Suspended particulate matter (PM) refers to a range of solid and liquid particles that are small enough and light enough to remain suspended in the air. Particulate matter can come from natural sources (pollen, spores) as well as from human activities (soot and smoke from fuel combustion, dust from construction projects). Scientists and environmental regulators classify particulate matter into two categories based on their size. Fine particulate matter, or PM-10, are particles with a diameter of 2.5 to 10 micrometers. (For comparison, a typical human hair is roughly 70 micrometers in diameter.) Ultrafine particulate matter, or PM-2.5, are particles smaller than 2.5 micrometers in diameter. Public health experts are particularly concerned about PM 2.5 pollution because smaller particles are more likely to enter deep into the lungs, where they can damage tissue and impair lung, heart, and brain function.