http://whqlibdoc.who.int/hq/2006/WHO_SDE_PHE_OEH_06.02_eng.pdf
WHO Air Quality Guidelines and their rationale
Particulate matter
Guidelines
PM2.5: 10 μg/m3 annual mean
25 μg/m3 24-hour mean
PM10: 20 μg/m3 annual mean
50 μg/m3 24-hour mean