There’s little in the way of hard data available for car air filters themselves.
That said, the clear consensus of informed opinion is that in newer vehicles, setting the AC to recirculate will remove most of the fine particulates (PM 2.5) from the cabin air in a matter of minutes.
I’m going to test that tomorrow, when I get my hands on an air quality meter that measures PM 2.5.
In the meantime, here’s what I can find on-line.
A jumble of slightly-related anecdotes
My exact question is “How good is the standard cabin air filter in a 2021 Toyota Prius”.
Point 1: No hard data from the manufacturer.
As it turns out, unless a vehicle manufacturer was specifically touting their air filtration (Volvo, Tesla), they just plain don’t state the filtration level of their standard cabin air filters. E.g., there’s no MERV rating for a Toyota cabin air filter.
I can find vague reference on deal websites to their ability to filter “soot”, which would be shorthand for fine particulates (reference). But that’s about it.
Point 2: You can buy aftermarket filters advertised as HEPA
HEPA would capture 99.97% of fine particulates in a single pass. These appear to be by legit manufacturers, meaning, they probably won’t screw up your car’s AC with excess back-pressure. So if you’re really worried, you can get one of these.
Point 3: Actual tests all seem to show very high particulate removal rates when in recirculation mode.
Smartair (my go-to on all things air filtration), 2010 Mazda 3, 99% PM 2.5 removal in five minutes (reference). “No need to add special car air purifiers to protect against particulate pollution. The air systems already in cars dramatically reduce particulate pollution.”
Society of Automotive Engineers study: “… recirculation ventilation can lead to high PM2.5 removal efficiency regardless of the status and performance of a cabin air filter …” (reference).
Scholarly article in PubMed, standard cabin air filter and recirculation mode resulted in roughly 90% reduction (my calculation) in airborne particulates. (reference).
Scholarly article in PubMed: “The ventilation system operating in the recirculation mode with the cabin air filter installed provided the maximum protection, reducing the cabin particle concentration exponentially over time and usually taking only 3 min to reach 10 microm2/cm3 (a typical office air condition) under medium fan speed. (reference).
Scholarly study via PubMed: “When ventilation settings were set to RC, UFP concentrations inside the vehicles … were 86% lower compared to outside air (OA) (23,496 pt./cm3) across a 30-min real-world driving route.” (Reference)
Conclusion
Best guess, if your car is relatively new, or if you’ve changed that air filter within living memory, you’ll get excellent particulate removal merely by running your car’s AC in recirculation mode.
Near as I can tell, that’s for any car, any filter. The analyses summarized above spanned a fairly wide range of cars, from the mid-1990s forward.
The obvious implication is that if you’re suffering from the effects of the recent air pollution, and don’t or can’t filter the air in your dwelling, you can get some relief by sitting in your car with the AC running, in recirculation mode.
