Dr. Harold Des Voeux wrote two papers about the effects of the London "smog" on people. This type of smog comes from coal smoke combining with the water vapor and liquid water in cool, humid or foggy air.

L.A. smog has been identified as coming from auto exhaust, primarily (we will see later that there is a significant "stationary" source). Originally, it was thought to be entirely from non-mobile sources, until Haagen-Smit of The California Institute of Technology showed otherwise.