Hong Kong air pollution

Worst smog of the year prompts Hong Kong government health warning

Low pressure system sends pollution levels rocketing across the city on the worst day of the year so far for air quality

PUBLISHED : Monday, 09 June, 2014, 3:03am
UPDATED : Monday, 27 April, 2015, 3:31pm

Hong Kong choked on its smoggiest day of the year so far yesterday as a trough of low pressure left the city coping with pollution that represented a "serious risk" to health.

At 7pm and 8pm, all 12 of the city's general pollution monitoring stations and all three roadside stations recorded pollution levels posing a "very high" or "serious" risk to health. All but two general stations and all of the roadside stations were at level 10+, the highest level on the government's Air Quality Health Index (AQHI), introduced last year.

At 2pm, the Tung Chung general station recorded 24-hour concentration levels of PM2.5 - fine suspended particles small enough to lodge deeply into the lungs - at 128 micrograms per cubic metre, more than five times the maximum safe level set the by World Health Organisation. Ozone concentration hit 269, against a WHO maximum eight-hour mean level of 100.

Watch: How to deal with Hong Kong's smog

Ozone concentration was even higher elsewhere, hitting levels of more than 300 in areas including Central and Western, Tsuen Wan and Yuen Long, which all recorded 10+ AQHI readings.

An AQHI level of 10+ triggers a warning from the Environmental Protection Department for people with heart disease or respiratory illnesses to avoid outdoor activity, especially in areas with heavy traffic.

A department spokesman said the city was experiencing higher regional ozone pollution and that health risks were now high. Weak winds were not able to disperse pollutants, the spokesman added.

The deteriorating air quality came as a trough of low pressure affected southern China.

Professor Wang Tao, chair professor of atmospheric environment at Polytechnic University, said high levels of air pollution were typical during low-pressure systems.

"Weak horizontal winds and descending air create 'stable conditions', rendering pollutants in the city and nearby unable to move," he said. "People tend to pay more attention to PM, while the dangers of ozone pollution, which is actually getting worse, tend to go unnoticed."

Ozone is not emitted but produced through chemical reactions between nitrogen oxides from power plants and car emissions, and volatile organic compounds.

Most of Hong Kong's ozone pollution is a mix of background pollution from external and local sources. Long-term exposure to ozone is linked to asthma, bronchitis, heart attacks and even death.

Clean Air Network chief executive Kwong Sum-yin said days with multiple 10+ recordings occurred in January during a northeast monsoon, but yesterday would have marked the most polluted day so far this year.

The Observatory forecasts showers and squally thunderstorms today. Shortly after midnight this morning, the Environmental Protection Department's website was predicting "moderate to very high" health risks from pollution this morning, with a "low to moderate" risk in the afternoon.

Watch: The dangers of air pollution - videographic