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Pollution Study Points To Dangers Of ‘Canyon Effect’

Cheung Chi-fai, SCMP – Updated on Feb 02, 2009

The “canyon effect” is to blame for the much higher level of ultrafine air pollutants at bus stops on “walled streets” in Central compared with those in more ventilated areas, a study has shown.

In one comparison, the number of pollutants nearly doubled. The canyon effect refers to the impact – such as poor ventilation and trapped heat – from the creation of canyon-like streets between walls of closely spaced tall buildings.

The study measured the number of ultrafine particles in every cubic centimetre of air, rather than the government’s pollution-monitoring method that tracks the weight of larger particles in every cubic metre of

Ultrafine particles can be as tiny as 20 nanometres in diameter – 2,500 times smaller than the diameter of a human hair.

Although there are no international standards on acceptable levels of the number of ultrafine particles in the air, overseas studies have found they can penetrate directly into blood vessels and lung tissue, causing
harm. Many scientists believe they may be the most harmful form of air pollution.

The Hong Kong study was conducted last year by students at the University of Science and Technology. They found the air at bus stops at sites between walls of buildings on streets with heavy traffic had more ultrafine pollutants than that at bus stops in open spaces, seaward streets and indoors.

The number of particles at the two eastbound bus stops outside Wing Lung Bank and the old Hang Seng building at Des Voeux Road Central were on average 90 per cent and 75 per cent higher than at the bus stop outside Statue Square in Central.

Measurements were taken during evening peak hours on six days between September and December.

Between 48,000 and 137,000 particles were recorded at the Wing Lung Bank bus stop, between 63,000 and 100,000 at the old Hang Seng Bank building bus stop, and a range of 22,000 to 82,000 at the Statue Square bus stop. For comparison, a benchmark reading of 20,000 was recorded at the researchers’ Sai Kung campus on a clear and fine summer day.

Des Voeux Road Central is surrounded by buildings on both sides while the Statue Square stop has more open space in its vicinity, although more buses pass by the square.

“The findings strongly suggest the presence of the canyon effect in Central,” said Lau Ngai-ting, the project’s supervisor. “The number of particles one inhales in the streets would be astronomical.”

He said that while the readings might have varied with changing weather conditions and ambient pollution levels, he believed poor ventilation contributed to higher pollution levels on the streets. Heavy traffic, such
as buses running on large diesel engines, was the key source of ultrafine particles, he said.

Researchers also measured particle levels along three different walking routes between Des Voeux Road Central and Statue Square.

On all three routes, the reading showed high levels of pollutants – hitting a high of 180,000 at one point on the road – but fell below 10,000 in elevated walkways, mall corridors and underground rail stations, where
ventilation was better.

Adrien Chen Kam-cheuk, a chemical and environmental engineering student who initiated the study, said: “Going above ground or underground seems to be a more desirable way for commuters to avoid street-level pollution.”

Mr Lau said regulating traffic through such means as electronic road pricing or low-emission zones might help reduce the pollution level.

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