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In-car smoke protection only as good as the vehicle’s filter

With all the wildfire smoke outside, I’m wondering whether we are protected in a car when we’ve got the windows up and the air conditioner running? To what level?

— Sally M., via email

When the outside air’s full of particulates, public health officials say closed windows behind the wheel is a healthful option. However, your protection is only as good as what’s filtering the air.

The Oregon Health Authority recommends drivers limit their exposure to wildfire smoke by keeping the windows closed with air conditioning set to its recirculate mode, according to the state agency’s website.

With the air reaching hazardous levels outside, now’s as good a time as any to check on the cabin air filter. Most cars made since the late 1990s have one — which is separate from the air the engine uses.

Manufacturers including Honda, Toyota and Ford typically say the cabin filter lasts about one year or 15,000 miles, according to owners manuals for the companies’ popular models, and should be changed more frequently in dusty or dirty environments.

According to the EPA, the ultrafine wildfire smoke particles we’re seeing in abundance are between 0.4 and 0.7 microns in size. A human hair is about 60 microns in diameter.

Automaker cabin filters don’t list a performance rating, but other aftermarket filters are designed for particles up to 5 microns — about the size of dust and pollen particles — though they may miss smaller smoke particles.

Aftermarket parts maker Bosch sells a line of cabin air filters that comply with the U.S. Department of Energy’s High Efficiency Particulate Air standard, meaning it filters particles as small as 0.3 microns. We haven’t spotted them stocked at local stores, but the Bosch HEPA filters are available online.

Any clean filter is better than a dirty one, and can be a straightforward and inexpensive replacement, particularly for those driving many Japanese makes. For example, no tools are needed to access the filter behind the glove box on many newer Hondas.

For those who aren’t mechanically inclined, replacing the cabin filter is a simple request at most oil change spots, repair shops and dealerships.

Send questions to “Since You Asked,” Mail Tribune Newsroom, P.O. Box 1108, Medford, OR 97501; by fax to 541-776-4376; or by email to youasked@mailtribune.com. We’re sorry, but the volume of questions received prevents us from answering all of them.