P. 248. (6th perek). There are two levels of klipos (shells) - the three that are completely evil and impure which cover Hashem's light completely. They are referred to as 'a stormy wind', a 'big cloud', and a 'fire burning from within'. Our avodah (work) is to completely reject these three klipos, not to try to fix them. The souls of the nations of the world come from these three klipos, as well as all forbidden animals and foods, and forbidden thoughts, speech, and actions. When the Torah says that something is forbidden it is because at its essence it is impure. Chametz is different in that during Pesach, it becomes encased in klipah. The essential choice we have is whether we want to draw from the three klipos or from Elokus (G-dliness).