AIR pollution was within health safety standards last month - unlike a year ago when they were exceeded by up to 50 per cent - but an official warned this was no sign air quality was improving. Wet and humid weather conditions enabled the pollutants to disperse last month rather than build up, according to Mr Raymond Leung Pak-ming, of the Environmental Protection Department (EPD). Yesterday's report on April's air quality revealed the safety levels for 24-hour exposure were not exceeded anywhere in the territory, but the more stringent limits for exposure over one year were surpassed, particularly at Mongkok. Vehicle pollution is the main problem and Mongkok has the only street-level-based station for monitoring air pollution. The EPD's readings have shown a slow but steady decline in air quality over the past few years, with the exception of sulphur dioxide levels which improved in industrial areas from 1990 after low-sulphur industrial fuel was introduced. Mr Leung said pollution varied, depending on the weather, and looking at one month in isolation was not an indication of air quality. In April last year the readings for respirable particulates, which are linked to cancer, and of total suspended particulates, which impair lung function, soared to 50 per cent above health standards over several days.