Frequently Asked Questions
The New AQHI System – Purpose and Use
Annual Air Quality Index
The New AQHI System - Purpose and Use
The Air Quality Health Index (AQHI) is a health risk-based air pollution index. It provides an estimate of the short-term (hospital admission) risk of heart and respiratory diseases from air pollution. The AQHI is reported on a scale of 1 to 10 and 10+, and is grouped into five health risk categories: low, moderate, high, very high, and serious.
The AQHI of the current hour is calculated from the sum of the percentage added health risk (%AR) of daily hospital admissions attributable to the 3-hour moving average concentrations of four criteria air pollutants: ozone (O3), nitrogen dioxide (NO2), sulphur dioxide (SO2), and particulate matter (PM) (respirable suspended particulates (RSP or PM10) or fine suspended particulates (FSP or PM2.5), whichever poses a higher health risk). The %AR of each pollutant depends on its concentration and a risk factor which was derived from local health statistics and air pollution data. The %AR is then compared to a scale to obtain the appropriate banding of AQHI. The equations are as follow:
%AR = %AR (NO2) + %AR (SO2) + %AR (O3) + %AR (PM)
where %AR (PM) = %AR (PM10) or %AR (PM2.5), whichever is higher
%AR(NO2) = [exp (β(NO2) × C(NO2)) – 1] × 100%
%AR(SO2) = [exp (β(SO2) × C(SO2)) – 1] × 100%
%AR(O3) = [exp (β(O3) × C(O3)) – 1] × 100%
%AR(PM10) = [exp (β(PM10) × C(PM10)) – 1] × 100%
%AR(PM2.5) = [exp (β(PM2.5) × C(PM2.5)) – 1] × 100%
%AR(NO2), %AR (SO2), %AR (O3), %AR (PM), %AR (PM10) and %AR (PM2.5) are the added health risk of NO2,SO2, O3, PM, PM10 and PM2.5 respectively;
C(NO2), C(SO2), C(O3), C(PM10) and C(PM2.5) are the 3-hour moving average concentration of the respective pollutants in microgram per cubic meter (µg/m3); and
β(NO2), β(SO2), β(O3), β(PM10) and β(PM2.5) are added health risk factors (technically known as regression coefficients) of the respective pollutants.
β(NO2) = 0.0004462559
β(SO2) = 0.0001393235
β(O3) = 0.0005116328
β(PM10) = 0.0002821751
β(PM2.5) = 0.0002180567
For further details, please refer to the Study Report.
In establishing the cut-points for the different bands, the AQHI system takes into account the Air Quality Guidelines (AQG) for air pollutants, developed by the World Health Organization (WHO), as well as the local hospital admissions risk. The health risk levels associated with the short-term WHO AQG for the air pollutants are used for determining the AQHI cut-points for the ‘high’ and ‘very high’ risk categories, corresponding to an increase in the risk of hospital admissions of 11.29% and 12.91% respectively. The cut-points for the ‘high’ and ‘very high’ categories respectively demarcate the air pollution levels for which people who are sensitive to air pollution and the general public are advised to take precautionary actions for health protection. Other AQHI categories are derived based on these cut-points, as shown in the table below.
|Health Risk Category||The label||AQHI||Added Health Risk
|Low||1||0 - 1.88|
|2||>1.88 - 3.76|
|3||>3.76 - 5.64|
|Moderate||4||>5.64 - 7.52||%AR of 5.64: 0.5 x threshold for people who are sensitive to air pollution (%AR of 11.29 ) to take precautionary actions|
|5||>7.52 - 9.41|
|6||>9.41 - 11.29|
|High||7||>11.29 - 12.91||%AR of 11.29: threshold for people who are sensitive to air pollution to take precautionary actions|
|Very High||8||>12.91 - 15.07||%AR of 12.91: threshold for the general public to take precautionary actions|
|9||>15.07 - 17.22|
|10||>17.22 - 19.37|
|Serious||10+||>19.37||%AR of 19.37 : 1.5 x threshold for the general public (%AR of 12.91) to take precautionary actions|
For further details, please refer to the Study Report.
The AQHI is a tool for communicating the short-term health risk posed by air pollution to the general public. It provides information on the possible risks to health from exposure to different levels of air pollution in the outdoor environment. This information allows people to make informed decisions on their outdoor physical activities. Individuals with heart or respiratory illnesses, who are on regular medication, may need to consult their doctors on adjusting the doses they take.
The previous Air Pollution Index (API) categorised air quality into different levels according to the values of the Air Quality Objectives (AQOs). The Environmental Protection Department (EPD) engaged a joint team of public health and air science experts from the Chinese University of Hong Kong and the Hong Kong University of Science and Technology to conduct a review of the API system. The study recommended replacing the API with a health risk-based AQHI system, which can provide better communication on health risk and thus improve the protection of public health.
The AQHI system features the following improvements over the API system:
(a) The AQHI system is a health risk-based reporting system based on the relationship between local air pollution and hospital admissions, thus providing a locally relevant index that is more useful a health reference than the older system;
(b) The AQHI takes into account the combined effects of the major air pollutants in Hong Kong;
(c) The use of 3-hour moving average pollutant concentrations in calculating the AQHI enables the changes in air quality to be closely followed, and hence will provide more timely health risk communication to the public.
