Surviving Smoke and Ash

How to use a mask to protect yourself from wildfire smoke & ash

Summary for the hurried:

  • Use an AQI app on your phone to notify you when the air gets gross.
  • When the AQI goes above 150, avoid going outside.
  • Consider filtering the air in your home.
  • When you have to go outside, wear a mask rated N95 or better (ideally P100).
  • Your mask must fit perfectly or it’s useless.
  • If it’s often smoky where you live, get a permanent mask.
Fires near Poudre Canyon, Colorado obscure the skies which glow red. Public domain image, by Senior Master Sgt. John Rohrer, 140th Wing Public Affairs

It’s apocalypse season again. Wildfires burn through forests and communities and clog the skies with smoke and ash. You might have noticed the blood-red sunsets through dusty skies. Perhaps you’ve been lucky enough to see ash raining from the heavens.

Fire-season sunsets are bright red because the air is full of tiny smoke and ash dust. This dust paints the sky beautiful colors but has a much less beautiful effect on your lungs. If you live within a few hundred miles of wildfire country, learn how to keep your air clean and safe.

Assess Air Quality

Smoke and ash dust distort light, so you can often make a decent guess at how dusty the air is by just looking through it. If the air seems hazy when you look at something half a mile away, it’s probably pretty dusty. You can make pretty precise measurements just by looking at things. But like all weather, it’s normally easier to rely on official measurements and forecasts.

Air Quality Index (AQI) is a single number from 0–500 which combines information about various pollutants. Basically: the lower the AQI, the safer it is to breathe the air outside. When it goes above 50, you should start thinking about ways to stay safer. When it rises past 150, you should avoid being outside.

Screenshot from AirVisual AQI app, showing very unhealthy conditions in San Francisco in November 2018.

You can find the local AQI by Googling “AQI”, looking at the EPA’s air quality site, or installing an app on your phone. An app on your phone lets you set alerts when the air gets bad. Then you don’t have to think about the air at all until it gets bad enough to do something about it.

Smoke and ash from fires is mostly made of dust particles smaller than 2.5 microns, called PM₂.₅. This dust is too small for your body to effectively filter and it can have all sorts of bad health effects. Some people will quickly notice symptoms like shortness of breath and irritation. But PM₂.₅ can permanently damage your lungs before you notice any of this. There’s no safe level of exposure to PM₂.₅ in the air you breathe.

Limit Exposure

The best way to protect yourself from hazardous dust in the air is not to breathe it too much. If you stay inside and close the doors, your air probably won’t get very dusty, even if it’s pretty dusty outside. Staying inside is the single best way to stay safe. Any kind of mask or filter or respirator is designed to reduce your exposure when you have to go out in less-than-healthy conditions.

Health authorities don’t often recommend that people use masks. Masks are inconvenient and make you feel like you’re really doing something to stay safe. Sadly, this leads folks to spend more time outside, because they feel safer. Don’t fall into this trap. Reduce your exposure first, then use a mask protect yourself during that reduced exposure. When the AQI goes up, go outside less, and wear a mask more of the time when you do.

Opening doors to go out lets some dust inside. But it’s not much compared to how much dust there is outside. Unless you can see or feel a draft, it’s probably not worth trying to seal your windows or doors with tape or plastic sheeting or the like. Air quality sensors can give you more information about the amount of dust in your home. But they rarely tell you anything you don’t know from just breathing the air.

An active air filter can help reduce the amount of dust in the air in your home. Look for a device which uses a HEPA filter — this means that the filter has been tested and certified to remove various pollutants effectively. It’s sort of like an N100 mask for your home. Be wary of a “HEPA-like” filter; those are not HEPA filters. The Wirecutter recommends a Coway AP-1512HH air purifier. You can also tape a furnace HEPA filter to a box fan — this is a relatively effective approach.

Mask up

Even if you reduce how often you go out, you still have to go out sometimes. You can reduce your exposure to pollutants in the air by wearing certain respirator masks. These do not keep you completely safe: they only reduce your exposure. Any time you spend outside in a mask exposes your lungs to more junk than time spent at home behind closed doors.

Your mask should meet air filtration standards and fit you extremely well. The best approach is a permanent face-piece with disposable filters.

Mask ratings

Credit: Caitlin Hillyard/California Healthline. Source: Environmental Health Investigations Branch, California Department of Public Health. Via @KaiserHealthNews on Twitter

Surgical masks, “dust” masks, and masks with only one strap do noting to protect you from PM₂.₅. There’s no benefit to wearing them. For protection against the sort of dust produced by wildfires, you need a mask which is certified to meet the NIOSH N95 rating or better.

