Nitrates are often used to cure and process meats,such as bacon, salamis, and sausages, being the main culprits. Nitrates are known carcinogens.Nitrites or nitrates, preservatives commonly found in luncheon meats and hot dogs, are also possible headache provokers.I got the aforementioned off of Atkins site, and also this following bit of information: "What are nitrates and why are they bad for you? And where can I find meat/bacon/cold cuts without nitrates?
Nitrates and nitrites are added to meats (such as bacon and cured ham) and smoked fish to produce an appealing color and inhibit growth of germs and poisons. While these chemical compounds are not themselves carcinogens, they can yield byproducts such as nitrosamines, which have been implicated in higher rates of cancer--particularly colon and gastric cancer--in animals. For this reason, we feel it is prudent to limit your intake of foods containing nitrates or nitrites. You can find nitrate-free products in health food stores and some supermarkets. Fortunately, an increasing number of nitrate-free meat products are available in stores or by mail-order including Coleman Beef (800-442-8666). Your local butcher or health food store may also carry nitrate-free products or stock them for you upon request"
Hope that has helped to answer your question~ have a great day~