askST: How do I interpret the new 1-hour PM2.5 readings?

A reader wants to know how he should interpret the new 1-hour PM2.5 concentration reading. Does an elevated band mean the air quality is unhealthy? How do the values in the 1-hr 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 hourly concentrations of PM2.5 on its haze microsite at www.haze.gov.sg. But many did not know what to make of these readings.

So in June this year (2016), the NEA introduced bands and descriptors for 1-hour concentration readings of PM2.5.

There are four bands in all, 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 in the same band for 24 hours.

For instance, the 1-hour PM2.5 concentration at 10am on Aug 30 ranged between 2 and 12 mcg/m3, which is in Band 1 and 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.

Similarly, if the 1-hour PM2.5 concentrations become "elevated", which mean 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 5) or Moderate air quality (PSI : 51 and 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 and 200).

Under unhealthy air quality conditions, healthy people should reduce prolonged or strenuous outdoor activity.

  • Band 3 (151 to 250 mcg/m3) readings will correspond to Very Unhealthy air quality ( PSI: 201 and 300).

Under very unhealthy air quality conditions, healthy people should avoid prolonged or strenuous outdoor activity.

  • Band 4 (251mcg/m3 and above) readings will correspond to Hazardous air quality (PSI: above 301).

Under hazardous air quality conditions, healthy people should avoid going outdoors.

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