Some 200,000 diesel engines pumping out nitrogen dioxide into windless conditions caused record air pollution levels yesterday, the Environmental Protection Department (EPD) said. The highest reading so far this year, triggered warnings for asthmatics, heart patients and elderly to stay indoors and avoid exertion. Many districts overshot the unhealthy threshold of 100 micrograms of respirable particles per cubic metre - although environmentalists calculate street-level rates could be 40 to 75 per cent over the limit. Pollution in urban areas such as Central, Western, Wan Chai, Eastern, Kowloon City, Shamshuipo, Wong Tai Sin and Yau Tsim Mong districts leapt to 117 micrograms per cubic metre. The industrial areas of Kwun Tong, Kwai Tsing and Tsuen Wan recorded 113 micrograms. The New Territories and outlying islands soared towards the danger zone, registering 92 micrograms. Air pollution readings were taken six storeys above ground level and measurements taken on the street where most people are breathing would have been much higher, said Citizens Party environmental spokesman Lisa Hopkinson. 'The EPD monitors air several storeys up so it's just common sense that it's going to be worse on the ground, whether it's 20 per cent or 50 per cent worse,' she said. EPD spokesman Pang Sik-wing said the pollution came mainly from diesel vehicles. Ms Hopkinson urged the Government to speed up liquid petroleum gas vehicle trials and take action on Hong Kong's diesel pollution. Friends of the Earth described the smog as a 'black tide in the air' and said there was no guarantee it would clear, given the proximity of the fires and haze sweeping Southeast Asia. Spokesman Plato Yip Kwong-to said the 'aggregate effects' of fires in Indonesia and the Philippines could drift as far as Hong Kong because of the summer monsoon winds. And that would send official readings up to at least 150 micrograms. Today's official readings could be lower, in the moderate range between 75 and 85 micrograms range.