Ultrafine particles

Ultrafine particles (UFPs) are particulate matter of nanoscale size (less than 0.1 μm or 100 nm in diameter). Regulations do not exist for this size class of ambient air pollution particles, which are far smaller than the regulated PM₁₀ and PM_2.5 particle classes and are believed to have several more aggressive health implications than those classes of larger particulates per unit mass. Although they remain largely unregulated, the World Health Organization has published good practice statements regarding measuring UFPs.