Purity and Pollution are like antonyms, opposite to each other. Purity is associated with the higher classes whereas impurity or pollution is associated with the lower classes. Purity is related to those things which are considered sacred. Thus some rituals in temples can be performed by only the Upper Castes like Brahmins. But those who do lower types of jobs, like cleaning, sweeping, etc. are considered impure and even their touch was avoided polluting (untouchability) by the Upper Classes.
