What is PM?
Particulate matter is a mixture of solid and liquid particles in the air. The smaller-sized particles - those 2.5 micrometers or less in diameter (PM2.5) - are of greatest health concern because they can pass through the nose and throat and lodge deep in the lungs. PM2.5 are sometimes called "fine" particles. For comparison, they are less than 1/25 the diameter of a human hair.