Although there are many types of emissions, such as from factories, these zones predominantly target exhaust fumes from vehicles, poisonous gases called nitrogen dioxide.
Smog is air pollution caused by a mixture of smoke, chemicals and especially, car exhaust fumes. It makes the atmosphere difficult to breathe and very unhealthy.
But far worse is the nightmare of travel in and around Los Angeles, where public transport does not exist and people are literally choking to death in their exhaust fumes .