Air pollution has plagued Southern California for as long as people have been writing about the region.
A blanket of haze hung over the land that would become Los Angeles on October 8, 1542, when Spanish sailors entered San Pedro (or perhaps Santa Monica) Bay and made the first written observations of the Southland. This early air pollution so impressed the the sailors and their captain, Juan Rodriguez Cabrillo, that they named the area "Baya de los Fumos, or "Bay of the Smokes." (The name, sadly, did not stick.)
The haze wasn't photochemical smog, of course; the internal combustion engine and modern industrial factories were still centuries away. Rather, it was smoke emanating from the dozens of Tongva Indian villages that dotted the coastal plain and inland valleys, rising in wispy columns only to flatten out against an invisible ceiling.
That invisible ceiling was formed by a persistent meteorological phenomenon that continues to threaten Angelenos' lungs: temperature inversion.
A product of the Southland's topography and its prevailing weather patterns, the inversion layer forms when ocean breezes draw cool marine air onshore beneath a mass of warmer air above. Held in place by the mountains that shelter Los Angeles on the north and east, the cool air then stabilizes, unable to rise through the warm air above.
In essence, the inversion layer acts as an atmospheric lid, trapping whatever pollutants—whether automobile exhaust fumes or smoke particulates—happen to rise from the ground below. On average, an inversion ceiling hovers over Los Angeles 260 days a year. (It's also responsible for the city's "June Gloom" marine layer.)
Short of building giant fans to blow the stagnant air over the mountains and into the desert, there's little the Southland can do to disrupt its natural inversion layer. But the region can control the pollutants it pours into the atmosphere. In recent decades, Southern California has made huge strides toward keeping its air clean. Between 1976 and 1980, for example, the South Coast Air Basin recorded an average of 112 stage 1 ozone alerts per year. It hasn't experienced one since 2003.
Another way to illustrate the change—in 1966, this was apparently an acceptable way to drive through Los Angeles: