Aimed to offer guidance in reducing the health impacts of air pollution, the World Health Organization made its first air quality guidelines (AQGs) in 1987. These have since been updated, with the latest edition of WHO AQGs for ambient air pollutants published in 2006. The WHO's guidelines are widely referred to as the global authoritative guide for air pollution hazard and precautions.
The WHO air quality guidelines for particulate matter is as follows:
Air pollutant WHO Exposure Guideline
PM2.5 5 µg/m³ annual mean
25 µg/m³ 24-hour mean
PM10 20 µg/m³ annual mean
50 µg/m³ 24-hour mean
The annual mean exposure guideline of 5 µg/m³ guideline is broadly referred to as the authoritative global guideline for PM2.5 exposure to be limited to, in order to minimise the risk of negative health effects.
By comparison, the US EPA’s Air Quality Index represents PM2.5 levels below 12 µg/m³ as within the "Good" category, which is very close to WHO standards. Thus, the two different standards are relatively comparable.
See the US air quality index for PM2.5 concentrations below: