Before I was born my family moved to Southern California. This was in a place called Orange County, directly south of LA County.
During the 1950's LA had very bad smog (aka air pollution). There was smog alerts issued depending upon the air quality
First Stage Smog Alert: All vigorous and strenuous activities should be reduced or shortened.
Second Stage Smog Alert: All forms of vigorous activity must be discontinued or cancelled.
Third Stage Smog Alert: Students, staff, and faculty should remain indoors and restrict movement as much as possible.
I recall first stage smog alerts and not being able to breath a deep breath without coughing. A yellowing brown sickly haze hanging in the sky.
stage one was 0.5 ppm ozone, two 1 ppm, three 1.5 ppm. On Sept. 13, 1955, the city of Vernon reached 0.9 ppm of ozone and 0.85 ppm downtown. So no stage 2 or stage 3 was ever issued.
Current standards for ozone list the stage one 0.20 ppm, stage two 0.35 ppm, stage three 0.50 ppm. In the year 2000 there were no stage one smog alerts.
Cleaning up the air, good thing.