People on Hong Kong buses are breathing air containing twice the level of fine pollutants as that deemed safe by the World Health Organisation, an environmental group said yesterday.
Friends of the Earth conducted tests in June and July, and found the average level of PM2.5 particles - those 2.5 microns or less in diameter - in buses on 13 routes was 53.11 micrograms per cubic metre. The WHO calls for a limit of 25.
Such particles are roughly 1/28th the thickness of a human hair and because they are so fine, can lodge deep in the lungs. Some respirable suspended particles can cause cancer.
Melonie Chau Yuet-cheung, senior environmental officer of Friends of the Earth, said such high levels of PM2.5 were a serious health threat to the passengers and drivers. "There is a perception problem. People think that entering a bus is like entering a protected area, comparatively safer [than on a polluted roadway], but that is not the case," Chau said.
The PM2.5 levels in buses were about the same as on the roadside because the vehicles' ventilation systems did not filter out fine particles, she said.
The study was limited to franchised buses, and so excluded minibuses. Buses on each route were tested at least four times to get a reliable average reading.
The highest level recorded was in a bus on Yee Wo Street in Causeway Bay - 171 microns per cubic metre - on Citybus route 5.
Dr Wong Tze-wai, a professor in occupational and environmental health at Chinese University, said: "These suspended particles can bypass the normal defence mechanism of the human body. They do not exist in nature … so they cannot be captured [by the fine hair and mucus in our lungs]."
Wong said US studies had linked fine particles to elevated death rates from lung cancer.
Kowloon Motor Bus said new buses were being equipped with electronic static filters, which can block PM2.5. By last month, 1,350 of its 3,900 buses had the filters. Citybus could not be reached for comment.
The government has not set a limit for PM2.5.