Particulate matter of mass median aerodynamic diameter (MMAD) less than or equal to 2.5 micrometers. 

Particulate Matter less than 2.5 micro gram in diameter (fine particles)

Small inhalable particulates under 2.5 micrometres in diameter. They can penetrate lungs more deeply than larger particulates, affecting sensitive groups like children and people who have respiratory difficulties.

Particles in the air less than 2.5 micrometers in diameter. Often referred to as "fine" particles.  Also see PM10.

Includes tiny particles with an aerodynamic diameter less than or equal to a nominal 2.5 microns. This fraction of particulate matter penetrates most deeply into the lungs. For more information, see o [..]

Particulate matter with an aerodynamic diameter of 2.5 microns and less. PM2.5 is a component of PM10. The smaller PM2.5 particles penetrate to even deeper portions of the lung, reaching individual al [..]

A US EPA standard that includes additional controls on particulates sized 2.5 micron and smaller.

tiny solid or liquid particules, generally soot and aerosols. The size of the particles (2.5 microns or smaller, about 0.0001 inches or less) allows them to easily enter the air sacs deep in the lungs [..]

- Particulate Matter with aerodynamic diameter 2.5 microns or less

Mass of particles with an aerodynamic diameter less than 2.5 microns

Airborne particulate matter with an aerodynamic diameter of less than 2.5 micrometres (i.e. <400th of a millimetre). In recent years attention has been turning towards monitoring the PM2.5 particle fraction, which is smaller than the more commonly quoted PM10 fraction. It is believed that these smaller particles can penetrate deeper into the lun [..]

P

Particulate matter that is 2.5 micrometers in diameter—one-thirtieth the diameter of a human hair—and smaller. Fine particle pollution can be emitted directly or formed in the atmosphere.