Particulate matter, or PM, is a mix of particles and droplets in the air. PM varies in shape and size, but those of 10 micrometers in density or smaller can adversely affect your health because they can be inhaled. PM 2.5 refers to fine particulate matter – with a density of two-and-one-half microns or less.

Sufficient exposure to PM can irritate the eyes, nose, throat, and lungs, leading to allergy-like symptoms and shortness of breath in otherwise healthy people. It can also exacerbate existing medical problems, such as asthma and heart disease. PM 2.5 is considered the world’s single biggest environmental health risk.

Indoor PM levels can be influenced by outdoor sources like vehicle exhaust, wildfires, and power plant emissions. But many indoor activities produce PM as well: cooking, burning fireplaces, and smoking are just a few common sources.