Hazards of Smoke and Tips for Cleaning After Fires
In response to the January 2025 fires in the greater Los Angeles region, Paul Wennberg, Caltech’s R. Stanton Avery Professor of Atmospheric Chemistry and Environmental Science and Engineering, summarizes recent research on air quality and wildfires, and provides guidance on how to reduce pollution in our homes, apartments, and offices.
Wennberg thanks his colleague Joost de Gouw, professor of chemistry at the University of Colorado Boulder, who did extensive sampling of homes following the 2021 Marshall fire in Colorado, for help in formulating this guidance.
The situation:
Most of the material burned in the Eaton fire was structural, not living biomass. Given the age of these buildings, there is a lot of lead from previous painting. Burning of cars also released lead and other metals. Some of the older homes contained asbestos. Consistent with this, preliminary data from particulate samplers located in Pico Rivera, south of Caltech's campus and Pasadena show significant amounts of both chlorine (from burning plastics like PVC) and lead in the air. (These samplers do not measure asbestos.)
For those who want to dive deeper, there is significant existing literature about the composition of both gases and particles from these types of fires. Here are two recent studies following the Marshall fire in Colorado and the Camp fire in California—both of which started with burning dry trees and grasses but then mostly burned structural materials like the Eaton fire.
- "Residual impacts of a wildland urban interface fire on urban particulate matter and dust: a study from the Marshall Fire"
- "Camp Fire Air Quality Data Analysis"
In addition to particulate, smoke contains a huge diversity of volatile organic compounds (VOCs). These are the source of much of what we are smelling in the air now. Again, for those who want to dive deeper into the research, here is a recent inventory of these chemicals, mostly from burning trees: "Parameterizations of US wildfire and prescribed fire emission ratios and emission factors based on FIREX-AQ aircraft measurements." The diversity of these chemicals will be even higher for structural materials burned in the Eaton fire.
During the fires, the winds carried both the particulate and gases far downwind (all the way out to the Pacific Ocean). The smoke entered homes through openings around windows and doors. This transport downwind was tracked with monitors that recorded the amount of particulate in the air. During the fire, the levels of particulate even miles away from the fire were 50–100 times larger than usual. Since the generation of local smoke from nearby active fires has basically ended, the amount of particulate (and the amount of lead in these particles) is back to levels similar to those before the fire.
I track the air quality near me in two ways: using the EPA website AirNow and viewing local data collected at Caltech by instruments we have installed on Caltech Hall at breathe.caltech.edu. When the air quality is good, I do not wear a mask outdoors and keep the windows in my home open to help remove the smoke (as long as the smell is worse inside than outside). Many homes that are within or adjacent to the fire, however, have very large amounts of contamination both inside and outside. Especially when it's windy, the regional air quality as indicated on the EPA site may not reflect how much particulate is in the air around these homes as dust and ash is stirred. So, I would recommend always wearing a high-quality well-fitted mask when in the fire zone.
Indoor air quality can be improved with air cleaners. Filters in forced-air HVAC systems should be inspected and changed. Renters should ask their property owners to do this. Stand-alone air cleaners can also remove particulate. Some of these cleaners also include an activated carbon filter that will remove many VOCs, but these filters have a relatively small capacity and so need to be changed frequently.
VIDEO: Learn to build a homemade air filter
This design can be enhanced for VOC filtering by adding activated carbon filters in addition to the MERV 13 particle filters.
Suggestions for cleaning homes downwind of the fires:
Note: If a home is very impacted with dust and ash, it may be best to hire professionals who use industrial-grade HEPA vacuum cleaners.
- Wear appropriate personal protective equipment (PPE). Cleaning often stirs up dust, so wear a high-quality well-fitted mask. If possible, clean with the windows open. But, if it smells worse outside than inside or if it is very windy and ash is being stirred up by open windows, close up the house. Make sure to wash your clothes after cleaning.
- Do not sweep. Hard horizontal surfaces should be wet mopped and wiped down to minimize resuspending the dust into the air. You will find some resources suggesting using TSP, an all-purpose cleaner, but diluted soapy water should work fine (and is less toxic).
- Upholstery and rugs should be cleaned with a HEPA-filtered vacuum. Do not use vacuum accessories such as mechanical brushes or beaters until you have vacuumed these surfaces several times. If possible, do this cleaning with the windows open or, even better, bring the rugs and furniture outdoors to clean.
- VOCs are very difficult to remove from homes and take a long time before they are cleared. Our homes, apartments, and offices may continue to smell of smoke for a month or more. Homes have lots of porous surfaces (and carpets/rugs), and semi-volatile compounds end up accumulating in these materials. It has been observed, for example, that using activated carbon filters in the HVAC system or in stand-alone air filters does remove these compounds from the air, but as soon as the fans are turned off, the concentrations of compounds return to nearly the same value.
Given this, the best approach to clear the VOCs is to keep the windows open once outdoor air is safe and thereby move as much air through our spaces as possible to repartition these chemicals first to the indoor air and then vent them outdoors. Data from the Marshall fire showed that the often-expensive professional cleaning services generally are not effective for VOCs because companies simply wash or vacuum surfaces, and most VOCs are embedded deeply within materials. There are several gadgets that claim to provide extra cleaning, e.g. hydroxyl generators, but these can actually make things worse. Some professional cleaning services spray terpenes (such as limonene), but these simply mask the smoke smell rather than remove the cause. - Given the ash and dust on the landscape, it is important to avoid tracking more contamination indoors. Thus, don't wear street shoes inside, wipe off dog paws, wash down hardscape (patios, driveways, etc.), and do not use leaf blowers. If you have a lawn service, make sure they do not blow.
Additional information gleaned by my colleagues in Colorado after the Marshall fire has been nicely assembled here: How to mitigate post-fire smoke impacts in your home.