How to mitigate post-fire smoke impacts in your home

In the aftermath of the destructive Marshall Fire, CU Boulder, CIRES and CDPHE experts have compiled a resource of post-wildfire indoor air quality facts and solutions to mitigate smoke impacts in your home or business.
(This information was updated January 13, 2022)

Click here to read Frequently Asked Questions.

Lee esta guía en Español.

Background

Health risks in fire-damaged areas include particles, asbestos, lead, and chemical residues.
Fires emit a lot of particles (also known as Particulate Matter, PM) and gases. Smaller sizes of PM penetrate deep into the lungs and are harmful to human health. Some of the gases emitted and created in fire plumes are Volatile Organic Compounds (VOCs). These compounds create the smell of a fire plume (and linger post-fire). Many of the gases emitted by active fires are also harmful to human health.
With the latest snowfall, the airborne PM should have all been deposited out or trapped in your air filters. Because the airborne particles are likely deposited in the snow – don’t eat it!

Post-fire smell – VOCs

The post-fire smell is from VOCs (Volatile Organic Compounds). Some (but not all) are bound to particles and can be trapped by a particle filter.
Those VOCs can deposit on walls and other surfaces, and will likely keep off-gassing for a long time (weeks to months). The off-gassing will be temperature-dependent: More VOCs will be emitted (off-gas) when it’s warmer, less when it’s cooler.
The release of the VOCs in your home after a fire should slowly reduce with time (since there is only a limited mass of VOCs that are adsorbed by materials). So, it will get better!
There are a whole lot of VOCs both in active fire plumes, and that remain after. Some of these compounds are toxic (e.g., dioxins, benzene, formaldehyde, PAHs).

What to do to clean the indoor air in your home

Protect your health.
- To protect your lungs, wear a well-fitted NIOSH-certified mask or respirator (such as an N95 mask or more protective respirator) when you clean up inside your home.
- Wear gloves, safety goggles, long-sleeved shirts, long pants, and shoes with socks to avoid contact with ash or debris. If you get ash on your skin, in your eyes, or in your mouth, wash it off as soon as you can.
- People with heart or lung disease (including asthma), older adults, and pregnant women should avoid cleanup activities as much as possible.
- Keep children and pets away from ash and cleanup activities.
- Boulder County has more information on how to safely clean up after a fire (also en español).
Bring in fresh air.
- Air out your indoor spaces as much as possible by opening windows and doors. If you have minimal smoke smell/damage inside your home – and the area outside smells worse – don’t take this step.
Check your HVAC or furnace.
- Change your furnace or HVAC filter monthly until the smell goes away. Use MERV 11 or 12 rated filters (for example, 3M Filtrete 1000 and 1500 filters available for $10-$15 in hardware stores, Target, on-line, etc.) and add 1” foam around the edges or tape over any gaps in the filter holder. Induce warm temperatures and get fresh air into the house.
- Clean your HVAC ducts. The evidence is limited on how this impacts general indoor air quality, but many ducts are poorly sealed, poorly maintained, and have poor filtration so can get contaminated.
Air cleaners can help remove particles and odors.
- Consider purchasing portable air cleaners with both a HEPA filter and activated carbon filter. The carbon filter is important as it will remove the VOCs from the air. The HEPA filter will remove 99.97% of particles and VOCs that are bound to particles. Keep air cleaners on until the smells go away, and change the filters according to the manufacturer’s instructions. See below for information and resources about air cleaners.
- You may need to change the carbon filters more frequently since they do get used up.
Deep clean the surfaces of your home.
- Vacuum floors/carpet/rugs, drapes, furniture.
- Following Boulder County recommendations, do not use harsh chemical cleaners or vinegar as they can react with chemicals in the ash. Soap and water are adequate to clean ash from hard surfaces.
- Replace smoke and carbon monoxide detectors as soon as possible.
- Detectors subjected to heavy smoke can lose sensitivity and put your family at undue risk from fire or carbon monoxide.
You may want to change out the cabin air filter in your car – if it also smells and was exposed to smoke.
More information about cleaning your home:
- For more advice about cleaning up the ash in your home, see: https://www.airnow.gov/sites/default/files/2021-07/protect-yourself-from-ash-factsheet.pdf
- For more advice about the overall cleaning up your home, see: https://www.servicemaster-dak.com/blog/remove-smoke-smell-after-fire

More about Air Cleaners

Adding an air cleaner (sometimes referred to as an air purifier) with activated carbon will help to remove the VOCs (that create the smell) from the air. Make sure to size the air cleaner so that it will work efficiently in the room you want to clean (see Shelly Miller’s blog post, which includes a calculator so you can determine what size air cleaner you'll need for your space).

Air cleaners with HEPA (High-Efficiency Particulate Air) filters remove airborne particles (fire smoke, other pollutants, cigarette smoke, coronaviruses, pollen, pet dander, etc.), and models with activated carbon filters will also remove VOCs and most odors. Consumer Reports (subscription required) and Wirecutter (from The New York Times) rate portable air purifiers, but generally just for their particle removal efficiency, noise level, cost, and features. Both Consumer Reports and Wirecutter recommend the Blueair Blue Pure 211+ ($300) for a large room (say 20' x 25' with a normal height ceiling), and the Blueair Blue Pure 411+ ($140) is appropriate for a small one (10' x 15'). These models include both HEPA and activated carbon filters, and dust pre-filters. Amway, Austin Air, Coway, Daikin, Honeywell, IQAir, LG, Panasonic, Philips, Samsung, and Sharp are also good brands with both types of filters. Most manufacturers do not carry models with standalone activated carbon filters, but HEPA filters address many other pollutants of health concern. Carbon pre-filters are also available for furnaces and HVAC systems; although less effective, they can help purify the air and are a low-cost option.

It’s important to select an air purifier that is appropriately sized for the room and use the highest speed when someone occupies the room.
It's very important to avoid models with technologies that generate unhealthy levels of ozone. These models, which sometimes falsely advertise odor removal and disinfection. include air cleaners that use ozonizers, ultraviolet radiation, or ionizers to remove VOCs.
The California Air Resources Board has a certification program (without this certification, it is illegal to sell an air cleaner in California) and information on ozone generators to avoid.
If you are on a budget, you can make an inexpensive air filter (aka, Corsi-Rosenthal box fan filter). This air filter was developed in response to COVID and is great for particles, but will not address gases/VOCs, which is what we smell.
- Update: There are Odor Relief Carbon Air Filters that could be used in this set-up that would remove particles and odors, but they have only a thin layer of carbon and have not been tested for their odor removal efficiency.
Here is a great blog post from Dr. Shelly Miller about air filters/purifiers for the home.

Other online references:

Authors: Christine Wiedinmyer (CIRES/CU Boulder Mechanical Engineering), Joost De Gouw (CIRES/CU Boulder Chemistry), Bart Croes (CIRES Visiting Sabbatical Fellow), Mike Hannigan (CU Boulder Mechanical Engineering), Shelly Miller (CU Boulder Mechanical Engineering), Colleen Reid (CU Boulder Geography), Leah Wasser (CIRES, Earth Lab), Kristy Richardson (CDPHE State Toxicologist), Tara Webster (CDPHE), Shannon Barbare (CDPHE)

NOTE: This information has been compiled as a resource for our community. The authors of this site are scientists/engineers/professors who have not been specifically contracted to perform this service, and are not experts on fire remediation. Readers should assess their own risk and common sense to determine actions that they want to take.

Do you have other resources to add here? Please email Christine.Wiedinmyer@colorado.edu

How to mitigate post-fire smoke impacts in your home

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