MANY drivers believe that winding up the windows and switching on the air-conditioning is enough to keep the haze out of the car.
With nearly 40 areas recording unhealthy Air Pollutant Index readings Oct 9, that belief is being put to the test on the daily commute.
But can your car's air-conditioning protect you from the haze?
Verdict:
TRUE
A car's air-conditioning can help protect you from the haze, but how much depends on the windows, the filter and the ventilation setting.
The fine particles in haze, known as PM2.5, have been linked to cardiovascular disease, and researchers have measured how much of them ends up inside cars.
Most of the protection comes from the cabin air filter in the ventilation system, which catches particles before the air reaches the cabin.
A study of 17 taxis in Los Angeles, led by researchers at the University of California, Los Angeles and found that closing the windows and fitting a high-efficiency cabin air filter cut PM2.5 inside the cabin by 37% compared with driving normally.
Closing the windows alone, with the taxis' original filters, did not make a significant difference.
A study of 10 cars on real roads in Birmingham in the UK, presented at the European Geosciences Union General Assembly in 2022, found that the right ventilation settings cut in-cabin PM2.5 by up to 80% compared with the air outside.
The reduction was similar with standard pollen filters and activated carbon ones.
Recirculation mode, which reuses the air already inside the car instead of drawing in air from outside, appears to help further.
A 2013 study in the journal Atmospheric Environment, by a team that included researchers from the Technion-Israel Institute of Technology, found that particle levels inside a passenger car were lowest in recirculation mode.
A 2019 study in the Journal of the Air & Waste Management Association also found the lowest ratio of in-car to on-road particles with the air-conditioning on and internal recirculation.
Recirculation has a catch, because the carbon dioxide you breathe out builds up in a closed cabin, and the Technion team said this limits the mode to very short stretches of driving.
A 2023 simulation study by researchers at Chalmers University of Technology in Sweden estimated that recirculating 70% of the cabin air lowered PM2.5 by 55% on average with a new filter, but by only 39% with an aged one.
The same study noted that an aged filter performs worse when the air outside is heavily polluted, which describes a haze week rather well.
Not all filters are equal either, with a Chalmers paper noting that efficiencies reported from different vehicles range from 20% to 90% and that conventional filters are weakest against some of the smallest fine particles.
Some car models do not have a cabin air filter at all, as the Los Angeles researchers found when one taxi model had to be left out of their study.
Haze is not only particles, because it also contains gases, and the Los Angeles study found that the high-efficiency filter did little to reduce the gaseous pollutants it measured.
None of these studies tested haze itself, so the results are best read as a guide to how cars handle fine particles in general.
The research points to a simple routine for haze days: keep the windows up, check that the cabin filter is not overdue for a change and use recirculation in short bursts, switching back to fresh air before the carbon dioxide builds up.
So winding up the windows and switching on the air-conditioning does help, but the filter is doing the heavy lifting, and one that has not been changed since the car left the showroom makes a poor bodyguard.
Sources:
2. https://pmc.ncbi.nlm.nih.gov/
3. https://doi.org/10.5194/
4. https://doi.org/10.1016/j.
5. https://pubmed.ncbi.nlm.nih.
6. https://research.chalmers.se/
7. https://research.chalmers.se/
8. https://acp.copernicus.org/
