Both Assertion and Reason are true and Reason is the correct explanation of Assertion. Smog is a type of intense air pollution. It is composed of nitrogen oxides, sulphur oxides, ozone, smoke and other particulates. Rubber is an organic compound and it is sensitive to number of chemical influences-oxidation, reduction etc. Excessive cracking of rubber products can be one of the indicators of the presence of atmospheric photochemical oxidants. Excessive cracking of rubber is caused by atmospheric ozone formed in the photochemical smog formation process.