As haze continues to affect air quality across Singapore in recent weeks, many households are retrieving air purifiers that may have been stored away for years. Key questions arise: Can these devices still perform effectively after prolonged storage? How frequently should filters be changed? And what is the financial impact of operating one throughout the day?
The National Environment Agency (NEA) advises the use of portable air purifiers to help reduce indoor haze particles. CNA consulted experts and retailers to provide guidance on maximising air purifier effectiveness while managing costs and maintenance requirements.
Can an Old Purifier Still Work Effectively?
According to Professor Steve Yim, director of Nanyang Technological University’s Centre for Climate Change and Environmental Health, an air purifier left unused for several years should generally remain functional if it was stored in a dry, pest-free environment.
Before reactivating the device, households should inspect the power cord and test all fan speed settings.
Regarding filters, unused high-efficiency particulate air (HEPA) filters that remain sealed in their original plastic packaging do not significantly deteriorate over time.
However, filters that have been opened or previously used may be affected by moisture, potentially promoting mould or bacterial growth, Prof Yim noted.
Activated carbon filters can also absorb odours and gases from surrounding air even when the purifier is switched off.
“Given the seriousness of the current haze situation, it is recommended to change (to) a new filter,” he advised.
Professor Rajasekhar Balasubramanian, NUS Keppel Professor in Sustainability Solutions at the National University of Singapore’s College of Design and Engineering, suggested that households keep spare HEPA filters on hand as a precaution.
Signs Your Purifier May Need Replacing
Indicators that the entire purifier may need replacement include weaker airflow, unusual noises, lingering odours or a visibly clogged filter, according to a Dyson spokesperson.
A Philips spokesperson noted that many of these warning signs could stem from a clogged filter rather than a faulty appliance. Consumers should therefore attempt filter replacement and unit cleaning before deciding whether to purchase a new device. If problems persist, the purifier itself may be worn out.
Storage Recommendations
When storing an air purifier between haze episodes, Dyson recommends switching it off, unplugging it and wiping it down. The filter should remain fitted to prevent dust from entering internal airflow paths, while the appliance should be stored upright in a clean, dry indoor location away from direct sunlight and humidity.
Before using it again, consumers should verify that airflow paths are clear and replace the filter if necessary.
How Often Should Filters Be Replaced?
There is no universal answer, as filter lifespan depends on usage frequency and operating conditions. Factors include outdoor PM2.5 concentrations, the amount of outdoor air entering the home, room size, filter quality and indoor activities such as smoking, said Prof Yim.
A Xiaomi Singapore spokesperson noted that filter lifespan also depends on the purifier’s operating speed and filter design.
During haze episodes, households may operate purifiers for extended periods and at higher fan speeds, increasing the amount of pollutants trapped by filters and potentially shortening their lifespan. Consumers should therefore consider both the filter status indicated by their device and their actual usage conditions when deciding whether replacement is needed.
Philips recommends replacing HEPA and carbon filters when the device’s filter replacement alert appears, typically every six to 12 months. Other signs include a noticeable drop in fan motor airflow and persistent strange smells, according to Blueair.
When Is an Air Purifier Needed?
Air purifiers become particularly useful when PM2.5 concentrations rise, although the need also depends on individual health circumstances.
Prof Yim explained that average PM2.5 concentrations of 55 micrograms per cubic metre (µg/m³) and below are considered within the normal range under Singapore’s air quality advisory bands for the general public. However, vulnerable groups—including infants, elderly people and those with chronic respiratory conditions—may benefit from using air purifiers at lower concentrations.
He “strongly recommended” using an air purifier when average PM2.5 concentrations exceed 55 µg/m³, entering the elevated range.
Even in air-conditioned environments, an air purifier remains necessary because air-conditioner filters only trap large particles and are not designed to filter fine particulate matter like PM2.5, said Prof Yim.
When selecting an air purifier, consumers should look for one with an effective HEPA filter designed to capture fine airborne particles. Prof Yim cautioned against relying on marketing labels such as “HEPA-type” or “HEPA-like,” which may refer to lower-grade filters. A “true HEPA” filter captures at least 99.97 per cent of airborne particles measuring 0.3 microns, he said.
Is One Air Purifier Enough for an Entire Flat?
Another important consideration is the clean air delivery rate (CADR), which measures the purifier’s effectiveness based on room size and the volume of clean air produced.
Prof Balasubramanian said consumers should choose a purifier with a CADR appropriate for the intended room. According to NEA, an air purifier should have a smoke CADR of at least three times the volume of the room, measured in cubic metres.
However, one powerful purifier cannot necessarily clean an entire HDB flat. “Walls and doors block airflow, so one central air purifier cannot clean an entire HDB flat,” said Prof Yim.
For a typical three-room HDB flat of 60 sq m to 70 sq m, he recommended one high-CADR unit in the living area and one in the main bedroom. For a four-room or five-room flat measuring 90 sq m to 110 sq m, two to three units are recommended, including one in the living room or dining hall and one in each regularly occupied bedroom.
Air purifiers that allow stacking of two filters—HEPA and carbon—are also beneficial during haze episodes, said Prof Balasubramanian. This is because haze events may produce strong odours from volatile organic compounds emitted by fires, which activated carbon can capture but HEPA filters cannot.
He cautioned against purifiers that rely on ionisers instead of particle filters, as these may produce secondary pollutants such as ozone.
Should You Leave an Air Purifier Running All Day?
Both professors indicated they would consider keeping the air purifier on continuously during periods of poor air quality.
Prof Balasubramanian said he would operate his purifier throughout the day because PM2.5 concentrations can fluctuate and spike. Prof Yim similarly recommended using a purifier for as long as PM2.5 concentrations remain at “a high level.”
Running an air purifier continuously on auto, low or sleep mode can also help ensure stable indoor air quality, as indoor pollutants can build up from cooking, dust, human activity and outdoor infiltration, he added.
Households should keep windows and doors closed when using an air purifier during haze episodes to minimise polluted outdoor air entering the room, he said. NEA similarly advises that air purifiers may be less effective when doors and windows are left open.
Placement matters too—air purifiers should not be positioned where their airflow is obstructed.
How Much Does It Cost to Run an Air Purifier 24/7?
Despite concerns about electricity bills, air purifier power consumption is “minimal” compared to air-conditioning systems, Prof Balasubramanian told CNA.
A typical air purifier consumes about 50 to 60 watts of electricity, he said. Based on an electricity tariff of about 31 cents per kilowatt-hour, running such a device continuously would cost roughly 40 cents a day—approximately S$11 to S$13 a month, depending on the purifier’s power consumption. Actual costs will vary according to model, fan speed and operating hours.
A Philips spokesperson said running an air purifier 24 hours a day typically consumes between 4 and 60 kilowatt-hours of electricity a month, depending on fan speed. At the same tariff, that translates to approximately S$1 to S$19 a month.
A Sterra spokesperson estimated that running one of its air purifier models for the whole day would cost about S$7 to S$10 a month. Blueair also estimated that running one of its models continuously would cost less than S$0.70 a day.
What Are Some Common Mistakes to Avoid?
One common mistake is choosing an inexpensive air purifier without checking its specifications, said Prof Balasubramanian. A purifier with insufficient CADR may struggle to clean the air effectively, particularly in larger spaces.
Other mistakes include neglecting to replace filters or attempting to wash disposable HEPA filters for reuse, he added.
Consumers should also avoid switching on the purifier only when there is an odour, as pollutants such as PM2.5 may not be detectable by human smell, said a Xiaomi Singapore spokesperson.
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