Since the Air Quality Health Index (AQHI) issued by Hong Kong Environmental Protection Department is not comparable to other regions in the world, Weather Underground of Hong Kong refers to the Air Quality Index (AQI) issued by U.S. Environmental Protection Agency and calculates the Air Quality Index based on air pollutant concentration data issued by Hong Kong Environmenal Protection Department. Reference Links: Hong Kong Air Quality and Health Risk Assessment Real-time Air Quality for Major Cities Real-time Global Air Quality Map Air Quality Guidelines by World Health Organization * Missing PM2.5 data from Environmental Protection Department Air Quality Index (AQI) Category Meaning 0-50 Good Air pollution risk is low. 51-100 Moderate Air quality is acceptable. 101-150 Unhealthy for high-risk group High-risk group may have health effects. General public is not affected. 151-200 Unhealthy High-risk group may have more serious health effects. Some of the general public may have health effects. 201-300 Very Unhealthy General public have health effects. 301-500 Hazardous Some of the general public may have more serious health effects. Air Quality Health Index (AQHI) Health Risk 1-3 Low 4-6 Moderate 7 High 8-10 Very high 10+ Serious