Improving indoor air quality during wildfires

In California and other U.S. western states, wildfires have become more frequent and intense, adversely impacting air quality and human health. Smoke from wildfires contains many toxins and irritants, including particles smaller than 2.5 microns in diameter, which, due to their size, penetrate deep into the lungs and contribute to cardiopulmonary and respiratory illness. Many health agencies and departments suggest people stay indoors during wildfires to limit their exposure to these particles. But how healthy is the air indoors during a wildfire, and can it be improved?