Carcinogens may cause cancer by altering cellular metabolism or damaging DNA directly in cells, which interferes with normal biological processes. Aflatoxin B1, which is produced by the fungus Aspergillus flavus growing on stored grains, nuts and peanut butter, is an example of a potent, naturally-occurring microbial carcinogen.

Cooking protein-rich food at high temperatures, such as broiling or barbecuing meats, can lead to the formation of many potent carcinogens that are comparable to those found in cigarrette smoke (i.e., benzo[a]pyrene).Pre-cooking meats in a microwave oven for 2-3 minutes before broiling can help minimize the formation of these carcinogens.