Particulate matter with an aerodynamic diameter of less than 0.1 micrometres (µm) is referred to as PM0.1.
PM can be emitted directly from a source (primary PM) or formed through the interaction of other pollutants (secondary PM). The National Atmospheric Emissions Inventory estimates emissions of primary PM only.
The health effects of these particle sizes are not well-understood, but ultra-fine particles in the PM0.1 range are believed to contribute to the overall toxicity of airborne particulate matter.