“During the past five years we have found the smog in Los Angeles is caused primarily by auto exhaust activated by intense sunlight. We call this photo-chemical smog. We realize crankcase emissions are a smog source, but they are only about 10% of the cause. Most of the rest is caused by exhaust emissions so let's not get too excited about eliminating crankcase vapors,” says W.L. Faith of the Air Pollution Foundation in San Marino.