SINGAPORE - From this month, Singapore's air quality will, more often than not, be classified in the "moderate" range.
That's because on April 1, the old Pollutant Standards Index (PSI) was tweaked to incorporate the 24-hour concentration of PM2.5, the smallest polluting particles.
This is particulate matter (PM) of 2.5 microns in diameter or less, a micron being a millionth of a metre. (The human eye can see particles as small as 40 microns or the diameter of human hair.)
PM2.5 is so tiny that it penetrates deep into and gets embedded in lung tissue to cause or aggravate various lung and heart diseases.
This is because particulate matter is actually made up of solid or liquid droplets, or both, carrying acids, organic chemicals, toxic metals, bacteria and viruses.
By contrast, the PM10 that was reported in the PSI in the past comprises larger particles of between 2.5 and 10 microns in diameter.
These are trapped only in the nose and throat, and so they get removed when we cough, sneeze or swallow.
Thus, while they can irritate the upper airways, they don't damage the lungs. So health-wise, PM2.5 matters, not PM10.
When the National Environment Agency (NEA) reported the PSI with only PM10 in the past, we had mostly "good" air days. Now that the PSI includes PM2.5, available historical data tells us we're likely to get reports of "moderate" air quality for more than 90 per cent of the year.
You can check the real-time air quality over different parts of Singapore at aqicn.org as well. This international site has always included PM2.5 and, in the first three months of 2014, it reported "good" air quality for only a few parts of Singapore and for just a few days.
For almost all the hours of all the days, most parts of Singapore had only "moderate" air quality - even without forest fires in Sumatra causing us grief.
The NEA said recently that people can carry on normal activities if air quality is in the "moderate" range. But if prevention be the better part of valour, people should avoid outdoor activities when the air quality is "moderate".
You know people are worried when you find no air purifiers left on the shelves. Demand has risen seven-fold since January compared with the same period last year, according to big retail chains such as Courts and FairPrice.
But are portable air filters that claim to remove particulate matter from indoor air worth the money? They sell for anything from $300 to $1,700.
There are two types of such devices.
Less common these days are electronic cleaners that spray out charged ions into the air. These ions attach themselves to PM in the air, giving it a charge. When thus charged, PM will stick to surfaces like walls or furniture.
These air cleaners are less popular as they may produce ozone, which damages the lungs.
Most units sold here now are mechanical air filters that come with high efficiency particulate air (Hepa) filters. The filter media are made of microscopic glass fibres that can remove almost all PM2.5, which sticks to the fibres.
Units that cost a bit more may include an activated carbon filter to remove gases and odours.
Most also have a fan to circulate the cleaned air. This can be noisy but research indicates that the fan has to be turned on fully - at its noisiest - to be of benefit. Conversely, units marketed as being silent because they have no fan are typically less effective.
What about health benefits?
Doctors tend to ask patients with asthma or allergies to use Hepa air filters, but there is no good evidence that they significantly affect health outcomes in these or other conditions such as chronic heart or lung disease.
In a review of studies, the Institute of Medicine - a non-profit organisation that is part of the United States National Academies and provides independent advice on health issues - said it was not possible to draw "firm conclusions regarding the benefits of air cleaning for asthmatic and allergic individuals".
The institute, which relies on scientists who volunteer to do the studies that are subjected to a formal peer-review system, said it was also not clear that cleaning the air would even be highly effective in reducing symptoms per se.
In a 2008 study published in the official journal of the Pittsburgh- based Air and Waste Management Association, researchers concluded that if a portable Hepa air cleaner was used, any effectiveness was limited to a single room and not the entire dwelling. So it was "not likely" to help asthmatic patients because of their exposure to allergens in other rooms where no cleaners were running.
That study also indicated that building Hepa filters into the ductwork of a centralised air conditioning system for an entire home was more likely to be of benefit than portable air cleaners.
For most of us lesser mortals, however, portable air cleaners will have to do. If you do decide to get one, check its efficiency and effectiveness first.
Efficiency tells you how much airborne PM it can remove. If its efficiency is 99 per cent, it removes 99 per cent of the PM in the air that passes through it.
Most units nowadays claim 99 per cent efficiency.
Its effectiveness tells you how much it can reduce PM within a specified space. This depends on the unit's efficiency, of course, but also the volume of air it has to deal with.
For example, if the cleaner can process only 10 cubic metres of air per minute, it is 10 times less effective than one that can process 100 cubic metres per minute.
But most academic studies do not identify air cleaners by brand.
If you want to know the effectiveness of different brands, the best resource is the searchable directory at www.cadr.org (run by the Association of US Home Appliance Manufacturers).
A recent study of the air filters identified by brand on this website found that those with moderate- to-high effectiveness ratings were sufficient for home use where PM was the main concern.
But once you've picked the portable unit you want, note that where you place it can affect its effectiveness too. Make sure that its air intake and output are not blocked by furniture, walls or other obstructions.
Also, for the best results, all the doors and windows of the room must be closed, so that means you will need to run the air conditioner as well.
Finally, Hepa filters need to be regularly cleaned and periodically replaced. But be careful not to release the PM back into the air when you do so. Handle the filters gently, don an N95 face mask when washing them and dispose of your old filter media in plastic bags with a tight knot.
This article was published on April 6 in The Straits Times.Get a copy of The Straits Times or go to straitstimes.com for more stories.