I was surprised to learn recently that there is a cheap and easy to take action that will reduce your exposure to air pollution while driving: Replace your passenger-compartment air filter with a HEPA filter! Most cars made in the last 15 years or so have an air filter right before the AC. You can, for slightly more money, replace this with a HEPA filter. I haven't tested, but this will probably be very effective, since the car is such a small space and there is so much pollution outside.

Replacing the filter is an incredibly easy job on most cars. You don't need any tools or even to open the hood. You just pull out your glove box, and then the filter slides in and out.

I honestly think that in terms of health, this might be the action that has the highest ROI. (Amount of health improvement per amount of time / money invested.)