fine respirable suspended particulate matter ultrafine respirable suspended particulate matter A portion of fine respirable suspended particulate matter is a form of particulate matter composed primarily of solid particles each with a diameter of 100 nanometers or less. Regulations do not exist for this size class of ambient air pollution particles, which are far smaller than the regulated PM10 and PM2.5 particle classes and are believed to have several more aggressive health implications than those classes of larger particulates.