Update on MERV-13 and masks for wildfire smoke

First, my heart goes out to so many people who have lost their homes or loved ones during this crazy wildfire season. It’s so hard to predict when a fire will blow and get out of control. For this reason, anyone who lives in the West really needs to be prepared for this type of disaster at any time. And climate change is clearly making these disasters worse.   

The air quality is horrible across the western U.S. I’ve been getting swamped with calls about how to protect yourself from bad air. To be clear, smoke particles are very fine, mostly smaller than 1 micrometer (um). So, it’s these very fine particles that get deep into our lungs that we have to protect against. I have four suggestions:

  1. If you have to be outside, wear a mask. The masks we are using for COVID help a little bit, maybe 20-30% depending on your mask. If you have a high-quality mask and it fits tightly onto your face, you could reduce your particle intake by up to 50%. 
  2. Stay indoors as much as possible. Our data show that, depending on your home, the air inside contains 10-80% less particulate matter than the air outside. That’s a big range, depends on your home and how tight it is.
  3. Run air conditioning if you have it.
  4. Run an air purifier or make your own box fan/filter combinations with MERV-13 filters, as shown in my previous blog. 

I have been using the box fan/MERV-13 filter combinations for the last week and the results are really good. I have tested these not only with my low-cost particulate sensor but also with a particle counter that counts the particles by size. So, I am very sure that the MERV-13 removes the fine particles in smoke. 

Here is data from this morning from my home office/guest bedroom. I shut off the fan/filter overnight and it fills up with bad air. This morning I came down and turned on the fan/filter and within about 30-45 minutes it cleans up really well. Again, this is with a $20 box fan and one MERV-13 filter.  The room is about 15 feet x 15 feet. This gives you an idea as to what you can do to improve your own home air quality. 

Good luck, keep cool, and smoke free!

Graph showing downward trend of 2.5ug/m3 and 0.5um particles over a few hours to almost negligible amounts.
PM2.5 and 0.5 um particle counts in my home office this morning. I turned on the box fan at about 8:40 am.
Graph showing downward trend of 2.5ug/m3 and 0.3um particles over a few hours to almost negligible amounts.
PM2.5 and 0.3 um particle counts in my home office this morning. I turned on the box fan at about 8:40 am.
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Thanks for this. I was proactive and found a neighbor that had an extra box fan and a merv 11 filter. I’m running it now Unfortunately there is nothing available in the stores. We need to make these available to everyone in these smoke storms. I wish Washington state Emergency management could distribute these.


The instructions for the DIY air cleaner mention kits… do those exist? Are they available?

Thomas Talhelm

Great tests! I’ve been testing DIY air purifiers for years, so it’s great to see more tests. One quick nitpick: high-quality masks that fit well provide far more than 50% protection. Fit tests (including some on my face!) routinely find fit effectiveness of over 99%: https://smartairfilters.com/en/blog/do-pollution-masks-work/


Can you any fan to make this? I.e. a Vornado air circulator fan?

Sarah V

I made a pathetic one with a circular fan, and cut the filter into three parts to fit around it. Worked like a charm but looked terrible!


I salute your ingenuity Sara, and I did something similar that worked so well I’m planning for it again this year. I cut a hole in a large cardboard box to match my circular fan, put the fan inside and put whatever wonky sized furnace fan I could find still in the stores across the top of the box. This year I’ve already bought 20x20x1 filters and a box fan, but will be re-creating the frankenfilter-box so that my circular fan can do double duty as an air-cleaner. Thanks Dan for the original post with actual data on performance..