Nearly half of China's cities have heavily polluted air, according to the State Environmental Protection Administration (SEPA).

Yesterday SEPA Deputy Director Zhang Lijun told an international seminar on air quality management in Beijing that in most cities with a population more than 1 million, the percentages of sulphur dioxide and particles far exceeded the standard.

In response to the problem US researchers have suggested coal power plants need to be tackled first.

"Coal-fired power plants are major sources of sulphur dioxide, and it would be a good idea to start with them in the air pollution control campaign," said John Chang, team leader of indoor air research with the United States Environmental Protection Agency.

Chang and 65 other US experts were in Beijing for the seventh Sino-US Technology and Engineering Conference. They also visited two demonstration power plants fitted with advanced desulphurization facilities.

"We were told that more coal-fired power plants would install desulphurization equipment," said Chang. "The government is determined to control air pollution, but the key will be whether these power plants permanently adopt the equipment."

The government plans to cap sulphur dioxide emissions at 23 million tons by 2010, according to the 11th Five-Year Plan.

Coal-fired power plants alone emit more than 60 per cent of the total sulphur dioxide pollution, with SEPA statistics showing that 16 million tons out of the total 25.5 million tons of sulphur dioxide was emitted by coal power stations.

China currently lacks a continuous emission monitoring system to provide data and a basis for implementing a reward-punishment system for power stations, said Chang.