Hong Kong choked under the worst smog ever recorded in the city Thursday, with residents warned to stay indoors, away from the blanket of toxic haze, officials said.

Air pollutant readings broke records going back to 1999, except for levels reached when a natural dust storm hit the southern Chinese territory two years ago, environmental protection department spokesman Y.F. Chau said.

"This is the worst air pollution reading we've seen since Hong Kong started recording air pollution in 1999, except for the dust storm," he said.

Hong Kong's famous skyline was shrouded in a dense blanket of toxic smog and the sky looked grey, although the weather was fine and sunny.

"People with heart or respiratory illnesses, the elderly and children should reduce physical exertion and outdoor activities," a government spokeswoman said.

Officials said the pollution had been exacerbated by the influence of Typhoon Saola, which killed four people as it lashed Taiwan some 700 kilometres (450 miles) to the east.

The storm's outer high-pressure air mass blanketed Hong Kong, bringing strong sunshine and high temperatures that pushed up ozone levels.

The pollution was particularly bad in the Central district of downtown Hong Kong, where luxury retail brands and multinational companies pay among the highest rents in the world.

Anti-pollution activists said Hong Kong could not keep blaming the weather or factories in neighbouring mainland China for its recurring pollution problems.

"If Hong Kong did not produce air pollutants, the weather conditions would not be able to exacerbate or cause further consequences," Clean Air Network campaign manager Erica Chan told AFP.

Emissions from local vehicles using old and dirty engines are among the main contributors to Hong Kong's air pollution, she said.

The network said monitoring stations in the bustling shopping district of Causeway Bay on Wednesday recorded levels of the most dangerous fine particles that were three times higher than World Health Organization (WHO) guidelines.

The government announced revisions to its air quality objectives for the first time in 25 years in January, after University of Hong Kong research showed pollution-related illnesses killed more than 3,000 residents a year.

The new objectives impose more stringent limits on the atmospheric concentration for seven pollutants including sulfur dioxide, nitrogen dioxide, carbon monoxide and lead.

For the first time the city has started measuring airborne particles smaller than 2.5 micrometres in diameter, known as PM2.5, which are more harmful than the larger particles.

Heavy polluting vehicles will be phased out, hybrid or electric vehicles will be promoted and more use will be made of natural gas.

But independent analysts say the measures are too little, too late, and fall short of WHO guidelines.