Damage from the Marshall Fire in Boulder County on Dec. 30. Credit: Anthony Albidrez.

Note: This story was updated on Jan. 16 at 4:26 p.m. with information regarding recommended air cleaners, an update on post-fire analysis efforts by CU Boulder scientists and NOAA, and an indoor air quality FAQ.

The aftermath from the Marshall Fire is coming into sharp relief across Boulder County. Indoor air quality for thousands of homes and businesses in and around the burn area, where people are starting to return, is among the myriad concerns right now.

That’s why environmental experts from CU Boulder and its Cooperative Institute for Research in Environmental Sciences (CIRES) have compiled a handy resource to address post-wildfire smoke impacts in your home. (Note: If you’re going back to a smoke-damaged home, Boulder County officials have also recommended getting the air quality assessed.) 

In addition to sharing mitigation strategies, scientists from CIRES, CU Boulder and NOAA have been installing devices in standing houses “to understand the effects of smoke on indoor air quality.” They’re also sending out a mobile laboratory to measure air quality outdoors.

Particles and gasses stirred up by the fire are the main culprits when it comes to indoor air quality, according to the scientists: 

“Smaller sizes of particulate matter (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.”

Post-fire snowfall should have helped clear out airborne particulate matter, or trap the particles in your home’s air filters. With that in mind, be sure not to ingest any snow, because that’s where these airborne particles have likely been deposited.

The Boulder Reporting Lab is breaking down the salient points from the resource to help you mitigate harmful effects indoors. Here’s what you need to know to keep the air in your home safe. 

What’s that smell?

Some VOCs will bind to airborne particles, which can evade air filters. The resource’s authors say active fire plumes contain “a whole lot” of these compounds, which can linger in the air after the flames die down.

So if you’re noticing a post-fire smell in your home, this may be the reason. And that’s not good, because some of these compounds are toxic — dioxins, benzene and formaldehyde, to name a few.

Here are a few key points to remember: 

  • VOCs can stick to the walls and other surfaces of your home. These compounds will probably continue to release harmful, smelly gases for weeks or even months. 
  • This “off-gassing” is affected by the temperature. More VOCs will enter the air as it gets warmer. 
  • This should reduce with time, because materials can only absorb so much from these compounds. 

Clearing the air 

You know about how VOCs can make their way into your home. Now what? 

Installing an air cleaner — which you might know as an air “purifier” — can help. Many of these devices remove VOCs (and the resulting smell) from the air. 

This blog post from environmental engineer Shelly L. Miller features a lot of good information, including an air cleaner sizing tool for schools. (Confused? Here’s a video.) 

And here’s more from the resource about how to clean the air inside your home:

  • Air it out. This might sound obvious, but open windows and doors can increase ventilation, which improves air quality. 
  • An important note: Don’t take this step if the area outside your home smells smokier than it does inside. 
  • Change the filter on your furnace or HVAC every month until the smell goes away. Resource authors suggest using a MERV 11- or 12-rated filter. Examples include 3M Filtrete 1000 and 1500, which you can find in most hardware stores.
  • Next, you’ll want to cover the edges with one inch of foam, or tape over gaps in the filter holder. Then turn up the heat in your house and let in some fresh air.
  • Lots of HVAC ducts aren’t sealed well, or they’re poorly maintained and filtered — which leads to contamination. So you’ll want to be sure and give them a good clean.
  • Give the surfaces of your home a good, deep clean. Vacuum floors, fabrics and furniture.
  • Replace your smoke and carbon monoxide detectors as soon as possible, as their sensitivity may have dulled from heavy smoke exposure. 

More on air cleaners 

You might want to buy portable air cleaners with a HEPA filter and activated carbon filter. Air cleaners with an activated carbon filter will help remove VOCs from the air. Resource authors say cleaners with a HEPA filter will “remove 99.97% of particles and VOCs that are bound to particles.”

Let the air cleaners run until you don’t notice the smell anymore, and change filters in accordance with manufacturer instructions. (Carbon filters tend to get used up quickly, so you’ll want to change those more frequently.)

Here are a few more things to consider when it comes to air cleaners:

  • Get one that’s the right size for the room, and use the highest speed when people are in the room. 
  • The CU experts encourage people to avoid models “with technologies that generate unhealthy levels of ozone.” They say these might falsely claim to disinfect and remove odors, but use ultraviolet radiation, ozonizers or ionizers to remove VOCs.
  • Here’s more on ozone emissions and consumer products from the California Air Resources Board. 
  • On a budget? Make your own air filter. The Corsi-Rosenthal box fan filter, developed in response to the Covid-19 pandemic, can help with particles, but it won’t be effective against VOCs.
  • However, odor relief carbon air filters could help in this context. But resource authors say the carbon layer is thin, and the filters have not been tested for their effectiveness at removing odors. 
  • CU experts point to the Consumer Reports-recommend Blueair Blue Pure 211+ ($300) for a large room (approximately 20′ x 25′ with a ceiling of average height) and the Blueair Blue Pure 411+ ($140) for a smaller one (10′ x 15′). Both models include HEPA and activated carbon filters, and dust pre-filters.
  • More brands with models using these filters include Amway, Austin Air, Coway, Daikin, Honeywell, IQAir, LG, Panasonic, Philips, Samsung and Sharp.

Dealing with fire debris 

Of course, cleaning the air is just one part of the process in making your home environment safe again. Here are a few tips from the CU experts  and also Boulder County Public Health about dealing with any fire debris.

  • The most important thing to remember when it comes to sorting through ash and debris? Don’t do it! 
  • From a recent BCPH press release: “Do not attempt to remove debris or clean-up properties that have been damaged or destroyed … under no circumstance should residents disturb ash regardless of what personal protective equipment (PPE) they may have.” 
  • For more on protecting yourself from ash, check out this EPA factsheet. 
  • For more on general clean-up after a fire, check out these resources from ServiceMaster Restore and Boulder County.

Further reading 

CU researchers who compiled the original resource documents: 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)

Archived work by Jezy Grazy for Boulder Reporting Lab.