How to interpret one-hour PM2.5 readings

A reader wants to know how he should interpret the one-hour PM2.5 concentration readings. Does an elevated band mean the air quality is unhealthy? How do the values in the one-hour reading correspond to PSI readings?

Reporter Audrey Tan explains.

PM2.5 pollutants are the main pollutants during periods of haze.

They are fine particles smaller than 2.5 microns in diameter, or a 30th the diameter of a human hair.

Since 2014, the National Environment Agency (NEA) has been providing readings of hourly concentrations of PM2.5 at, but many did not know what to make of them.

So in June, the NEA introduced bands and descriptors for one-hour concentration readings of PM2.5. There are four bands, from normal to very high.

For instance, the range of 0 to 55 micrograms per cubic metre (mcg/m3) is described as "normal", while anything above 250 mcg/m3 is considered "very high".

These bands do not show health impact, unlike the five categories - good, moderate, unhealthy, very unhealthy, hazardous - determined by the 24-hour Pollutant Standards Index (PSI).

This is because there are no scientific studies to show the short-term impact of haze on health. But as a gauge, the hourly PM2.5 concentration bands will correspond to the air quality ranges determined by the PSI, provided they remain the same for 24 hours.

For instance, the one-hour PM2.5 concentration at 10am on Aug 30 ranged between 2 and 12 mcg/m3, which is in Band 1 and is described as "normal". If concentrations of PM2.5 persist in this same band for the next 24 hours, it will correspond to good air quality.

If the one-hour PM2.5 concentrations become "elevated", which means its hourly concentrations range between 56 and 150 mcg/m3, it will correspond to unhealthy air quality, as determined by a 24-hour PSI reading of 101 to 200.

In a nutshell, if hourly PM2.5 concentration bands are maintained for 24 hours:

• Band 1 (0 to 55 mcg/m3) readings will correspond to good air quality (24-hour PSI reading between 0 and 50) or moderate air quality (PSI : 51 to 100). Under good or moderate air quality conditions, normal activities can be carried out.

• Band 2 ( 56 to 150 mcg/m3) readings will correspond to unhealthy air quality ( PSI: 101 to 200). Healthy people should reduce prolonged or strenuous outdoor activities.

• Band 3 (151 to 250 mcg/m3) readings will correspond to very unhealthy air quality ( PSI: 201 to 300). Healthy people should avoid prolonged or strenuous outdoor activities.

• Band 4 (251 mcg/m3 and above) readings will correspond to hazardous air quality (PSI: above 301). Healthy people should avoid going outdoors.

A version of this article appeared in the print edition of The Straits Times on September 03, 2016, with the headline 'How to interpret one-hour PM2.5 readings'. Subscribe