Why does haze in Singapore seem worse than PSI readings suggest?
As the haze situation worsens in Singapore, many people have flocked to Facebook and Twitter to express their irritation over the poor air quality, as well as their doubts over whether the official PSI updates are reflecting the haze situation accurately.
“No way is the PSI 40 right now. Can barely see past Sheares Bridge from downtown. #sghaze,” said Twitter user Anna Hamill.
“The PSI is 83 now. How can it be so low when it is so smelly? Burning smell all over the house. #sghaze,” said another Twitter user Serene Ho.
And it looks like some of the skeptics have moved to follow @SGHazeAlert, an account claiming that it’s PM2.5 readings are “accurate”.
“We use PM2.5 only as it's more harmful to us than PM10. NEA uses a mix,” said in @SGHazeAlert’s profile. This account was set up by the founders of scoo.ps, a twitter account that tweets stories related to Singapore.
So why do the PSI readings appear low when the visibility is so poor?
NEA has provided a brief explanation on its website: “When a 3-hour PSI reading is released during the smoke haze periods, it is for air quality for the immediate past three hours. It therefore may not correspond to what you may observe at the point when you hear the PSI announcement.”
The difference between 24-hour PSI, 3-hour PSI and PM2.5
NEA’s 24-hour PSI readings are based on a “24-hour average of PM10 concentration levels”, which is the “dominant pollutant during haze episodes”. The 3-hour PSI measure a 3-hour average of PM10 concentration levels.
PM 2.5 refers to particulate matter that is smaller than 2.5 microns and is also taken into consideration in the PSI readings. NEA also publishes PM2.5 readings separately.
Particles smaller than 2.5 microns have been deemed to be more dangerous than PM10.
According to the United States Environmental Protection Agency, “Particles less than 2.5 micrometers in diameter (PM2.5) are referred to as "fine" particles and are believed to pose the greatest health risks. Because of their small size, fine particles can lodge deeply into the lungs.”
More Twitter reactions below:
