Smog is essentially caused by the presence of

 Smog is of two types: 
Classical smog: It is produced under low temperature conditions and reducing environment. It is called sulphurous smog or London smog or reducing smog. It contains only primary pollutants. The classical smog contains smoke or dust particles, H₂S, SO₂ and water vapour. 

Photochemical smog: Photochemical smog was first noticed over Los Angeles in1946. It is, therefore, also called  Los Angeles smog. It is a smog containing secondary air pollutants called photochemical oxidants. In the areas of intense solar radiation, smog is brown in colour and is called brown air.. The products are ozone, PAN, aldehydes and phenols. Sulphur is absent. Photochemical smog is highly injurious to plants, animals, humans and human assets.