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WHO air quality guidelines and national air pollution legal thresholds for annual average concentrations (https://apps.who.int/iris/handle/10665/345329, https://www.epa.gov/criteria-air-pollutants/naaqs-table).
| Air pollutant | WHO 2005 | WHO 2021 | EU threshold | US threshold |
|---|---|---|---|---|
| Nitrogen dioxide (•NO2) | 40 μg/m3 | 10 μg/m3 | 40 μg/m3 | 100 μg/m3 (53 ppb) |
| PM2.5 | 10 μg/m3 | 5 μg/m3 | 25 μg/m3 | 12 μg/m3 |
| PM10 | 20 μg/m3 | 15 μg/m3 | 40 μg/m3 | (50 μg/m3) canceled in 2006 |
