From May, Singapore will have a new Pollutant Standards Index (PSI) that includes tiny, hazardous particles referred to as PM2.5.
The National Environment Agency (NEA) will also provide hourly updates that are more current on PM2.5 levels in the air. Now, the updates are given hourly but are averaged from readings in the previous 24 hours, said Minister for the Environment and Water Resources Vivian Balakrishnan during Tuesday's debate on the ministry's budget.
We look what is PM2.5, and why it matters.

What is PM2.5?
PM2.5 is a fine particulate matter less than 2.5 microns in size - or a thirtieth of the diameter of a human hair.
Over-exposure to it increases the risk of heart and lung illnesses and can reduce an individual's lifespan. Because the particles are so small, they can penetrate deep into the lungs, and this makes them more toxic. Once these fine particles enter the bloodstream, they go anywhere in the body and it will be very difficult for the body to cleanse it.

What contributes to PM2.5?
PM2.5 can come from many sources, such as vehicles and burning plants. It can also be formed by chemical reactions in the atmosphere.

Isn't PSI good enough?
PSI measures sulphur dioxide, nitrogen dioxide, ozone, carbon monoxide and PM10. The PSI does not include PM2.5 as a standalone pollutant, even though it is measured as a subset of PM10, which are particles smaller than 10 microns in size. However, the fraction of PM2.5 within PM10 may vary.

Do other countries track PM2.5?
Countries like the United States use the Air Quality Index, which specifies PM2.5 concentrations. Malaysia uses the Air Pollution Index which also measures the PM2.5 levels in the air.
The NEA has been publishing PM2.5 readings separately, in addition to the PSI readings. It used to publish the annual average PM2.5 readings, but has been reporting it hourly since June last year, when Singapore was hit by the worst haze in its history.

What are Singapore's PM2.5 levels like?
Singapore's annual mean PM2.5 level has hovered between 16 and 19 micrograms per cubic metre (µg/m3) over the past six years, which does not meet the World Health Organisation standard of 10 µg/m3. The NEA aims to cut this to 12 µg/m3 by 2020.

What difference will the new PSI make?
Based on the present PSI, each of the past five years had between 91 and 96 per cent of "good" air quality days, and just 4 to 9 per cent of "moderate" days.
But with the revised PSI which includes PM2.5, the figures would be 1 to 4 per cent of "good" days each year and 92 to 98 per cent of "moderate" days.