deaths from late January through October 3, 2020?

  12. All of this could maybe reduce the risk, but it will not eliminate it. Is it worth the risk to have any kind of indoor event with people from outside your household? Enforce strict 14-day quarantines leading up to the event if you must, but the far better option would be to not have the event all. You can catch Covid outdoors. You can catch it indoors. You can catch it anywhere your are close to breathing humans. Covid picked up at Thanksgiving could put a loved one in the hospital by Christmas, or worse. Personally, I would not rely on any ordinary residential heating system to protect me or my family from getting Covid. There are too many variables with home heating systems. Air circulation would be my number one concern. Most systems are prescriptively sized at 100 cfm per register, but most likely never achieve that rate and air circulation is generally poor due to register layout. Does every corner of your house feel warm and toasty in the winter? If not, your air circulation might not be great. Plus the indoor air is just recirculated by typical systems. For basic ventilation you need at least 7.5 cfm of outside air per person continuously. Most systems don't pull in any outside air at all. As far as filtration, older systems might not be able to pull air through a MERV-12 or 13 filter, and if they do, already poor circulation would be made worse by the drop in velocity. Bottom line, I'd recommend having your Thanksgiving party outdoors or don't have it at all, with strong lean towards don't have it at all. Good Luck!

    1. Jev... that is why the box fan approach is very good...particularly if your heating system can't handle MERV-13.. Some (many) folks are having thanksgiving dinners indoors and they might as well be as safe as possible.....cliff

  13. Used the box filter when your smoke drifted up to BC. Worked smashingly well.


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