Also known as PM, particulate matter is a mixture of solid and/or liquid inhalable airborne particles that range from 2.5 microns to 10 microns in diameter. Once inhaled, PM can affect the lungs, heart, and cause other health issues.

While, in general, ventilation is a helpful way to exhaust polluted indoor air and replace it with fresher outdoor air, there are times when it's a bad idea; notably when outdoor air is dirtier than indoor air. The American Lung [...]