The AQHI is reported hourly at each general (ambient) and roadside station. The AQHIs reported at general monitoring stations are referred to as ‘General AQHIs’, while those reported at roadside monitoring stations are referred as ‘Roadside AQHIs’. The EPD calculates and releases the AQHIs every hour; it also provides forecasts of roadside and general AQHIs for the next 12 to 24 hours in two time blocks, i.e. both a.m. and p.m. sessions.
There are a number of ways that you can get the latest hourly AQHI and forecast:
(i) Visit the EPD website at http://www.aqhi.gov.hk by using a personal computer or a mobile device such as a smart phone;
(iii) Call an AQHI hotline through an interactive voice recording system at 2827 8541 for verbal updates, or obtain a copy of this information by the fax-on-demand service.
All of the above methods can provide you with AQHI information 24 hours a day. There are also updates provided at regular intervals by the mass media, on different TV and radio channels.
The General AQHI reflects the level of air pollution to which you are exposed most of the time. It comes from measurements at thirteen general air quality monitoring stations in the EPD’s air monitoring network.
The Roadside AQHI tells you the level of air pollution specifically at the roadside, with very heavy traffic and tall surrounding buildings. Measurements are taken from the three roadside air quality monitoring stations.
The health effects of air pollution result from exposure to a combination of air pollutants, in different concentrations, over a period of time. Exposure to moderately high pollution levels for a short time normally will not lead to significant problems. If you spend most of your time away from the roadside, the General AQHI is more relevant. The Roadside AQHI is more relevant to people who spend most of their daily activities on the roadside, near heavy traffic and surrounded by tall buildings.
Areas of similar land uses, traffic conditions and levels of urban development would tend to have similar air pollution levels. You may refer to the AQHI of those stations with similar development characteristics closest to the area you live in. If you live in a district that does not have an air quality monitoring station, you may refer to the AQHI measured by a neighbouring station, as shown in the table below:
|District||Correlated Monitoring Station|
|Hong Kong Island|
|Central & Western||Central/Western|
|Southern||Central/Western or Eastern, whichever is nearer|
|Kowloon City||Sham Shui Po|
|Kwun Tong||Kwun Tong|
|Sham Shui Po||Sham Shui Po|
|Yau Tsim Mong||Sham Shui Po|
|Wong Tai Sin||Kwun Tong|
|Kwai Tsing||Kwai Chung|
|Sai Kung||Tseung Kwan O|
|Sha Tin||Sha Tin|
|Tai Po||Tai Po|
|Tsuen Wan||Tsuen Wan|
|Tuen Mun||Tuen Mun|
|Yuen Long||Yuen Long|
The new AQHI aims to give short-term health advice. The long-term impact of air pollution on the general population is communicated by means of the annual Air Quality Index (AQI).
Different health advices are given to people with different degrees of susceptibility to air pollution, including:
(i) People who are sensitive to air pollution, i.e. (a) people with existing heart or respiratory illnesses, and (b) children and the elderly;
(ii) Outdoor workers, and
(iii) The general public.
These terms are used to express the different extent of curtailment of strenuous outdoor activities. ‘Reduce’ refers to a lowering of the intensity or duration of these activities, while ‘avoid’ means to refrain from such activities completely. To ‘reduce to the minimum’ is to cut down to only those activities that are essential.
You should reduce exposure to air pollution in the outdoor environment and avoid outdoor physical exercise. If you stay indoors, make sure the indoor environment is not polluted by sources such as cigarette smoking, cooking fumes, and open flames, or by air pollutants emitted from furniture, carpets, and other household items. If you are not fit, or have heart or respiratory diseases, consult the doctor on your medications and on the amount of physical exercise you should perform.
Annual Air Quality Index
The annual Air Quality Index (AQI) aims to communicate the health risks caused by long-term exposure to air pollutants. The index is derived from the ratio of the annual mean concentration of an air pollutant to that of the corresponding WHO annual AQG. An annual AQI of one means that the air pollutant concentration is equal to the WHO annual AQG level. An AQI greater than one would indicate that the health risk resulting from long-term exposure to an individual air pollutant is higher than that caused by exposure to the WHO reference value, whereas an value below one means a lower risk. The annual AQIs are calculated for those air pollutants with annual WHO AQGs, i.e. NO2 and PM10 or PM2.5, based on 12-month moving average concentrations of these pollutants.
For those air pollutants with annual WHO AQGs, i.e. NO2 and PM10 or PM2.5, the annual AQI tells us how good (or poor) the Hong Kong air quality has been in the past 12 months, as compared with the WHO AQGs.
An annual AQI of an air pollutant of greater than one means residents in that district are at a higher risk to the long-term health effects caused by that air pollutant. For example, a long-term concentration of 35 µg/m3 for PM2.5 implies a 15% increase in the risk of death from heart and respiratory diseases, including respiratory cancer, compared to the risk if the concentration of PM2.5 were 10 µg/m3 (at which level a small risk still exists). It should be noted that the risks to health differ for different air pollutants and in different age groups.
As a whole, the greater the index, the more we as a community need to do to improve our air quality by reducing the release of air pollutants from various sources.
For decisions relating to daily physical activities, you should refer to the hourly AQHI and consider the relevant health advice. The annual AQI is relevant only if you would like to assess the health risks caused by long-term exposure to air pollution.
Please feel free to contact us by writing to the following address or giving us a call or a fax.Air Science Group, Environmental Protection Department, 33/F., Revenue Tower, 5 Gloucester Road, Wan Chai, Hong Kong. Telephone: 28278541 Fax: 28278040