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Poorly designed buildings, congested traffic and limited legislation make Hong Kong's 'street canyons' a serious health hazard
25 May 2006
South China Morning Post

FOR THE PAST 18 years, optician Raymond Lai Wing-chiu has watched the scene outside his small shop in Causeway Bay evolve. Buildings have gone up; buildings have come down; streets have been re-routed; and businesses have changed hands. But through it all, two things have steadily worsened: Lai's lungs. "We won't have a long life working here," says the 50-year-old Jardine's Bazaar shopkeeper, as exhaust fumes from the street's idling mini-buses waft into his shop. "But you need [to make] a living."

According to environmental experts, the site of Lai's shop, Times Optical Trading, is particularly hazardous. It's located in a "street canyon" - where the presence of numerous tall buildings prevents free circulation of air and traps pollutants at ground level, creating a "canyon effect".

The phenomenon occurs when the height of buildings on both sides of a street are twice the width of the road, with no passages between the wall of high-rises at ground level. Hong Kong is particularly susceptible to the canyon effect. Vehicle exhaust is common to every city, but Hong Kong's narrow streets and closely packed high-rises make it much harder for the fumes to escape. And in areas with the most densely packed structures, such as Causeway Bay, Central and Mongkok, air quality has now deteriorated to a critical level, experts say.

"If the pollution is in the atmosphere, it can be diluted," says Liu Chun-ho, a professor in the Hong Kong Polytechnic University's buildings department, who has studied the street-canyon effect. "But the air pollutant inside the street canyon has a very limited chance to dilute to an acceptable level."

Jimmy Fung Chi-hung, an academic at Hong Kong University who has analysed the canyon effect using computer modelling, says the phenomenon is starkly evident during a stroll through Causeway Bay. Walking from Hysan Avenue towards Hennessy Road with two of his students, Fung notes how the streets narrow and the air becomes stifling. Nearing Sogo, the air quality worsens: more pedestrians cover their noses with their hands or handkerchiefs; street vendors cough and hack. Standing in the crush of people waiting to cross Hennessy Road, Fung says they're all breathing in the exhaust from the slow-moving traffic. "Just by walking here, you can see what we're trying to measure with our computer models," he says.

Hong Kong residents take similar walks every day without giving it a thought, says one of Fung's graduate students, environmentalist Marty Man Yu-kit. "You can see how small the street is and how tall the buildings are," Man says, pointing to Jardine's Bazaar. "The pollution from the automobiles gets trapped in there and we breathe it in."

Fung cites three examples of street canyons in Hong Kong: Hennessy Road running through Causeway Bay; the inner streets of Central, and Nathan Road in Mongkok. Hang out on any of these busy streets long enough, and you might feel your throat and eyes getting itchy. And staying inside doesn't offer much more protection, says Fung, a mathematics expert who has been conducting environmental research since 1996.

"No building is completely airtight," he says. "The pollution will find a way in."

Ho Mei-kwan knows this better than most people. The 27-year-old still lives in the same fourth-floor apartment in Hennessy Road that she grew up in, and the pollution has been a constant irritant to her and her family. Exhaust fumes from cars and grime from nearby construction sites regularly make the apartment "very dusty", Ho says. "It has always been the same, and it's never improved much." Her father suffers most. "My dad has respiratory disease and he feels worse whenever the air-pollution index goes up," she says. Ho's family has considered moving over the years, but high property prices mean they'll probably stay.

Ho and her family are most likely to inhale two groups of pollutants at their Causeway Bay home, Man says. The first are nitrogen oxides (NOx), the generic term for a group of highly reactive gases that include nitrogen dioxide (NO2), characterised by its reddish-brown colour. Even short-term exposure to nitrogen oxides can lower a person's resistance to normal respiratory infections, and it can aggravate existing conditions such as asthma. The second group is respirable suspended particulates (RSP), which can lead to respiratory illness, reduced lung function and, in certain cases, increased risk of cancer.

The Environmental Protection Department (EPD) says that Hong Kong's roadside pollution has been reduced with the improvement of vehicle-emission technology. Among its air-quality objectives is to ensure that the level of nitrogen dioxide, a common indicator of pollution, over any 24-hour period should not exceed 150 microgrammes per cubic metre (µg/m3) more than once a year.

However, having taken measurements at the Causeway Bay end of Hennessy Road, Man says that levels of nitrogen oxide have exceeded 400µg/m3 on numerous occasions. Because the gas combines with oxygen in the air to form nitrogen dioxide, he says, this means the government has failed to achieve its goal.

Man says RSP levels in Causeway Bay are only slightly higher than the officially acceptable level of 180µg/m3 over a 24-hour period. But compared with less than 50µg/m3 on the outlying island of Tap Mun, it's a smog.

Not everyone lives at the heart of the problem, but the majority of Hongkongers work, eat, shop and relax in street canyons such as Ho's at some point during the week. And escape is difficult. "There really isn't anything to do - but reduce the source of the pollution," says Liu.

Reducing traffic - especially large vehicles such as buses - is the most obvious way, experts say. Moves such as a recent proposal to introduce electronic road pricing might alleviate congestion. An annual cap on buses, might also help, says Fung. "Just look around and see how many empty buses are driving around this city."

Planting more trees, which produce oxygen, would ameliorate air quality, says Edwin Chan Han-yun, a professor at the Polytechnic University's building department. But most important, he says, is for developers to create passageways at the ground level to allow air circulation. The open, ground-floor lobby of the HSBC headquarters is a prime example. Air is allowed to flow under the building, rather than being trapped at street level.

Liu says new developments in Kowloon such as the Arch exacerbate the street-canyon effect. Although the building features a prominent opening, it's built on large podium block which prevents an easy flow of air at ground level.

The reason more developers don't pay attention to the canyon effect is because the Buildings Department hasn't introduced regulations to ensure wind flow around buildings, says Chan. "Ground-floor space is a priority for developers," he says. "Not the pollution [trap] that buildings with no airflow create."

The Buildings Department says it encourages developers to leave openings at the ground level to allow for airflow. "There are guidelines on {hellip} open space and breezeways to improve the micro-climate and to allow effective air movements into the urban area," the department said in a statement. "These guidelines would be applied wherever possible in new development areas and comprehensive development or redevelopment areas."

There are also some guidelines to improve airflow, they say. For instance, site coverage above 15 metres of a domestic building is generally restricted to 33.33-40 per cent, and 60-65 per cent on a non-domestic building.

The proposed new government headquarters at the Tamar site in Central is another point of contention. Many green groups say it will exacerbate ground-level pollution in an already congested area. Steve Yim Hunh-lam, whose PhD thesis is on the street-canyon problem, says the government's plan for Tamar will further slow air flow and lead to pockets of pollution building in the streets behind the site.

"Even if the buildings are low," Yim says, "the wind will slow down and the pollution will get trapped." The government insists this will not occur as the area is spread out and the buildings are not tall.

It seems unlikely that Yim and the environmentalists will win their battle over Tamar. The government has run its own tests and says the canyon effect isn't a reason to stop the project. But Fung still hopes they can minimise the commercial buildings attached to the site. He doesn't want to see more congestion in the city centre. "Hong Kong can do better," he says.

Lai says he has become resigned to the bad air. He endures the noise from the demolition of a nearby building, but says he's aggravated the most by the mini-buses waiting for passengers outside his shop. "I wish there was a law to make them turn off their idling engines," Lai says. But he's not holding his breath.
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