The first letter in the rating shows what sort of particles the mask captures. Masks can either be Not oil resistant, Resistant to oil, or oil Proof. The number indicates what fraction of applicable particles the filter catches. 95 means 95%, 99 means 99%, and 100 means 99.97% of particles 0.3 microns or larger are removed. Most masks rated this way also remove many particles smaller than 0.3 microns too.

So an N95 rating means that the mask will filter out 95% of non-oily particles larger than 0.3 microns when fitted and worn correctly. That part about fit is really important. Individual fit matters so much that masks rated above 95% are only certified for use when the wearer is individually fit-tested on a regular basis. Permanent masks are normally rated to P100, which means they protect you from at least 150 times more dust than a disposable N95 mask. That’s a huge difference.

Mask fit

Your mask needs to be fitted with two loops around your head. It should be tight enough that when you take it off after use, you notice a small red depression where it met your face.

It’s much easier to fit a permanent respirator than a disposable one. If you live somewhere that sees wildfire smoke with an AQI above 150 even a few days out of the year, you should get a permanent P100 respirator. Disposable masks are an emergency stopgap while you get your permanent mask.

Pay attention to how the mask feels when you breathe in and out. When you inhale, it should pull in and stick to your face. You shouldn’t feel air moving past your nose or eyes. All the air should enter through the filter of the mask. When exhaling, the mask shouldn’t separate from your face along the line below your eyes. If your glasses fog up, you need to adjust the fit. If you don’t feel extra resistance when you breathe, you need to adjust the fit.

You can test the fit of a permanent respirator by removing the filters and covering the intakes. When you inhale, it should stick to your face. If it falls off, adjust the fit. Test your mask’s fit every time you put it on. Adjust it before going outside. If your mask does not fit perfectly it’s useless.

Other considerations

Very few masks work well when you exercise. Your skin gets flushed and sweaty which ruins the seal. You breathe more heavily which exerts more pressure than the filter is designed for. When you move around a bunch, the fit gets all messed up. If you need to walk or bike to get where you’re going, look for masks which are specifically designed for active use. But if the AQI is above 150, you’re much better off avoiding walking or biking.

No respirator works with a beard. Any respirator will fit worse if you have stubble. The only solution is shaving. Unlike many other dangerous pollutants, wildfire dust is not particularly hazardous to the small nicks and cuts made while shaving.

Even in a permanent mask, the filters don’t last forever and have to be replaced. The more durable part is the assembly which attaches to your head and makes a seal around your face. That’s what you want to keep in good working order. Get spare filters, and change them as directed. If you leave them in the packaging, filters can last for years. Once you open them, one filter probably lasts for several weeks of everyday use if you’re minimizing your time outside. Look for obvious signs of discoloration or wear & tear, and definitely replace filters after a month. But if you need to use a mask for multiple months in a row, it’s probably a good idea to look for somewhere new to live.

Disposable details

The hardest part of a disposable mask to fit is the line below your eyes where it goes over your nose. You may need to spend some time adjusting the flexible metal piece here so that it fits you well. Depending on your face and skin, you might need to make other alterations so that the nose piece stays put as your breathe. This can take a fair bit of work.

Some disposable masks and all permanent respirators have one-way valves which make it easier to exhale. These are great because they help keep the mask sealed around the edge of your face. When testing, try covering the valves if you want to check where the fit is weakest.

Disposable masks are only okay. They’re much harder to fit correctly than a permanent mask. And they wear out very quickly even if you only put them on and take them off a few times. If the AQI where you live exceeds 150 for even a few weeks out of the year, a more durable mask probably costs the same amount as the box of disposable N95 masks you’d otherwise use.

Filter Details

Wildfire smoke and ash consists mostly of PM₂.₅. Any filter rated to P100 is more than enough to keep you safe. Most respirator facepieces—including the 3M 6000 or 7000 series—use a standard bayonet fit and will work with any filter. Any 3M filter labeled 209x is rated to P100. These filters are all colored bright pink/magenta. The 2091 filter is the simplest and least-expensive of these.

3M cartridges labeled 609xx are rated to P100 and provide additional protection from vapors of various kinds. These cartridges are also colored pink/magenta. Vapor protection isn’t needed for wildfire smoke since smoke is basically just PM₂.₅ dust. 609xx cartridges make it appreciably more difficult to breathe than a 209x filter, a significant drawback. For wildfire smoke, a 3M 2091 P100 filter is the best choice.


[Ed note: I was leery for a while about including specific product recommendations in this post. But the most common questions I get are about specific recommendations, so I’ve yielded and added them. None of the recommendations in this post are sponsored. None of the links in this post are affiliate link. I get no compensation or other benefit if you use the specific things I recommend, except the knowledge that you’re staying safe, which is priceless.]