A reading of 51 to 100 is considered high, and more than 100 is considered very high. People with heart and breathing problems are advised to avoid congested areas.

An Environmental Protection Department spokeswoman said minimal wind yesterday made the dispersion of particles in the air difficult, resulting in the high pollution levels.

The Hong Kong Observatory said a slight wind of 1 to 2 metres per second was recorded at its Central station. Visibility was 16km at Tsim Sha Tsui. On its website, the department warned people with heart or respiratory illnesses to avoid spending too long in congested places and to keep physical exertion to a minimum if they had to be in areas with heavy traffic.

The pollution was worse than on the same day for the previous two years.

Last year, noon roadside readings were 89 in Causeway Bay, 79 in Central and 83 in Mong Kok, and they were 72, 61 and 69 in 2004.

Today, pollution is expected to remain high to very high.

The fact that the skies have been obscured by haze or smog for a record number of hours - totalling about a third of the year - has further worried environmentalists.

They have seized on the figures as further evidence of worsening air pollution and called for stricter controls on emissions. In the first 11 months of the year, airport visibility was reduced to less than 8km for 2,617 hours, exceeding the annual total for last year by 7.3 per cent.

The government says most air pollution measures are improving and its existing and planned emission reduction actions - including an agreement with Guangdong - will see air quality improve further.