At 4pm yesterday, the roadside pollution readings in Central and Mong Kok peaked at 210 and 205 respectively, mainly due to the high concentration of nitrogen dioxide. The indices fell slightly to 209 and 204 respectively at 6pm.

The readings in Central remained above the severe level of 200 all day yesterday. Under such severe levels, people, especially those with heart or breathing problems, are asked to stay away from areas of heavy traffic.

An Environmental Protection Department spokesman warned pollution levels would remain high today, but would start to improve gradually later in the day as the wind was expected to pick up.

It was forecast that roadside air pollution indices would still range at the very high level of between 120 and 190 today, while readings at the 11 general stations would range from 65 to 130, or "high" to "very high" levels.

Friends of the Earth senior environmental affairs officer Melonie Chau Yuet-cheung said: "The government can hardly blame China this time. The wind was light and the high concentration of nitrogen dioxide means serious vehicle emissions." She urged the government to speed up the phasing out of aged heavily polluting vehicles.

The number of vehicles in Hong Kong jumped from 541,000 in 2005 to about 653,000 in 2012, with private cars accounting for close to 70 per cent of the total.

" There are too many high-rise blocks in the city centre. Even when there is wind, it is blocked and can't disperse pollutants," Chau said.

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