Pollution readings were "very high" in business and shopping districts such as Central, Western, Causeway Bay and Mongkok, air monitoring stations showed, surpassed only once in March 2010 when a sandstorm in northern China covered Hong Kong in dust.

"Bad air is trapped here. But even though external circumstances can't blow away the pollutants, the problem still lies fundamentally in vehicular emissions in Hong Kong itself," said Patrick Fung of Clean Air Network, a local pro-environment group.

The view of the cramped skyscrapers of Hong Kong Island and the Peak was obscured from across the harbour in Kowloon.

Air pollution in Hong Kong, a former British colony which returned to Chinese rule in 1997, is a major source of worry for local citizens and foreign businesses, which increasingly see it as compromising the quality of life.

In a recent survey by human resources consultancy ECA International, Hong Kong distinguished itself as a place where its air quality was among the worst in the world.

The pollution comes largely from coal-fired power stations and traffic, though a significant contribution wafts down from the tens of thousands of factories in China's neighbouring manufacturing heartland of the Pearl River Delta.

Under intense lobbying, the government has been gradually tightening its air-quality objectives and monitoring measures to meet World Health Organisation standards, but these remain far short of global guidelines, green groups say.

(Reporting by Tan Ee Lyn; Editing by Nick Macfie)

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