Answering charges that not enough is being done to battle rising air pollution, Secretary for Environment Works and Transport Sarah Liao said Friday natural gas as a fuel source is impractical and foreign countries are gradually overcoming many technological issues for hydrogen use, such as developing tanks that can safely store hydrogen.

The big question now, she said, is infrastructure. "We need to better prepare ourselves by developing suitable infrastructure, including finding the right places to store hydrogen tanks and gas filling stations," she said.

The hydrogen fuel cell vehicle could take decades to perfect as the technology is still very new and expensive and there are no hydrogen fuel cell vehicle refilling stations in Hong Kong, only a handful in Japan and one in Singapore, according to Fuel Cells 2000, a group promoting the technology.

In the United States, automakers, along with federal officials, announced a joint program to develop vehicles that run on hydrogen two years ago.

Automakers expect to see hydrogen- powered cars mass-produced for the US market within the next 10-15 years.

Liao also said the joint measures agreed by Guangdong and Hong Kong to combat pollution in the Pearl River Delta have started to bear fruit with the recent installation of desulfurization devices at five power plants in the province at a cost of about 0.15 yuan (0.14 HK cents) per kilowatt hour.

Liao dismissed calls for introducing natural gas as a vehicle fuel, saying the government will have problems in finding locations for filling stations.

"It is difficult for us to find places to store natural gas and operate gas fill- up stations in a very crowded city like Hong Kong. We need to keep the natural gas tanks away from the [liquefied petroleum gas] tanks," Liao said.

The government has spent HK$1.2 billion on retrofitting LPG taxis and other vehicles while introducing ultra- low sulfur diesel to combat emissions from diesel vehicles.

According to an Audit Commission report in April, Hong Kong's air is among the worst in the developed world. The delta's rapid industrial development has been the leading cause of pollution, with about 80 percent of air pollutants in Hong Kong coming from the mainland.

Democrat Martin Lee said pollution has reached an intolerable level.

"Earlier I went to The Peak and I still saw thick smog clouding Hong Kong. Secretary Liao has explained that it was due to an increase of diesel vehicles in Guangdong. This is an unacceptable excuse," Lee said. "I think what Hong Kong people want are concrete results, not excuses."

Jasper Tsang of the Democratic Alliance for the Betterment and Progress of Hong Kong also criticized the government for not erecting noise screens on roads close to residential areas.

Liao said: "We need scientific ways to resolve the problem. We already have plans to tackle the noise problem. I hope you can be tolerant because we still need time to put the measures in place."