"The air in Metro Manila is still dirty but significantly improving," Ramon Paje, an undersecretary at the environment department, told reporters. "But we hope to meet the national standard by 2010."

Since 2000, a year after a clean air law was passed, the country has been using Euro 1 standards as a benchmark to gauge the cleanliness of the air in the sprawling capital of 12 million and four surrounding provinces.

"We're slowly moving to the Euro 2 standard and will probably hit the highest level at Euro 4 by 2010," Cesar Siador, head of air quality management at the environment department, told Reuters.

Siador said the department had been in close consultation with other government agencies and the private sector to revise the air pollution standards, matching the benchmarks used in Malaysia, Singapore and Thailand.

For instance, permitted emissions from motorcycles will be lowered to 4,500 parts per million (ppm) from 7,800 ppm by next year and cut further to 3,000 ppm by 2007.

For vehicles using unleaded gasoline, the carbon monoxide level was being reduced to 3.5 percent and hydrocardon content to 600 ppm, based on Euro 1 standards.

The environment department was also seeking assistance from the United States to acquire a radar system for Manila's streets to catch smoke-belching vehicles, a method deemed more effective than the current practice of roadside arrests.

Siador said car manufacturers would be encouraged to start selling engines that pass the stringent Euro 2 to Euro 4 levels by 2006 as part of efforts to reduce emissions.

Paje said 70 to 80 percent of air pollution in the Philippines was caused by vehicle emissions. The remaining 20 to 30 percent was caused by factories, construction work and burning of garbage.

Citing studies by the health department, Paje said there was a direct relationship between the rise of respiratory diseases in Manila and the poor quality of air, making transport workers more vulnerable to respiratory diseases such as asthma and sinusitis.