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 synthetic compounds break down into nitrosamines, thought to induce the cancers.
This is why there has been a trend toward consumers purchasing uncured meats, or those cured with alternative substances (such as celery powder).
What most people don't realized, and is sometimes pointed out to downplay the risks of these compounds, is that many fruits and vegetables (especially leafy greens) 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.
If you consume a lot of synthetic nitrates/nitrites, it may be wise to make sure you are getting a lot of vitamin C or other antioxidants at the same time.