They have different chemicals which can damage the ozone layer making it thinner that is why it is related to pollution, because it harms mother earth. 
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The answer of samcat is right,in addition to that,it affects also because of the garbage materials burned,depending of the materials burning,while the smog is the mixing of air pollution present around in the environment while having a moist around of the particular place,in that case the pollution were merely  severe in a way that it makes us to see a cloudy like images around and make it difficult for us to see clearly what is around us