Top of the hit list are the two main pollutants particulate matter (PM2.5) and sulphur dioxide, both of which are known to cause breathing problems and aggravate respiratory diseases such as asthma.

The Government hopes to reduce PM2.5 emissions from the current annual mean of 16 micrograms per cubic metre of air to 12 micrograms per cubic metre by 2020.

PM2.5, which are fine particles in air, are emitted by diesel vehicles and has been linked to respiratory and cardiovascular diseases.

Besides keeping PM2.5 emissions in check, the authorities also hope to cap sulphur dioxide levels at an annual mean of 15 micrograms per cubic metre by 2020.

Sulphur dioxide is known to worsen respiratory conditions. It is currently hovering at about 11 micrograms per cubic metre of air, but this level is expected to increase over the following years due to economic growth and increased demand for energy.

After 2020, the aim is to sustain these cleaner levels for another 10 years at least despite expected growths in the economy and vehicle population.

Minister for the Environment and Water Resources Yaacob Ibrahim said these targets were well within similar standards set by the United States' Environmental Protection Agency.

To meet these emission goals, the Government will work with transport operators and key industries such as oil and petrochemicals.

To tackle pollution on the roads, the Land Transport Authority (LTA) will carry out a trial of hybrid diesel buses with public and private bus operators.

Read the full report in Tuesday's edition of The Straits Times.