This story was first published in The Straits Times on March 13, 2014.
Air purifiers and masks are being snapped up as Singapore's air quality worsens.
But how useful are these products in filtering out pollutants, especially the tiny and hazardous PM2.5 particles?
Some, such as N95 masks and certain models of air purifiers, can be effective depending on how successfully they can remove the smaller microscopic particles.
The N95 mask, for instance, is "at least 95 per cent efficient against fine particles that are about 0.1 to 0.3 microns" in size, according to the Health Ministry's website. It added that the N95 mask is 99.5 per cent efficient against particles such as those 0.75 microns or larger.
The PM2.5 particle, which is 2.5 microns, is an air pollutant associated with vehicle emissions and the haze. It will be included in the Pollutant Standards Index (PSI) from May.
Whether an air purifier is effective enough to trap that and smaller particles cannot be determined just by price, distributors said.
Ms Joey Feng, a sales manager at Air and Odor Management, which sells purifiers from brands such as Daikin, said: "Price also depends on other factors, such as brand, capacity and the type of filters and technology used."
Instead, she advised consumers to look at model specifications as a clearer indicator of how useful the machines are in getting rid of particles.
Air purifiers which come with high-efficiency particulate air (Hepa) filters, for example, can get rid of particles of 0.1 to 0.3 microns in size. They have strong filters which trap particles within fine meshes of fibreglass.
This means the larger PM2.5 particles can be filtered out easily, said Mr Raymond Eio, 49, an assistant manager at Massmark International, which distributes Honeywell and Blueair purifiers.
He added that prices of air purifiers at his firm range from $499 to $1,699, depending on model, brand and capacity.
Dr Jason Phua, head and senior consultant in the division of respiratory and critical care medicine at the National University Hospital, advised the public to "consider an indoor air purifier that is fitted with a Hepa filter".
Mr Joe Eades, a member of the Health, Safety and Environmental Technical Committee at The Institution of Engineers, Singapore, said consumers should be cautious when buying air purifiers, adding that only those "complete with Hepa filters" can completely remove PM2.5.
He also advised: "Consumers should avoid selecting ionisers as they are only good for removing bacteria and viruses but have no effect on removal of particulates."