Residents in more than half of Sham Shui Po breathe in more pollutants than the World Health Organisation deems safe and government figures are not showing the real picture, an environmental concern group says.
The Clean Air Network based its comments on monitoring of seven roadside locations in the district over a one-month period.
It found that average daily concentrations of PM2.5 - fine airborne particles small enough to enter the lungs and blood - at four sites were higher than the WHO's recommended level of 25 micrograms per cubic metre of air each day.
Shek Kip Mei, Nam Cheong and Cheung Sha Wan recorded PM2.5 levels of 26, 28 and 35 respectively.
Yen Chow Street had an average daily PM2.5 level of 41, with 60 per cent of hourly occurrences above the WHO limit.
The highest reading recorded there in a day was 67 - four micrograms per cubic metre higher than the Environmental Protection Department's reading, the Clean Air Network said.
Four other areas also recorded average hourly concentration levels that, over 24 hours, were higher than the department's readings at its general air-quality monitoring station on Yen Chow Street, the group added.
"The government should look into installing more roadside monitors in bigger districts to give a more accurate assessment of air quality," the group's chief executive Kwong Sum-yin said.
She added that the highest concentration levels were usually recorded between 3pm and 6pm when traffic was at its worst.
The group is also urging the government to set up low-emission zones in the district to help speed up the phasing out of old pre-Euro IV polluting diesel commercial vehicles.
In a survey that the group conducted with 349 Sham Shui Po residents, 70 per cent believed district-wide air quality was either "bad" or "very bad".
About half strongly believed that the main sources of air pollution in the area were diesel lorries and franchised buses.
Carman Ng Mei, chairwoman of the Sham Shui Po district council's environmental protection working group, called on the government to tackle traffic problems, which she said were the main cause of air pollution in the area. The dense population and tall buildings compounded the problem, she added.
The Environmental Protection Department said districts with more diesel-vehicle traffic and that were closer to marine vessel fairways, such as Sham Shui Po and Kwun Tong, could experience more air pollution.
The department says it expects to announce the creation of pilot low-emission zones for buses, which will see only cleaner buses allowed into busy traffic corridors.