LAX is especially bad because of all the avgas burned. I've seen studies done where being directly downwind of lax relative to the other parts of southla not as downwind have marked increases in health issues. Port of LA is also a bad area. So is san bernardino and riverside county as the prevailing coastal breeze tends to push quite a bit of pollution off the la basin and into the san gabriel valley. You get up high to the highest reaches of hilly neighborhoods like bel air, you might actually be high enough to be totally above the usual inversion layer and have a dramatically different air quality than those maybe 10 minutes downhill on your street.
Every time I fly into LAX from northern Canada I get a punch in the lungs - it's horrible. There must be some severe long-term health impacts.