Utah State University Extension
Particulate matter two-point-five (PM2.5) is a tiny suspended particle of air pollution that can penetrate deeply into our lungs and potentially enter the bloodstream. The 2.5 unit means that the particles are smaller than 2.5 micrometers (a micrometer is one millionth of a meter). In Cache Valley and along the Wasatch Front, these tiny particles form and accumulate to unhealthy levels when air is trapped in the bottom of the valley during cold winter days.
Martin, Randy; Coulombe, Roger; and Brain, Roslynn, "Utah Air Quality: PM 2.5" (2016). All Current Publications. Paper 784.