What's with all the "nitrite free" labels on meat packages these days? Used as a preservative to protect against botulism-causing bacteria, sodium nitrite—a compound formed when nitrogen is exposed to oxygen—is added to everything from bacon to salami to hot dogs, and is to thank for keeping cured delicacies looking fresh.

Here's the problem: When heated above 350 degrees, nitrites in processed meat form nitrosamines, which can be carcinogenic if consumed frequently and in large quantities. Nitrites are also believed to cause migraines in people with chemical sensitivities. Don't worry, however, about the nitrites found naturally in spinach, lettuce, and celery—the vegetables' high levels of antioxidant vitamins C and D inhibit nitrosamine formation.