Strategies to Reduce Exposure Indoors
Strategies to Reduce Exposure Indoors
The most common advisory issued during a smoke episode is to stay indoors. During wildfire events, indoor PM2.5 levels tend to be lower than outdoors, but higher compared to non-wildfire periods (Liang et al. 2021; O’Dell et al. 2023; Shrestha et al. 2019; Walker et al. 2023). Your patients can follow these steps to reduce exposure indoors:
Close windows and doors: Keep doors and windows closed to keep smoke out. Use fans and air conditioning to stay cool. If a patient does not have an air conditioner and it is too warm to stay inside with the windows closed, they should seek shelter elsewhere.
- Staying indoors is more effective in some homes than others. In general, indoor PM2.5 levels will be around 55% to 60% of outdoor levels during wildfire smoke events when doors and windows are closed and portable air cleaners are not in use. However, this can vary greatly across homes, from as low as 30% to as much as 100% (Barn et al. 2008; Xiang et al. 2021). Newer homes and homes with air conditioning tend to keep ambient air pollution out more effectively than older homes (Liang et al. 2021; Walker et al. 2023; Xiang et al. 2021).
- In any home, if doors and windows are left open, particle levels indoors and outdoors will be similar (Shrestha et al. 2019).
Do not add to indoor pollution: Do not smoke or vape; burn candles; use gas, propane or wood burning stoves or fireplaces; or use aerosol sprays. Avoid cooking, especially frying and broiling and vacuuming.
Adjust the HVAC system to keep smoky air out and filter the indoor air: If your patient’s home or room air conditioner has a fresh air intake, they should close it or set the system to recirculate mode.
Upgrading to a filter rated MERV 13 or higher in the HVAC system is recommended during smoky periods. More highly rated filters effectively remove particles in smoke from the indoor air.
To maximize the filter’s effectiveness, patients should set the HVAC fan to run continuously by switching the thermostat from “Auto” to “On.” Check the filter often and replace it when it appears dirty (dark brown or black).
- Upgrading to a high-efficiency HVAC filter may decrease PM2.5 concentrations by about 50% when running the system fan continuously.
- Even with a typical low-efficiency filter, running the system continuously may reduce particle concentrations by about 24% (Fisk and Chan 2017).
Use a portable air cleaner: Your patient should run a portable air cleaner as often as possible on the highest fan speed. Check the filter often and replace it when it appears dirty. Portable air cleaners can be used with central air systems with higher-efficiency filters to maximize the reduction of indoor particles. See the Prepare for Fire Season (pdf) (260 kb) fact sheet for more information about choosing a portable air cleaner.
- Portable air cleaner effectiveness in reducing indoor air particle concentrations depends on several factors such as size of the air cleaner, area to be cleaned, filter efficiency, frequency of use and fan speed.
- Using a portable air cleaner during wildfire smoke events can reduce PM2.5 infiltration rates considerably (Barn et al. 2008; Xiang et al. 2021). HVAC filters and portable air cleaners can both reduce indoor PM2.5. However, portable air cleaners may cost less to operate continuously (Fisk and Chan 2017).
If portable air cleaners are not available or affordable, DIY air cleaners can be used as an alternative. See EPA’s DIY Air Cleaner to Reduce Wildfire Smoke Indoors infographics for directions on how to build one.
- DIY air cleaners have been shown to be safe and effective in a laboratory setting (UL 2021; Holder et al. 2022) and can effectively reduce PM2.5 in homes and schools (Eykelbosh 2023). One study found that a DIY air cleaner in a larger room in a home reduced PM2.5 by about 56% after 90 minutes and by about 99% in a smaller room within 60 minutes during a wildfire smoke event (May et al. 2021).
- DIY air cleaner CADRs vary based on their design. Those with shrouds, thicker filters and multiple filters tend to be more effective overall (Holder et al. 2022).
Create a cleaner air room in your home: Spending as much time as possible in a cleaner air room can help reduce exposure to wildfire smoke while indoors. Choose a room with no fireplace and as few windows and doors as possible. Use a portable air cleaner in the room and avoid activities that add to indoor particle levels. For more information see EPA's page Create a Clean Room to Protect Indoor Air Quality During a Wildfire.
Air out the home when air quality improves: Open windows or the fresh air intake on the HVAC system to air out the home when smoke clears, even temporarily.
Using air quality sensors for smoke: Some patients may have access to air sensors (also called portable, low- or lower-cost air quality sensors). Air sensors provide localized air quality readings that can help people make decisions about personal activities during wildfire smoke events. The fact sheet, Using Air Quality Sensors for Smoke, provides more information about choosing and placing an air sensor and about using and understanding sensor data.
To help control indoor temperatures in spaces without air conditioning:
- During the day, before it gets hot outside, close windows and window coverings such as shades, blinds and awnings.
- At night when it is cooler, open the windows and use fans to exhaust hot air from rooms or draw in cooler air.
- Once the air indoors is cool, close doors and windows. Use a portable air cleaner or a DIY portable air cleaner (i.e., box fan with a high-efficiency filter) to filter the indoor air.
- For extended heat events, where the temperature stays elevated at night, or when humidity is high, consider leaving the area or purchasing an affordable air conditioning unit.
For more information about indoor air quality, see the Wildfire Guide fact sheet Indoor Air Filtration (pdf) (129kb) and EPA's web page on Wildfires and Indoor Air Quality.
Cleaner Air Shelters and Spaces
Inform your patients about public spaces where they can find relief from wildfire smoke and heat.
- Cleaner air shelters are public spaces that are set up by sheltering organizations or agencies for people who are displaced by wildfire or smoke. They may be located in places like malls, libraries or schools. People who take refuge in these shelters may need to stay one night or for extended periods. During a heat event, people should look for cleaner air shelters that also offer cooling.
- For short periods (hours) of respite from smoke and heat, people can seek out cleaner air spaces in their community. Examples could include libraries, museums, shopping malls, theaters, indoor sports arenas, senior centers or any indoor area with effective particle filtration and air conditioning. Finding temporary relief from smoke (as well as heat or cold) for several hours is a good option for people who cannot keep the air in their home cleaner.
More information about the indoor air quality requirements for cleaner air shelters and spaces can be found in the Wildfire Guide. State, local or Tribal governments may provide additional information about the locations and availability of cleaner air shelters and spaces.
Evacuation
The most common reason for evacuation during a wildfire is the direct threat of engulfment by the fire, not exposure to smoke. However, leaving an area of thick smoke is generally a good protective measure for members of at-risk groups.
Evacuation is not an attractive option to many people. It can cause stress, and most people do not want to leave their homes. It is often difficult to predict the duration, intensity, and direction of smoke, making the need to leave uncertain. Smoke can reduce visibility on the roads making driving hazardous. If smoky conditions are expected to continue for weeks, it may not be feasible for a large percentage of the affected population to evacuate. Therefore, it’s important for your patients to know how to reduce exposures when outdoors, what steps they can take to keep air cleaner in their homes, and where they can spend time in cleaner air spaces during the day.