Bad air days: three Hong Kong districts rank worst for air pollution

Those who live and work in Tuen Mun, Kwun Tong and Tung Chung spent more hours exposed to bad air in the first 11 months of the year than anywhere else in the city, according to a green group's preliminary analysis.

On average, the duration of bad air days recorded by the Air Quality Health Index was longer than what was recorded on the previous air pollution index, which was in effect between 2000 and 2013. Kwun Tong recorded the longest consecutive amount of time with "serious" air pollution - the highest level possible - at 48 hours.

In terms of recording "high" readings or above, Tuen Mun and Tung Chung ranked first and third for the second consecutive year, based on monitoring stations in those districts. They clocked 395 and 334 hours respectively. Kwun Tong was second with 344 hours, ousting Yuen Long, which came second last year.

The government says a reading of "high" on the new index means that children, elderly people and those with heart or respiratory illnesses should reduce physical exertion and outdoor activities, and they should stay indoors if a "serious" alert is in place.

Cheng Luk-ki, head of scientific research at non-profit group Green Power, said ozone pollution - a hazardous secondary pollutant formed by volatile organic compounds reacting with nitrogen oxides - was most severe in western parts of the city.

In a separate analysis of Environmental Protection Department data between 1999 and last year, the group identified a link between warmer weather and ozone formation.

By plotting daily maximum ozone concentrations against maximum daily air temperatures, Cheng calculated that a one-degree rise in temperature would lead to an increase in ozone by 17.4 micrograms per cubic metre in Tung Chung and 13.2mcg in Yuen Long district.