Rain almost as acidic as vinegar fell in Yuen Long, Central and Western last year. The level of acidity was the worst in five years and even worse than that recorded in Guangdong, Environmental Protection Department figures revealed. The average annual pH level of rain - a measure of its alkalinity or acidity - was 4.39 in Central and Western, and 4.29 in Yuen Long last year, more acidic than the 4.71 average recorded in Guangdong. Rain collected in Yuen Long showed levels as severe as 3.1, while 3.2 was recorded in Central and Western. Rain with a pH level below 5.6 is regarded as acidic, with three or below indicating an acidity similar to vinegar. The department's principal environmental officer, Raymond Leung Pak-ming, said the average acidity was 'moderate' and comparable to levels in places such as London. He said that the differences in the levels over the past five years were not significant. The current levels of acid rain - which contains pollutants such as sulfates, mainly produced by the burning of fossil fuels - would not have a significant impact on the ecosystem, Mr Leung said. But Ho Kin-chung, of the Open University's environmental science programme, said that while acid rain in Hong Kong did not pose an immediate threat, it might change the ecosystem in the long term. 'It will change the soil textures and, eventually, affect the biodiversity of plant species,' he said. Lee Ho-yin, acting director of the architectural conservation programme at the University of Hong Kong, said acid rain also posed a direct threat to heritage buildings as it would damage their fired clay, timber and lime plaster. The damage would occur in a relatively short time. 'In Yuen Long, most of the heritage buildings are of traditional Chinese construction using materials that are more vulnerable than modern materials to acid rain,' Dr Lee said. 'Acid rain will dissolve these organic materials over a relatively short period - in years, rather than decades. Many of the timber and plaster surfaces are decoratively painted in mineral paint, and acid rain will ruin the paintwork quickly.' Mr Leung said the department could not conclude that the acid rain was caused by local power generation, as it might also be attributed to the mainland. 'The result is not unexpected as acid rain is more than just a local issue.' Professor Ho agreed, saying more evidence had to be collected to determine the source. 'Less rainfall last year might make the rain more acidic because of a weakening of the washing effect.'