Greenpeace said official measurements of smog in the southern Chinese autonomous region were misleading and their own studies showed pollution in some districts was as much as three times the level accepted in other cities.

"Hong Kong air quality has exceeded international safety levels by 200 percent over the past nine days," a statement from the organisation said.

Greenpeace says the government has set the levels of acceptable pollution -- or the air quality objective -- far too high, meaning that daily readings by environmental protection officials are artificially lower.

On October 27, for instance, it said monitoring stations showed breathable particles were double to triple European Union safety levels, but the official index declared that the air was safe and there was no need for a health warning.

"With the current loose and outdated (base level) in Hong Kong the public receives no warnings from the government to stay indoors," assistant campaigner Edward Chan said in a statement.

To reinforce their message, Greenpeace activists handed the report to environmental officials while wearing gas masks.

Pollution in Hong Kong has been a big concern in the past few months as smog from the heavily industrialised Pearl River Delta in neighbouring Guangdong province has hung over the city, often obscuring the famous skyline.

Scientists at another green group, Friends of the Earth Hong Kong, claim some 90 percent of all pollution in the city drifts in from the rest of China.

Pollution readings are typically higher in autumn as the calm weather prevents the smog from being blown out to sea.

Greenpeace claims that most of the sulphur dioxide in the air, among the most dangerous of pollutants, is produced by the city's own power stations.

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