Smog is a combination of fog and pollution. The common cause of a widespread thick fog is a temperature inversion. If there is a significant source of pollution under the inversion, it gets trapped along with the fog.
The famous London pea-soupers were produced by mixing fog with the smoke from several million open hearths burning coal. The coal used contained a large amount of sulphur, so the smoke contained lots of sulphur dioxide. This nasty yellow tinged fog killed hundreds of people in the 1950s; the resulting public outcry led to a ban on burning this type of coal. London still gets very thick fog at times, but it doesn't hurt the lungs any more.
Los Angeles has an even bigger problem. Their pollution source is car exhaust emissions, especially oxides of nitrogen. . The photo-actinic effect of their continual strong sunlight makes the pollution worse. Banning cars is politically impossible, so the state of California has done the next best thing, with the strictest emission standards in the world.
The problems caused by smog vary according to the pollution type. They include : Lung damage from bronchitis to cancer; severe eye irritation; blackened buildings; smelly hair and clothes.