Haze Singapore readings : PSI ( Pollutant Standards Index ). PM 2.5 or AQI ( Air Quality Index) ?



Got haze or not?



24 hour PSI from NEA is at 80

That is within the healthy range. Yet, when you look out the window and take a breath, it smelt a lot smokier than what a 'healthy' range would feel like. Should you trust the readings and head out for your morning run or trust your nose and stay indoor with the air purifier on?




The 24 hours PSI readings had always been controversial since it was introduced back in 2015. Prior to that in 2013, the 3 hours PSI was taken as the standard gauge. Since the haze is back in 2019, other indicators such as AQI had emerged as alternative readings to gauge the severity of the haze, Despite NEA indication that the 24 hours PSI is the correct way to measure air quality, it may not necessarily be the one to rely upon if you intend to step out of the door at the moment.
Source: NEA

Currently, there are at least 3 other alternatives to NEA 24 hour PSI to assess the haze. Here is a breakdown of what they are and what are they measuring.

NEA 24 hour PSI
This measures the average haze level over 24 hours. Since it is a measurement of 24 hours of haze activity, it is generally lower than 1-hour measurement especially when air quality deteriorates significantly over a short period of time. The rationale for recommending this measurement is that one-hour exposure to air may not be meaningful because "scientific findings ( on the effects of particulate matter on a person's health) are based on 24-hour exposure to PM2.5" 

In truth, many would disagree with the statement especially when there is a thick cloud of haze enveloping the skies.
Source: NEA

It may be more useful to use the 24 hour PSI to plan ahead for activities rather than use it to gauge the current haze outlook


NEA 24 hour PSI can be found here

NEA 1 Hour PM 2.5
PM2.5 are fine particles in the air. Particles in the PM2.5 size range are able to travel deep into the respiratory tract, reaching the lungs. Exposure can cause short term health effects such as eye, nose, throat and lung irritation among other symptoms.  

The readings bands are as follow
1-hr PM2.5 reading (µg/m3)BandDescriptor
0 - 55INormal
56 - 150IIElevated
151 - 250IIIHigh
Above 250IVVery High
As this is a one-hour measurement, it would be a better gauge to use for immediate activities. Unfortunately, NEA does not specify which levels are considered unhealthy. At elevated band, the equivalent PSI reading is usually above 100 (unhealthy). It would be better if NEA made the readings uniform to the 24 PSI reading for easier reference for everyone.

NEA 1 hour PM 2.5 readings can be found here.

AQI (Air Quality Index)
The AQI uses a measurement matrix similar to PSI. It is used by the United States Environmental Protection Agency (EPA). Data is extracted from 4 sources: The citizen weather observer programme, NEA Singapore, the World Meteorological Organisation and Malaysia's Department of Environment.  It is not ascertained that the readings are base on hourly or 24-hour average PM 2.5 readings.

AQI Readings can be found here.


Other Sources
There are apps such as Sg Weather , Haze@SG and websites such as Originally US that uses their own methodology to derive the PSI reading. It is interesting to note that most use NEA's official PSI formula to deduce the 'actual' hourly PSI measurements. Given that knowledge, it would still be best to rely on NEA readings at the end of the day. On that note, we think the app Haze@SG is one of the better apps as it pulls information directly from both NEA and AQI and displays the readings direct from the source.

The best gauge to use for immediate activities would be the hourly PM 2.5 readings from NEA. The next best gauge would be your own nose and eyes. IF for some reason the PSI readings are at the healthy range but all you see ahead of you is a mist of smoke and every breath you take smells of fumes, put on the N95 mask ( not the surgical one ) before you head outdoors. Even if you are indoors, do not assume that the air is free of pollutants. It may be wise to get a purifier than leave it to chance.

Regardless, let's hope the haze would clear up soon...


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