Singapore haze: Is your old air purifier still effective, and how much does it cost to run one?

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SINGAPORE: With haze affecting air quality in Singapore in recent weeks, households may be dusting off air purifiers bought years ago.

But can an air purifier that has been sitting unused still work effectively? How often should its filters be replaced, and how much would it cost to keep one running throughout the day?

CAN AN OLD PURIFIER STILL WORK EFFECTIVELY?

An air purifier that has been left unused for several years should generally still work, provided it was stored in a dry, pest-free environment, said Professor Steve Yim, director of Nanyang Technological University's Centre for Climate Change and Environmental Health.

Before switching it on again, households should check the power cord and test all fan speeds, he advised.

As for the filters, unused high-efficiency particulate air (HEPA) filters that remain sealed in their original plastic packaging do not significantly degrade over time.

However, filters that have been opened or previously used may be affected by moisture, which can encourage mould or bacterial growth, Prof Yim said.

Activated carbon filters can also absorb odours and gases from the 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 said.

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.

But how do you tell whether the entire purifier needs replacing?

Signs to look out for include weaker airflow, unusual noises, lingering odours or a visibly clogged filter, a Dyson spokesperson said.

A Philips spokesperson noted that many of these warning signs could be caused by a clogged filter rather than a faulty appliance.

Consumers should therefore try replacing the filter and cleaning the unit before deciding whether to buy a new one. If problems persist, the purifier itself may be worn out and need replacing.

When storing an air purifier between haze episodes, Dyson recommends switching it off, unplugging it and wiping it down.

The filter should be left fitted to prevent dust from entering the 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 check that the airflow paths are clear and replace the filter if necessary.

HOW OFTEN SHOULD YOU REPLACE THE FILTERS?

There is no one-size-fits-all answer, as the lifespan of an air purifier filter depends on how frequently the device is used and the conditions in which it operates.

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 said filter lifespan also depends on the purifier's operating speed and filter design.

During haze episodes, households may run their purifiers for longer periods and at higher fan speeds. This increases the amount of pollutants trapped by the filters, potentially shortening their lifespan.

Consumers should therefore consider both the filter status indicated by their device and their actual usage conditions when deciding whether a replacement is needed, Xiaomi said.

Philips recommends replacing HEPA and carbon filters when the device's filter replacement alert appears, typically every six to 12 months.

Other signs that a filter needs replacing include a noticeable drop in fan motor airflow and persistent strange smells, said Blueair.

WHEN DO YOU NEED AN AIR PURIFIER?

Air purifiers become particularly useful when PM2.5 concentrations rise, although the need for one also depends on an individual's health.

Prof Yim said 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 an air-conditioned environment, an air purifier is still needed as the filters in air-conditioners only trap large particles and are not designed to filter fine particulate matter like PM2.5 particles, said Prof Yim.

When choosing an air purifier, consumers should look for one with an effective HEPA filter, which is 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 room they intend to use it in.

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.

But can one powerful purifier clean an entire HDB flat?

Not necessarily, said Prof Yim. "Walls and doors block airflow, so one central air purifier cannot clean an entire HDB flat."

For a typical three-room HDB flat of 60 sq m to 70 sq m, it is recommended to have one high-CADR unit in the living area and one in the main bedroom, he said.

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, he added.

Air purifiers that allow stacking of two filters - HEPA filter and carbon filter - are also good for haze episodes, said Prof Balasubramanian.

This is because in a haze event, besides fine particles, there may also be strong odour caused by certain volatile organic compounds that are emitted by the fires. Activated carbon can capture such gases that HEPA filters would not. 

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 Prof Balasubramanian and Prof Yim said 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 the amount of 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, as the devices are generally rated for rooms without a continuous influx of pollutants.

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, the power consumption of air purifiers is “minimal” compared to an air-conditioning system, 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, Prof Balasubramanian added. 

That works out to about S$11 to S$13 a month, depending on the purifier's power consumption.

Actual costs will vary according to the 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 the fan speed setting.

At the same electricity tariff, that translates to approximately S$1 to S$19 a month.

Meanwhile, a Sterra spokesperson estimated that running one of its air purifier models for the whole day would cost about $7 to $10 a month, depending on the electricity tariff.

Blueair also estimated that running one of its models continuously would cost less than S$0.70 a day, based on the prevailing tariff rate.

WHAT ARE SOME COMMON MISTAKES TO AVOID?

One common mistake is choosing an inexpensive air purifier without checking its specifications, said Prof Balasubramanian. 

For instance, a purifier with an insufficient CADR may struggle to clean the air effectively, particularly in a larger space.

Other mistakes include neglecting to replace its 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.