While shintô places emphasis on purity and avoiding pollution, it isn't very clear on what pollution is and why it occurs. Instead it tends to list proscribed actions and the circumstances under which those actions should be avoided.

Pollution is seen as more spiritual than physical. Because it is viewed as contagious, the main concern with pollution is that it not be spread across the ritualistic boundries that circumscribe the various parts of Japanese culture — uchi (the inside) and soto (the outside), hare (the formal or extraordinary) and ke (the informal or ordinary).

Here are three of the more commmon terms for pollution:

fujô — Pollution and Impurity
Impurity obscures our ability to sense the kami within and without. Like dirt on a window, it makes hard for us to see what lays beyond.

It's best to avoid being polluted in the first place, but fortunately, if you can't, pollution is not permanent — it can be removed.

tsumi — Pollution Arising From Our Actions
Actions that disturb the harmony and peace of the family and the community are a source of this pollution.

To regain purity in these cases, it is necessary to restore the harmony and peace, and to make restitution to any you may have injured.

kegare — Pollution From Contact With The Environment
Contact with something that is out of its normal place — blood outside the body, premature death, even childbirth.

Purity can be regained by engaging in harai, either by yourself, or at the hands of a purified individual, spiritual leader, or priest.