Even after a wildfire has been put out, particulate matter in the air can pose a risk to your health—even when you’re indoors. In the video above, Joe Allen, associate professor of exposure assessment science at Harvard T.H. Chan School of Public Health, describes four methods for reducing your health risk when returning to a burn zone after a fire.

1. Control indoor air by increasing the level of filtration. For central air systems, Allen recommends a filter with a MERV rating of 13 or higher. A portable air purifier with a HEPA filter is also effective.
2. Control indoor dust. Avoid tracking in soot and ash from outdoors. Clean surfaces with a damp cloth and use a vacuum with a HEPA filter.
3. Measure air quality. Low-cost air quality sensors can provide information on how many particles are in the air.
4. For people returning to homes close to a burn area, take additional precautions. Wear a well-fitting mask or respirator. Allen recommends a P100 mask with a multi-gas cartridge.