Nitrates and nitrites are the subject of controversy regarding their safety. Substances called nitrosamines form when foods containing nitrates or nitrites are subjected to very high heat, such as when bacon is fried. Nitrosamines are know to cause cancer. Using only nitrites to cure foods usually avoids this problem because the nitrites break down quickly and are not usually a factor when the food is cooked. Nitrates, on the other hand, because they remain in meats longer, should not be used for curing bacon for this reason.
Some people feel that all use of nitrites and nitrates should be avoided because of the nitrosamine factor. However, if only nitrites and not nitrates are used in foods that will be exposed to extreme heat, the risk is low -- much lower than the risk of botulism. So far, we know of no adequate substitute for these chemicals in curing pork and other meats.