SINGAPORE - What you see is not what you get, at least when it comes to the haze, as poor visibility could be due to water vapour instead of haze particles.
On Monday (Sept 23) and Tuesday, some hazy conditions were observed even though air quality measures did not suggest the haze had suddenly worsened.
While people might be tempted to gauge air quality on how clear the air seems, it is actually not reliable, a researcher pointed out on Tuesday.
The National Environment Agency (NEA) also clarified as much last week and, again, on Monday.
Air pollution researcher Erik Velasco told The Straits Times: "Do not just trust your visual perception as hazy skies can also be triggered by mist. The one-hour PM2.5 readings are a better indicator of the actual air quality."
Dr Velasco added that for specific haze events such as those caused by wildfires on neighbouring islands, PM2.5 readings could even be better than the 24-hour Pollutant Standards Index (PSI), which reflects air quality during the previous 24 hours.
At 6am on Wednesday, the one-hour PM2.5 reading was 18-24 micrograms per cubic m, in the normal range, while the 24-hour PSI reading was 71-83, in the moderate band.
NEA also explained on Sept 19 that poor visibility does not necessarily mean that the air is more polluted.
"A significant factor of visibility is the amount of water vapour in the air. Water droplets scatter light, resulting in lower visibility," said the agency.
This means that on more humid days, air quality can look a lot worse than it actually is.
The environment agency added that some larger air pollutant particles, such as smoke, also attract water molecules, saying: "This results in larger particles that scatter more light. Such water-bound particles cause poor visibility to persist for a longer time."
In a haze update on Monday, NEA said the increased moisture in the atmosphere coupled with the slight haze on Monday morning reduced the visibility in some parts of the island to between 3km and 5km. This gradually improved to between 7km and 8km over most areas in the afternoon.
Experts have said that larger air pollutant particles are less worrying than the PM2.5 pollutants, as the smaller PM2.5 particles can become trapped deep in the lungs and are tiny enough to pass through linings into the bloodstream.
PM2.5 particles are so called because they are no larger than 2.5 microns, or a thirtieth the diameter of a strand of human hair.
With long-term exposure on a regular basis, there is an increased risk of death from heart and lung complications such as lung cancer or heart disease.
The 24-hour PSI measures concentration levels of six pollutants for the past day, with PM2.5 levels having the most influence on PSI readings during transboundary haze episodes.
The other five pollutants PSI measures are particulate matter (PM10), sulphur dioxide (SO2), nitrogen dioxide (NO2), ozone (O3) and carbon monoxide (CO).
NEA has recommended that the public use PM2.5 concentration readings to make decisions on whether to pursue outdoor activities at a particular point in time.
It said: "The one-hour PM2.5 concentration readings may be used to decide on your immediate activities, such as going for outdoor exercise. The readings reflect the average PM2.5 level over the past one hour, and indicates the current air quality."