This type of smog was first observed in London in 1952. It is formed due to presence of SO2 and humidity in the air which combine to form H2SO4 fog which deposits on particulates. It involves smoke and fog. It is formed in the months of winter particularly in the morning hours when the temperature is low. It causes bronchitis irritation i.e., problem in the lungs. As it contains C and SO2 , it is reducing in nature and therefore, called as reducing smog.
