02 June 2011

World cities' air quality comparison using satellite data


Bad air in Hong Kong
Hong Kong, together with other east Asian cities, has some of the worst air quality in the world. In terms of high GDP per capita, Hong Kong is the worst performer. NASA data shows how far we have to go to make our air fit for breathing.

NASA satellite data
The most updated satellite data from NASA were retrieved. Based on aerosol optical thickness (AOT), the amount of light absorbed by particles in a defined area of atmosphere above different places on earth can be estimated. According to NASA, if the value of AOT is less than 0.1, visibility is the highest and a clear blue sky can be seen in daytime; on the other hand, if the value is 1, visibility is the worst and the sky is very hazy.

Measurements of aerosol particles at different heights of the atmosphere (<2 km, 2-4 km, >4 km) around the world, including East Asia and southern China, showed clearly that the closer to the ground the higher the density of aerosol particles. This indicates that using AOT to estimate near-ground particulate air quality caused by anthropogenic combustion is relevant and should be valued by policy-makers (Clarke and Kapustin 2010). We have also applied this kind of satellite data in epidemiologic studies and showed associations with damage to children’s respiratory system in Hong Kong (Lai et al 2010).

The AOT level in Hong Kong (Figure) in April 2011 was very high, indicating poor air quality with hazy sky conditions. Although it was slightly better than Guangzhou, Macau, and Bangkok (Thailand), it was even poorer than Shanghai and Beijing in China, as well as Seoul (South Korea), New Delhi (India), Mexico City (Mexico), Tokyo (Japan), and very much poorer than London (United Kingdom), Paris (France), Athens (Greece), New York (United States), Rome (Italy), Helsinki (Finland), and Sydney (Australia).

The monthly average PM10 in April in 2011 in Hong Kong was 59 μgm-3, higher than that last year (41 μgm-3) and about 200% above the World Health Organization guideline for safer air quality. If AOT is linearly associated with particulate concentration (Lai et al 2010), the AOT level in April corresponding to the WHO annual AQG level of 20 μgm-3 should be approximately around 0.27 (dotted line). In Hong Kong, it is over 0.8.

The latest epidemiologic evidence from other regions showed that exposures to particulates and ozone were associated with increased inflammation in early pregnancy (Lee et al 2011), potentially damaging the mother and unborn child. Exposure to SO2 and PM10, much elevated pollutants in Hong Kong, particularly during the first 3 months of pregnancy, may increase risk of born-dead foetus (Hwang et al 2011). Serious harm to maternal and child health is a signal for urgent and effective action by government to protect the public's health. It is clear this is not yet happening in the HKSAR.

Air quality in Hong Kong at present cannot protect our health at all. We should not allow the impression to be created that Hong Kong has acceptable air quality or that it is comparable to other jurisdictions with better air quality.

Edited by AJH

References:
Clarke A, Kapustin V. Hemispheric Aerosol Vertical Profiles: Anthropogenic Impacts on Optical Depth and Cloud Nuclei. Science. 2010;329:1488-1492.
Lai HK, Ho SY, Wong CM, Mak KK, Lo WS, Lam TH. Exposure to particulate air pollution at different living locations and respiratory symptoms – an application of satellite information. International Journal of Environmental Health Research. 2010;20:219-230.
Lee PC, Talbott EO, Roberts JM, Catov JM, Sharma RK, Ritz B. Particulate Air Pollution Exposure and C-reactive Protein During Early Pregnancy. Epidemiology. 2011 Apr 21. [Epub ahead of print]
Hwang BF, Lee YL, Jaakkola JJ. Air Pollution and Stillbirth: A Population-Based Case-Control Study in Taiwan. Environ Health Perspect. 2011 Mar 28. [Epub ahead of print]

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