Yuen Long registered the worst monthly average air quality five times. For the year, Yuen Long averaged 41.29, sixth of 11 districts installed with general stations for the air pollution index. All four areas are among the districts with lowest average income in Hong Kong.

The bad air quality in those areas could be due to a higher number of old diesel vehicles running on the roads, compared to other areas, said Thomas Choi Ka-man, Friends of the Earth's senior environmental affairs officer. 'The narrow roads at Sham Shui Po and heavy trucks going to the container terminal at Kwai Chung also add to the problem,' he said.

Wendy Wong Siu-bing lived in Sham Shui Po for five years before moving to Lam Tin last year. An asthma sufferer, she said the air had seriously affected her. On two occasions the medication could not help her when she had asthma attacks. 'Once, I even could not call an ambulance by myself,' Wong said.

After she moved to Lam Tim, she had fewer asthma attacks, she said.

The 11 general stations are placed on the rooftops of buildings that are four to six storeys high. Roadside stations in Causeway Bay, Central and Mong Kok, by contrast, measure air quality closer to the ground.

The yearly average general stations' readings all lie in the medium level of the index, meaning air quality is acceptable by the bureau's standard. But Choi said that did not mean air in those areas was fine, because there were months in which air quality indexes of the areas exceeded 50. At that level it would be harmful to breathe the air for long.

The group also commissioned the Hong Kong Institute of Vocational Education to measure air pollutants in 10 districts last year by detecting respirable suspended particles and nitrogen dioxides in three or four places in each district.

Sham Shui Po had the highest concentration of nitrogen dioxides, measuring 326 parts per billion (ppb) and 466 ppb in two places. Central followed with readings of 85.03ppb and 5.6ppb. The centre of Sham Shui Po near Yen Chow Street had the most pollutants, Choi said. For respirable suspended particles, a spot in Sha Tin had the highest level (440ppb), followed by one in Kwun Tong (405ppb).

Friends of the Earth wants the government to tighten its air pollution index standard to meet the World Health Organisation's standard and speed up the retirement of older buses to reduce emissions.

According to the Environmental Protection Department, between 2005 and 2010 the average sulphur dioxide level measured at general stations has dropped by 45 per cent and respirable suspended particulates concentration by 18 per cent.

Breathtaking

API levels at general monitoring stations

Medium 26-50

No action by public is required.

High 51-100

Few people will notice any immediate health concerns, but there could be long-term effects if they are exposed to such levels for months of years.

Very hight 101-200

People with heart or respiratory illness are advised to stay indoors and reduce physical exersion.

*The API gives advice on the quality of air and any possible health risks. It is calculated hourly on a scale from 0 to 500 and measures concentrations of five pollutants - nitrogen dioxide, sulphur dioxide, ozone, carbon monoxide and respirable suspended particulates. Low 0 to 25, Medium 26 to 50, High 51 to 100, Very high 101 to 200, Severe 201 to 500