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

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.