When Singapore adopted the Euro II vehicle emission standard in 2000, the initial plan was to raise this to Euro III and then Euro IV.

But in March 2004, the Environment and Water Resources Ministry announced it would go straight to Euro IV by Oct 1 last year, citing growing concern over a fine airborne pollutant called PM2.5.

The term describes fine particulate matter that is 2.5 microns or smaller in size. These particles have been found to be a serious health hazard because they can get lodged in the deep recesses of our lungs.

Studies conducted by the National University of Singapore found that diesel vehicles contribute to half the PM2.5 in the air here.

The ministry said while the air here meets most pollutant standards, the PM2.5 content is high, 40 per cent higher in fact than the level recommended by the US Environmental Protection Agency (USEPA).

Singapore is not alone. Most cities in the world fail the USEPA standard and many are taking steps to address it.

India and China have been replacing aged diesel city buses with compressed natural gas (CNG) models, which emit very little or no PM2.5.

In Hong Kong and Tokyo, the taxis run on liquefied petroleum gas (LPG).

Thailand's ubiquitous tuk-tuks now run on LPG, too.

In 2002, fuel stations in Hong Kong became the first in Asia to start selling ultra-low sulphur diesel, which produces less PM2.5 when spent.

London is trying out a fleet of diesel-electric hybrid public buses, while New York took delivery of America's first hybrid school bus last July.

Singapore's attempts to switch to CNG as a transportation fuel have met with several hurdles. Transport operators have found CNG vehicles to be costlier to buy and costlier to run.

The situation has not been helped by the absence of any CNG refuelling stations on the main island.

Enter the Euro IV plan. Euro IV vehicles emit about 90 per cent less particulate matter than Euro II models.

In announcing the roll-out, the ministry dangled carrots to businesses and vehicle owners who switched to Euro IV models earlier than October last year.

To defray the intrinsically higher running cost of Euro IV vehicles, reduced forms of these incentives will run till the end of this year.

These incentives were as high was 100 percentage points off the vehicle tax for cabs, but a mere 5 points off for commercial vehicles. Hence only some companies made an early switch to Euro IV models.

In embracing Euro IV, the ministry hopes to meet the USEPA's PM2.5 standard by 2014.