The one-hour PM2.5 pollutant reading reached a record high on Monday but experts are divided on how harmful that will be to health.
While some say even a one-time event could be considered potentially hazardous according to standards set by the United States Environmental Protection Agency (EPA), there are others who believe that only prolonged exposure to high PM2.5 concentration levels should be a cause for concern.
On Monday, the National Environment Agency (NEA) said the PM2.5 concentration level of 471 in the western areas of the country at 11pm was the highest recorded so far this year.
Conditions on that day had deteriorated sharply in several parts of the island after denser haze from areas south of Singapore was blown in by the prevailing winds.
Senior research scientist Santo Salinas at the Centre for Remote Imaging, Sensing and Processing at the National University of Singapore said that according to the EPA, PM2.5 concentrations higher than 65.5 micrograms per cubic metre are considered "unhealthy limits and potentially hazardous."
"I would say anything above 65 micrograms per cubic m is potentially harmful and steps should be taken to protect the more vulnerable," said Dr Salinas.
PM2.5 pollutants are smaller than 2.5 microns in diameter, or a 30th the diameter of a human hair, and unlike coarser particles, the body is not equipped to filter them out.
Long- term exposure to them on a regular basis has been linked to increased risk of death from complications such as lung cancer or heart disease.
The short exposure to PM2.5 concentrations over one or two hours at a level like Monday's exacerbates respiratory and cardiovascular illnesses in addition to irritation of the throat and nose, said research scientist Erik Velasco from the Singapore-MIT Alliance for Research and Technology.
"My recommendation is to check constantly the one- hour PM2.5 concentrations reported every hour by NEA," said Dr Velasco.
"If the current concentration is over 150 micrograms per cubic m, try to stay indoors."
The NEA has stated that only the 24-hour Pollutant Standards Index (PSI), which takes into account the PM2.5 concentration levels, is used as a basis for the health advisories issued by the Ministry of Health.
Associate Professor Richard Webster, from the Nanyang Technological University's School of Physical and Mathematical Sciences, said the spike is only really significant if it lasts for several days.
Compared with cigarette smoking, said Prof Webster, the total amount of PM2.5 breathed in over 24 hours' exposure to a concentration of 471 micrograms per cubic m would still be less than half the PM2.5 found in one cigarette.
He said the true health effects of PM2.5 from environmental samples are not yet accurately known.
"Cigarette smokers experience astronomically higher PM2.5 levels than what is given on the PSI scale and many live to old age, or at least the effects of smoking cigarettes do not catch up with them for many years. This would imply that the classifications on the health effects of the PM2.5 (unhealthy, hazardous, etc) are considerably overestimating the seriousness of the problem, at least in the short term," he said.
"The best thing to do is to carry on as normal and not to worry about it too much. However, people who exercise vigorously might like to slow down when the PSI level gets very high, since the amounts that they breathe in will increase proportionally to their breathing rate."
In its advisory yesterday, NEA said the 24-hour PSI for the next 24 hours is expected to be in the low to mid sections of the unhealthy range, and may improve to the high end of the moderate range if winds are favourable.
As of 7pm yesterday, the 24-hour PSI reading was at 93-167.
This article was first published on October 21, 2015.
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