AIR pollution soared to unhealthy levels in some areas last month, the Environmental Protection Department (EPD) reported yesterday. Low winds and stable weather were the aggravating factors because they trapped the pollutants and prevented them from dispersing, the EPD said. Readings at Mongkok, where the only street-level air quality monitoring station is found, showed nitrogen dioxide levels exceeded the health safety limit of 150 micrograms per cubic metre over 24 hours. High levels of the pollutant can cause respiratory problems, including an increased risk of infection. Raymond Leung Pak-ming of the EPD, who is responsible for monitoring, said the area was troubled by high vehicle emissions and weather conditions made these worse. Pollution readings at two other EPD monitoring stations did not exceed the health limits, although they were high. The EPD will soon publish a technical memorandum detailing its air quality standards and how it determines whether factories breach these standards. The memorandum is in response to a court case several years ago which the EPD lost because the defendant challenged its air quality standards and methods of establishing whether an offence had occurred. Air quality standards are open to some debate because there is not 100 per cent proof that pollution harms health, and a reading can be skewed depending on when and where it is taken. Legislative Council input will soon be sought on a special trading fund which would prevent sewage charges being swallowed up in the Government's general revenue. The move would ensure the charges paid by households, industry and commerce were spent only on sewage services and not on other government projects. Sewage charges, of an average $15 to $30 a month for households, were announced in September and endorsed by Legislative Councillors in December, after a heated debate, by a 26-16 vote. The charges would cover operating costs and are expected to be introduced from August 1. The administration hopes to have the trading fund in place by then. The fund will also be the clearing house for the Government's cash injection into sewage programmes to cover the capital costs of new sewers and treatment works. About $12 billion will be spent in the next three years on projects expected to reduce pollution in Victoria Harbour by 70 per cent.