When haze lowers outdoor air quality, many people rely on their home air conditioner, assuming it will keep the indoor air clean. Knowing the real limits of a typical wall‑mounted split AC can help you protect your health.

The U.S. Environmental Protection Agency says conventional wall‑mounted split air conditioners generally do not effectively remove fine particles. Their filters are mainly designed to catch larger dust, not the tiny PM2.5 particles.

PM2.5 particles are 2.5 micrometres or smaller – about thirty times thinner than a human hair. Because they can travel deep into the lungs and even enter the bloodstream, they raise the risk of respiratory and cardiovascular diseases.

Liu Zheng of Nanyang Technological University explains that household AC units mainly recirculate and cool indoor air, while their filters target larger dust. Rajasekhar Balasubramanian of the National University of Singapore adds that conventional AC filters are generally ineffective at removing fine particulate matter.

In Singapore, where more than four in five homes have air conditioners, the National Environment Agency warns that during haze episodes PM2.5 can enter buildings through outdoor intakes and gaps, building up to unhealthy levels. The agency recommends high‑efficiency particulate air (HEPA) filters and suggests looking at an air purifier’s clean air delivery rate (CADR) to choose the right size.

The EPA advises using a portable air purifier or upgrading to a filter that meets MERV 13 or higher if your system can accept it. Relying only on a standard AC filter may leave you exposed to harmful PM2.5 during haze.