Nitrates and Nitrites
These compounds are most often used to preserve meats such as deli meats, hot dogs, and bacon. They are associated with an increased risk of cancer that becomes greater as more of these foods are consumed. For example, the World Health Organization says that every 50 gram portion of processed meat eaten per day increases the chance of colorectal cancer by 18%. This is because the compounds break down into nitrosamines, thought to induce the cancers.
What most people don't realize, and is sometimes pointed out to downplay the risks of these compounds, is that many fruits and vegetables (especially leafy greens and notably celery) contain large amounts of nitrates. However, there has been no such correlation to cancers in these, because these foods naturally contain other compounds (such as vitamin C and other antioxidants) that prevent the formation of nitrosamines and lack the meat proteins for the conversion.
There has been a trend toward consumers choosing "uncured" meats, notably processed with celery powder instead of synthetic nitrites. Celery contains a high amount of nitrates, which is processed with bacteria to produce nitrites as a substitute for synthetic nitrites. Unfortunately, nitrites from any source (including celery powder) react with proteins from meats in your intestinal tract to create damaging nitrosamines; the source makes no difference in the digestive tract.
Because celery powder is not officially recognized as a curing agent, foods using it can be labeled "organic". But you are still getting the same amount of nitrites, and maybe even more since it is more difficult to maintain precise concentrations than it is for the synthetic types.
If you consume a lot of processed meats, it may be wise to make sure you are getting a lot of vitamin C or other antioxidants within 30-60 minutes after the meal.
For more information, see: Nitrites
See also: What Are Nitrates and Are They Bad For Your Health?