Author: Antonio Da Roza
Visibility as a measure of air pollution
Poor visibility is directly related to trends and episodes in air pollution. Loss of visibility is also a direct measure of illness and deaths caused by air pollution. 
The U.S. Environmental Protection Agency (EPA) identified reduced visibility as the best indicator of all environmental effects of air pollution.  For every one kilometre loss in visibility below 20km, there is an increase in mortality risk of 0.36% - 0.55%.
Visibility trends in Hong Kong
In the 1980s there were typically no hazy days in summer in Hong Kong and on average only five hazy days per month in winter. The number of hazy days per month went up to about four in summer and 15 in winter during the 1990s, and eight in the summer and 25 in the winter from 2000 onwards.
Since 2003, the number of hazy days (visibility less than 8km) has doubled.
- ↑ ‘Air Quality Objectives Review Public Consultation’, Environmental Protection Department Hong Kong, July 2009 - last accessed 5/8/10
- ↑ ‘The air that we breathe’, CLSA, April 2005 - last accessed 7/8/10
- ↑ ‘The Sustainable Development Council Invitation and Response Document: Clean Air-Clear Choices - Will High Air Pollution Alert Days provide an efficient path to health protection?’, Department of Community Medicine, HKU, The Air Quality Objective Concern Group - last accessed 7/8/10
- ↑ ‘Air Pollution: costs and paths to a solution’, Department of Community Medicine, HKU, Department of Community and Family Medicine, CUHK, Institute for the Environment, HKUST, Civic Exchange, June 2006 - last accessed 5/8/10
- ↑ ‘Boomtown to gloomtown – The implications of inaction’, CLSA – Christine Loh, James Paterson, September 2006 – last accessed 5/8/10