I visited Santa Monica (basically LA unless you live in LA) several times and walked almost everywhere, only taking a Lyft when I really wanted to go across town (the Blue Bus comes eventually, but it isn't fast). Air quality was fine and there was a 4-mile beach to walk on if you wanted to really Walk.
I know people who live in Downtown LA and don't own cars.
I get the impression there are several parts of LA you can live in without regularly driving, and still have a very normal lifestyle. Not sure about the air quality downtown but my friends didn't complain about it.
It isn't great, but it is not any worse than other large cities since the 90s. You can see the current and forecasted cities with the worst air quality here. https://airnow.gov/
Its not like there are black clouds of smoke hanging in the air or anything... Your average person is more likely to have allergies and asthma, but it's not like breathing the air hurts or your skin burns or anything...
I know people who live in Downtown LA and don't own cars.
I get the impression there are several parts of LA you can live in without regularly driving, and still have a very normal lifestyle. Not sure about the air quality downtown but my friends didn't complain about it.