When wildfire smoke fills the air, public health officials advise going indoors. However, the tiny, toxic particulate matter in wildfire smoke, known as PM2.5, can enter our homes through cracks, doorways, and heating and cooling systems. Consequently, public health officials increasingly emphasize that merely staying indoors when it’s smoky out is not enough; proactive measures are necessary to ensure clean indoor air. Recirculate the airIn general, the next step for ensuring healthy indoor air is ventilation, or bringing fresh, outside air in, said Steven Emmerich, a mechanical engineer in the indoor air quality and ventilation group at the National Institute of Standards and Technology. Note that during wildfire smoke events, these supplies can sell out, so preparing early is important.