Long-suffering Hongkongers accustomed to covering their noses to ward off pollution had a welcome break last month. The air quality in July was the best recorded in 11 years and the trend has continued this month, apart from a short break early on. Steady winds from the south, where there are no pollution sources, and frequent heavy rain are the reasons for the clean air, environmentalists say. Not one of the 11 general air monitoring stations recorded a high air-pollution index - above 50 - at any time last month. The readings were low - 0-25 - for 5,974 hours or more than 70 per cent of the time and medium - 26-50 - for the rest. A total of 22 days had low air pollution, while the number of days with medium pollution was nine. This was better than both the previous month and the same month last year. In June, there were 649 hours, or 8.23 per cent, with an air-pollution index higher than 50. In July last year, a total of 919 hours, or 12 per cent of the time, had an index higher than 50 and there were four hours above 100. Roadside air pollution also improved. The three roadside stations in June recorded 148 hours, or 7 per cent, with the air pollution index reaching very high, between 101 and 200. But in July, there was no such high figure. The Observatory did not record any reduction in visibility from haze in July. Visibility is said to be reduced when it is lower than eight kilometres with no natural causes such as fog, mist or rain. High readings were recorded for several hours on August 4, 5 and 6 by some general stations, but apart from these the indices for all other days were below 50. Dr Cheng Luk-ki, division head of scientific research and conservation of Green Power, said the good air quality was due to the sea wind from the south, where there was no pollution source. 'The wind speed is relatively high, which can disperse air pollutants. The rain in July also helped wash away the pollutants,' he said. According to the Observatory, the total rainfall in July was 469.4mm, compared to 389.4mm in July last year. Cheng said Green Power records showed that when typhoons came from the east, the air pollution index often exceeded 100. 'When a typhoon with a relatively high central speed enters an area 1,300 kilometres east of Hong Kong, the air pressure will begin to drop. There will be no wind or rain. This will bring poor air quality to the city,' he said. Both of this year's typhoons so far had come from a southerly direction with not such a strong central speed, which prevented the air quality problem occurring. He said the southwest monsoon, which brings showers, also helped maintain air quality this month.