Pollution was the worst on record on Tuesday, despite air quality officially being described as good. The pollution level meant people with heart or respiratory diseases should have been advised to reduce physical exertion and outdoor activities, but no warnings were given. The official Air Pollution Index for Tuesday was 95. Health alerts are issued when it reaches 100. But environmental officials admitted yesterday the index would have been 150 - the highest ever - if the ozone level, at 320 microgrammes per cubic metre, was included. The high pollution was caused by a lack of wind and strong sunshine trapping nitrogen dioxide and hydrocarbons from vehicle emissions and industry to form secondary pollution, known as ozone, said Acting Assistant Director of Environmental Protection, Raymond Leung Pak-ming. He said air quality was determined by a combination of weather and pollution. Unusually, ozone began to form late in the afternoon and disappeared by 6 pm, he said. 'Very late in the afternoon we had very stable air conditions [and] a lot of sunshine. Nitrogen and hydrocarbons - all these pollutants will react with each other to produce ozone. As the wind picked up, air pollution returned to a moderate level.' Among the ailments caused by ozone pollution are eye irritations and coughing. Air Pollution Index calculations are made from 4 pm to predict conditions for the following day. Yesterday, air quality improved to 64 in urban areas, 61 in industrial areas and 63 in new developments. Today, levels are expected to further improve.
