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 2021.
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
15 µg/m³ 24-hour mean
PM10 15 µg/m³ annual mean
45 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 minimize 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:

For more information on how this information is gathered click the link below: