LA has averaged over the past 5 years roughly 90 days of "unhealthy" air, mostly in the summer, including an average of 2-3 days each summer when the air is rated "very unhealthy" by the EPA index.

SLC has averaged over the past 5 years roughly 30 days of "unhealthy" air, mostly in the winter, but hardly ever has a day where the air is rated "very unhealthy" by the EPA index.

The stereotype of Utah having "the worst air in the nation" is because the Media latch on to the fact a few times a year SLC's particulate levels are higher than anywhere else. But claiming that makes the air the worst in the nation is like claiming SLC has the wettest climate in the nation because it has more rain than anywhere else a few days out of the year.