Based on the existing evidence of adverse health effects at low levels of exposure, WHO revised its AQG for PM in 2005 (1). For PM2.5, the new AQG values are 10 μg/m3 for the annual average and 25 μg/m3 for the 24-hour mean (not to be exceeded for more than 3 days/year). The corresponding guidelines for PM10 were set as 20 μg/m3 and 50 μg/m3.
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