Industrial waste gases, smoke and vehicle fumes reduced visibility to record lows in the southern Chinese enclave.

"In 2004, low visibility occurred some 18 percent of the time, the highest on record," Assistant Director of the Hong Kong Observatory Yeung Kai-hing said in a statement.

Hong Kong is inundated by smog from southern China's heavily industrialised Pearl River Delta. Some 80 percent of its pollution originates from the delta region.

Green groups say the problem was worsened last year by a power shortage on the mainland, which forced the reopening of old generating plants mothballed because of poor emissions standards.

The filthiest day of the year fell on September 14 when the air pollution index nudged for the first time above the critical 200 level: a reading of more than 100 is classified as "very high" and triggers an automatic health warning.

The pollution issue is a politically sensitive one that gets little governmental attention in Hong Kong, where Beijing-appointed leaders are reluctant to be seen as demanding costly emission cuts on the mainland.

Smog levels are usually worst in spring and winter, when the still weather keeps pollution trapped within the former British colony's urban canyons and rural valleys.

However, a government statement accompanying the latest pollution figures said the problem had worsened in each season.

"Before the mid-1980s, reduced visibility seldom occurred in (autumn). After the mid-1980s, reduced visibility became increasingly frequent in autumn, reaching 10 percent of the time in the last few years."

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