Particulate matter, also known as particle pollution or PM, is a mixture of extremely small particles. The size of particles is directly linked to their potential for causing health problems. The smaller these particles are, the more hazardous they are to your health. Particulate matter can be grouped into two categories:
Inhalable particles, with an aerodynamic diameter of up to 10 μm1 (all smaller particles are included). Particles of this size
cause health problems, because these are the particles that generally pass through the throat and enter the lungs2.
Fine particles with an aerodynamic diameter smaller than 2.5 μm. Particles of this size or smaller can have effects to the respiratory and cardiovascular system.
1 μm symbol represents a micrometre; one thousandth of a millimeter
2 World Health Organization Air quality guidelines for particulate matter, global update 2005