The European Union’s (EU’s) Air Quality Directive, the Directive on Ambient Air Quality and Cleaner Air for Europe (2008/50/EC) , is one of the key legislative instruments in place to address air pollution under the European Commission’s Thematic Strategy on Air Pollution. It was the first EU directive to include limits on ambient concentrations of PM2.5. In the Directive a new approach for PM2.5 was introduced in recognition of evidence suggesting that there is no clear concentration of particulate matter below which health effects do not occur. This new approach aims to achieve a reduction in the overall exposure of the population to PM2.5, based on the concept that greater benefits could be obtained from a general reduction in exposure than by a policy aimed at reducing concentrations in geographically-limited “hot spots”.

European Union Air Quality Standards:

PM 2.5 :  25μg/m³ annual average

PM 10 : 50µg/m³ 24 hour average

PM 10 : 40µg/m³ annual average

http://ec.europa.eu/environment/air/quality/standards.htm

World Health Organisation Air Quailty Guidelines (2005) :

PM 2.5 : 10µg/m³ annual average

PM 2.5 : 25µg/m³ 24 hour average

http://apps.who.int/iris/bitstream/10665/69477/1/WHO_SDE_PHE_OEH_06.02_eng.pdf

USA National Ambient Air Quality Standards (1990):

PM 2.5 : 12µg/m³ annual average

PM 2.5 : 35µg/m³ 24 hour average

PM 10 : 20µg/m³ annual average

PM 10 : 50µg/m³ 24 hour average

https://www.epa.gov/criteria-air-pollutants/naaqs-table