The air pollution is worse than official data shows, research by an air pollution concern group has found.
The Clean Air Network recently completed a three-month study of air quality at 12 locations in Sham Shui Po - covering shopping centres, markets and housing estates. It collected data on the concentration of PM2.5 using a portable air quality monitoring device.
PM2.5 refers to airborne particles that are the product of combustion, such as in cars or power plants, and industrial production. The small size - 2.5 microns across - means such particles can easily lodge deep in the lungs.
The group found the level of PM2.5 in eight out of the 12 locations was higher than what the government's general air quality monitoring stations recorded over the same period of time.
At Woh Chai Street in Shek Kip Mei and beneath the West Kowloon Corridor, the concentrations were 30 per cent higher than what the government recorded.
All locations recorded PM2.5 concentrations at least 50 per cent higher than that which the World Health Organisation deems safe, which is an average of 25 micrograms per cubic metre of air across a 24-hour period.
"The data collected by general monitoring stations does not accurately reflect how bad the air quality is on the ground, particularly along busy roads and bus stops," said Kwong Sum-yin, the group's chief executive.
Kwong said the government's general air quality monitoring stations were all built 11 to 25 metres above the ground, so the data collected might not represent the pollution that pedestrians were experiencing.
Sham Shui Po is one of the most densely populated districts in Hong Kong, with roads often congested with traffic.
Kwong urged the government to set up more roadside monitoring stations to accurately reflect residents' exposure to respirable suspended particles (RSP), and to establish low-emissions zones to restrict high-polluting vehicles from entering populated areas.
"The public should avoid going out to the streets when the concentration of RSP reaches a high level," said Daniel Ng Kwok-keung, president of the Society of Paediatric Respirology.
Ng said RSPs damaged health as the respiratory system was not able to prevent smaller particles from entering the blood stream, which can inflame blood vessels, leading to heart disease.
A spokeswoman for the Environmental Protection Department said the density of development in urban areas in Kowloon was similar, so readings from the roadside station in Mong Kok could represent the roadside air quality in Sham Shui Po.
The city has two other roadside stations, in Central and Causeway Bay.