Well there is only a narrow band of Redwoods in southern Oregon. You find Redwood forests In California, outside of the Pacific Northwest.
And it is feasible to live in Pacific Northwest forests, in a populated area. Portland, Oregon, comes to mind.
For a metropolitan area, Seattle is pretty good. My in-laws from China call it “one GIANT oxygen bar.” Admittedly that’s coming off of a low base.
We get some pretty amazing air in da UP, coming in from over Lake Superior, and it’s rarely muggy or overly humid.
Just from a logical perspective this is what I would have suppossed. There’s such an enormous distance from the last conceivable polluting source. The last time this air was breathed it probably was in Japan or Kamchetka.
Ketchikan, Sitka, Homer, they’re all good.
Traverse City, MI
Wisconsin Dells?
Portland, OR
My brother told me the air in the forests of Northern California and Oregon was the best he has ever experienced. And he has traveled a lot within the US. So whatever large city that is near the forests in those areas.
You can also try an ionic air purifier. Those make the air feel fresher and less stuffy when I use them.
Right here in God’s Country…the U.P. of Michigan!!! 
Third vote for Portland, OR.
Fort Collins, Colorado
Bout 50 miles west of here. (Denver…so past the foothills a bit)
Make that a hundred miles for me. The air may be thin up here, but it sure tastes good.
Finding air that meets my qualifications of really fresh would be difficult near a large city, but I am about 150 miles from Boise up in the mountains and my air is wonderful. I’m at about 5800 feet so it does take a day or so to adjust to, but oh, it is soooooo nice.
Mendocino, California. Great air, close enough to the Bay Area that you aren’t cut off from civilization.
Second vote for Mendocino or somewhere in Humboldt, like Eureka, Arcata or oh I know, Ferndale. After that on the west side of Mt Tam in Marin.
Hawaiian islands. Middle of the Pacific ocean with no sources of air pollution upwind. Otherwise northern CA coast, OR coast, WA coast - all close enuf to large towns and civilization.
Away from the city.
How about the Columbia River Gorge?
Right here in Wyoming.
The air there can be very smoky in parts. Oh, it’s a special kind of smoke, not air pollution, but still, it can cause your head to get foggy…
Places to absolutely STAY AWAY FROM: any of the California farming towns in Central Valley and outside, e.g. Bakersfield, Fresno, Modesto. Some of the worst pollution in the US. LA of course, too.