First, what is particulate matter (PM)? Particulate matter is a mixture of solids and/or liquids suspended within the air. These solids and liquids are too small for the eye to see, however, if they're in a high enough concentration it will often look like a haze in the air. The particles that make it up can be anything from pollen and dust to even molecules of water. The number at the end (ex. the 10 in PM10) is the upper limit of the particles’ diameter. A size comparison produced by the US EPA can be seen below.