Table 1.

Recommendations and thresholds.

	WHO recommendations	EU thresholds	Chinese Ministry of Environmental Protection∗	United States EPA
PM2.5	10 μg/m3 annual average	25 μg/m3 annual average	35 μg/m3 annual average	35 μg/m3 averaged over 24 hours
	25 μg/m3 24-hour maximum		75 μg/m3 daily average
PM10	20 μg/m3 annual average	40 μg/m3 annual average	70 μg/m3 annual average	150 μg/m3 averaged over 24 hours
	50 μg/m3 daily average	50 μg/m3 daily average	150 μg/m3 daily average
NO2	40 μg/m3 annual average	40 μg/m3 annual average	40 μg/m3 annual average	53 parts per billion (ppb) annual mean
	200 μg/m3 1-hour maximum	200 μg/m3 1-hour maximum	80 μg/m3 daily average 200 μg/m3 1-hour average
O3	100 μg/m3 8-hour maximum	120 μg/m3 8-hour average	160 μg/m3 8-hour average 200 μg/m3 1-hour average	0.070 parts per million (ppm) averaged over 8 hours
SO2	20 μg/m3 daily average	125 μg/m3 daily average	60 μg/m3 annual average 150 μg/m3 daily average	0.5 ppm averaged over 3 hours
	500 μg/m3 10-minute maximum	350 μg/m3 1-hour maximum	500 μg/m3 1-hour average

Numbers in parentheses are the maximal numbers of allowed exceedances per year. ^∗Values applying for urban areas are shown. Stricter standards are required for special regions such as national parks. Numbers are according to the WHO [1], the EU [2], the Chinese Ministry of Environmental Protection [5], and the United States Environmental Protection Agency (EPA, values retrieved from https://www.epa.gov/criteria-air-pollutants/naaqs-table).