An aircraft comes in to land at Chek Lap Kok airport yesterday through dense smog that cloaked the area and produced a record high air pollution reading of 201 in nearby Tung Chung. It was the first time the Air Pollution Index had topped 200 in the Lantau new town and the Environmental Protection Department issued the first severe air pollution warning since air quality monitoring was introduced in 1995. The reading came on a day when air quality was poor across the city and the index at roadside stations in Mongkok and Central stayed above 100 for most of the time. The record reading was taken at 3pm but it dropped to 182 in the next hour. It was attributed to ozone pollution, which topped at 402 micrograms per cubic metre at 3pm, 67 per cent higher than levels laid down by the Environmental Protection Department. World Health Organisation guidelines say exposure to an ambient ozone concentration of more than 120 micrograms per cubic metre for eight hours will result in health problems. A high concentration of ozone can lead to eye irritations, coughing and even chromosome changes. Alexis Lau Kai-hon, associate professor at the Centre of Coastal and Atmospheric Research at the University of Science and Technology, said he was pessimistic about the deterioration of air quality. 'It is not just a reading of 201 that makes me worried. I am also worried about the increasing frequency of high readings,' he said. Data provided by Professor Lau shows there have been 14 days of pollution readings above 100 so far this year in Tung Chung, compared with 12 last year, 13 in 2002, seven in 2001 and five in 2000. Professor Lau said he observed pollutants drifting into the city from the mainland in a weak northwesterly wind. He added that development in the Pearl River Delta had worsened regional ozone pollution. A spokesman for the department said the bad air quality was due to calm and sunny weather.