Haze In Singapore: How Is PSI Calculated?

With the recent spike in PSI readings such that the MOE had to call for a closure of all Primary and Secondary Schools on 25 September 2015, have you ever wondered how the various agencies determine the PSI readings and levels?

In other words, how is PSI calculated?

The PSI is based on six pollutants particulate matter (PM10), fine particulate matter (PM2.5), sulphur dioxide (SO2), carbon monoxide (CO), ozone (O3) and nitrogen dioxide (NO2). For each pollutant, a sub-index is calculated from a segmented linear function that transforms ambient concentrations onto a scale extending from 0 through 500.

The breakpoints used in defining each of the six pollutant sub-indices are listed as follows:

Using the following equation:

Finally, the overall index is calculated as the maximum of sub-indices:

PSI = maximum (I1, I2, I3, I4, I5, I6)


To put it simply, here is an example of the PSI calculation.

So this is how PSI readings are computed by National Environment Agency. This is for those who are curious but nevertheless, we should all be aware and always take precautionary measures under such unpredictable hazy conditions. HEALTH IS WEALTH!


Source: http://www.haze.gov.sg/docs/default-source/faq/computation-of-the-pollutant-standards-index-(psi).pdf