Pollutant | 2005 WHO AQG | 2021 WHO AQG | |
PM2.5 | Annual average concentration 24 h average concentration | 10 µg·m−3 25 µg·m−3 | 5 µg·m−3 15 µg·m−3 |
PM10 | Annual average concentration 24 h average concentration | 20 µg·m−3 50 µg·m−3 | 15 µg·m−3 45 µg·m−3 |
O3 | Peak season (long-term exposure) 8 h average concentration | 100 µg·m−3 | 60 µg·m−3 100 µg·m−3 |
NO2 | Annual average concentration 24 h average concentration | 40 µg·m−3 | 10 µg·m−3 25 µg·m−3 |
SO2 | 24 h average concentration | 20 µg·m−3 | 40 µg·m−3 |
CO | 24 h average concentration | 4 µg·m−3 |
CO: carbon monoxide; NO2: nitrogen dioxide; O3: ozone; PM2.5: particulate matter with a diameter of 2.5 microns or less; PM10: particulate matter with a diameter of 10 microns or less; SO2: sulfur dioxide. Reproduced and modified from the following with the permission of the World Health Organization: https://www.who.int/news-room/feature-stories/detail/what-are-the-who-air-quality-guidelines.