Hormel lists the ingredients of Spam canned meat as:

Chopped pork shoulder meat with ham meat added.
Salt (for binding, flavour, and firmness)
Water (to help in mixing)
Sugar (for flavour)
Sodium Nitrite (for colour and as a preservative)

If you're worried about the sodium nitrite, here is a great site that explains why it is absolutely necessary for slow cured meats and more, including details on the concern about nitrites and cancer: http://www.askthemeatman.com/how_and_why_to_use_sodium_nitrite.htm