Author: Antonio Da Roza

Date: 7/8/10

Health effects from air pollution are mainly caused by a combination of the concentration of air pollutants and the amount of time you are exposed to the air pollution. So a tool is needed to help indicate the air pollution levels. [1] The public is notified of air pollution levels based on a scale – the Air Pollution Index, which is easy for the public to understand. [2]

Calculating the Index

The API is calculated by the conversion of ambient pollutant concentrations measured by the Government’s monitoring network over a 24 hour period to a scale of 0 – 500. A subindex is taken of respirable suspended particulates, sulphur dioxide, carbon monoxide, ozone and nitrogen dioxide. The subindex level of 100 is corresponded with Hong Kong’s Air Quality Objectives. The API is calculated by taking the maximum of the subindices to indicate overall pollution level . [1]

Using the Index

Two indices are provided to the public: the general API and the roadside API. The general API is intended to represent the air pollution we are exposed to most of the time, whilst the roadside API is naturally higher on a given day due to close proximity to vehicle emission sources; it is thought of as less relevant as we spend quite short periods of the day in busy streets and roads. [1]


News report on the sandstorm episode showing the API figures (22/3/10) - Alex Hofford (

There are mainly 5 different air pollution levels, namely “severe”, “very high”, “high”, “medium” and “low”, and people are advised to make different responses to the respective levels. [1]

People may get the information on the API and its forecast through newspapers, radios, televisions, the API Hotline and the website of the EPD. [1]

When the index reaches 200 or higher, an air pollution alert is triggered. [2]

Relationship with the Air Quality Objectives

The Air Quality Objectives are used as a baseline for the Air Pollution Index, and corresponds to 100 on the API scale of 0 – 500. Hence, levels of air pollution above 100 indicate that the Air Quality Objectives have been exceeded.

The current Air Pollution Index (API) is derived from the AQOs. The outdated AQOs lead to lower API values, which are in turn misleading for the public as they under-represent the health impacts of the ambient air. [3]

An API of 50 corresponds to 55µg/m3 in Hong Kong. However, the same concentration translates to an API of 100 in the EU and California. Moderate or high API readings for Hong Kong would become very high and severe API readings if the indices were calculated based on tighter air quality standards used elsewhere. [4]

Air pollution levels on the majority of days in Hong Kong are between 51 – 100 on the local API, which still produces significant long term health effects. [5]


API forecasts are provided which serves to alert the public before the onset of any serious air pollution episodes, apart from reporting real-time hourly APIs. [1]

See Also

  1. The Air Quality Objectives
  2. Government monitoring of air pollution in Hong Kong


  1. 1.0 1.1 1.2 1.3 1.4 1.5 API & Air Quality, EPD website - last accessed 7/8/10
  2. 2.0 2.1 ‘The air that we breathe’, CLSA, April 2005 - last accessed 7/8/10
  3. ‘The impact of air pollution on population health, health care and community costs’, Department of Community Medicine, HKU, 27 November 2006 - last accessed 7/8/10
  4. ‘Boomtown to gloomtown – The implications of inaction’, CLSA – Christine Loh, James Paterson, September 2006 - last accessed 5/8/10
  5. ‘A Price Too High: The Health Impacts of Air Pollution in Southern China’, Civic Exchange, HKU Medical Centre, Department of Community and Family Medicine, CUHK, Institute for the Environment, HKUST, June 2008 - last accessed 5/8/10