- Greenpeace study tested level of nitrogen dioxide, a pollutant released from vehicle exhausts and electricity generation, at 63 locations over two weeks
- Findings came despite a 34 per cent fall in nitrogen oxide – a combination of nitric acid and nitrogen dioxide – emissions recorded between 2001 and 2017
Concentrations of a major air pollutant can reach "concerning" levels around public parks and primary schools in Hong Kong, a survey by a green group has found.
But two scientists, who both endorsed the need to control the city's roadside air pollutants, cautioned against the survey's methodology, saying the findings might not be as alarming as they sound.
The Greenpeace air quality survey measured the level of nitrogen dioxide, a major pollutant released from vehicle exhausts and electricity generation, at 63 locations over two weeks.
The findings came despite a 34 per cent fall in nitrogen oxide " a combination of nitric acid and nitrogen dioxide " emissions recorded between 2001 and 2017. Such emissions were at their highest in 2003 at about 140,000 tonnes, dropping to less than 90,000 tonnes in 2017, official statistics show.
Shipping, electricity generation and road transport were the top three sources of Hong Kong's nitrogen oxide emissions, accounting for 37, 27 and 20 per cent of the total respectively in 2017.
"We focused on nitrogen dioxide as it is a local issue which can be easily tackled by the government," Greenpeace campaigner Tom Ng Hon-lam said on Monday, urging the administration to take action by quickly switching to electric buses and controlling the number of private vehicles on the roads.
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Exposure to nitrogen dioxide can weaken the respiratory immune system, increase bronchial reactivity and trigger asthma symptoms in the short term, according to the World Health Organisation. In the long term, it can increase risks of lung and brain cancers, as well as harming children's lung development.
From July 13 to 27, Greenpeace collected air samples from 15 public parks, 20 schools, 17 roadside lamp posts and 11 pedestrian bridges in Wan Chai, Yau Ma Tei, Tsim Sha Tsui, Mong Kok, Kwun Tong, Sham Shui Po and Ma On Shan. Samples were sent to a lab in Britain for analysis.
The group then created an index to categorise pollutant levels using Hong Kong's air quality objectives, which set one-hour and annual concentration limits for several pollutants including nitrogen dioxide.
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Under the objectives, nitrogen dioxide levels should not exceed 200 micrograms per cubic metre of air every hour, while the annual limit is set at 40mcg/m³ per hour an average.
Greenpeace's index was based on the annual limit, and classified nitrogen dioxide concentration exceeding 80mcg/m³ as "serious", between 40 to 80mcg/m³ as "concerning" and less than 40mcg/m³ as "acceptable".
Results show that nine of the locations, including three schools in Wan Chai and Yau Tsim Mong district, had "serious" nitrogen dioxide concentrations. The group declined to name the affected schools. Most of the other serious locations were along roads.
Meanwhile, levels at 40 locations including near parks in Yau Tsim Mong, Kwun Tong and Sham Shui Po and pedestrian bridges were "concerning".
In terms of districts, four locations in Kwun Tong were "serious", mainly near roadsides, making it the district with the highest concentration of nitrogen dioxide.
However, two scientists not connected to Greenpeace or the survey said the results were not that alarming.
"The index they used compares a two-week data collection against a 52-week concentration benchmark," said Wong Tze-wai, an adjunct professor at Chinese University who specialises in public health and environmental health.
"The changes in seasons and climate need to also be taken into account. But, of course, nitrogen dioxide should still be taken seriously. You just cannot base conclusions off a two-week study."
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Alexis Lau Kai-hon, a Hong Kong University of Science and Technology professor specialising in air pollution and its health impacts, agreed with Wong. "Using a long-term index to measure a short-term project is scientifically inaccurate," he said.
Lau also said diffusion tubes used by Greenpeace were less accurate tools and there were more advanced instruments which could accurately measure nitrogen dioxide concentrations.
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