The average home in a mildly polluted area will actually generally have clean indoor air without filtration. Even with windows open. Don't ask me how this works, but I've verified it using a PM 2.5 monitor, a laseregg 2. With indoor and outdoor measurements on the same day.
I recommend indoor filtration for other reasons (allergies, diseases, cutting down pollution from cooking). But if someone already isn't producing much cooking PM 2.5, an indoor filter won't actually cut their exposure much.
A car with good filtration and a mask outdoors would be the most effective things one could do short of moving. Note that even lower quality masks such as surgical cut air pollution a fair bit. People somehow convinced themselves they have zero impact, because they lack perfect impact.
No one but the most concerned loonies are gonna start wearing a mask though and how do you select for a car with good filtration among the countless other decision parameters for a car?
It really depends where you live. What's the pm 2.5 in your city? Mine isn't high enough to worry about outdoor pollution, and if it was I would move.
But people in high smog cities do wear masks. They have regular smog warnings too; people treat it like the weather. See this pre-pandemic article showing people wearing masks during a smog outbreak: https://www.ctvnews.ca/lifestyle/beijing-locals-don-colourfu...
For cars, I think the biggest factor is choosing a good filter and making sure it remains in good working order.
