Bacon will kill you!
Yes, this is a real fear. Many (though not all) cured meats contain sodium nitrite (NaNO2), which is used for a couple of reasons. One is that it's responsible for the distinct flavor of bacon, ham, and many sausages, as well as reacting with the myoglobin in the meat to keep it pink even when cooked. Another is that it acts as a retardant to the growth of Clostridium botulinum, the bacterium responsible for the existence of botulinum toxin, one of the most toxic substances known to science; in fact, "botulism" is actually a Latinization of the German "Wurstvergiftung", which literally means "sausage poisoning".
The fear has some basis in reality. For one thing, sodium nitrite has an LD50 in the vicinity of 180 mg/kg (oral rats); although it's not the worst thing in the world for you, a teaspoon or so would be enough to give a healthy average adult male a very, very bad day. Nitrites are also thought to cause migraines in some people. While this is blindly accepted by most doctors, and the US FDA, real research is limited or non-existent. In addition, nitrites tend to react with proteins to create substances called nitrosamines, which can be carcinogenic. Many people have a general fear of preservatives, and sodium nitrite is well-known by the public to be a preservative. Unfortunately, as with all such situations, a little knowledge can be worse than none at all.
 What exactly are these chemicals there for?
In any high-end food store, especially one with pretensions of health food, you'll usually see a lot of meat goods — bacon, ham, hot dogs — labeled "uncured". Strangely, they usually look no different from their, um, cured counterparts, and frequently taste more or less the same as well. Although "uncured" certainly works as a nice marketing shorthand, and meats so labeled are usually higher-quality than their mass-market counterparts, it's actually required by the United States FDA as a code word for when no nitrites are used in the curing process. In fact, all such meats are by definition cured, and although certain preparations (especially dry hams like prosciutto di Parma or Serrano) use only salt and occasionally smoke in the curing process, there's a distinct difference when it comes to sausage.
You know how you're not supposed to cook a hamburger less than medium? Well, that's because in meat, any pathogens are usually on the surface of the meat. However, when you make hamburger (or sausage), you create a product that's almost entirely surface, mixing any pathogens on the meat deep into it. This is where nitrites (and nitrates like saltpeter (KNO3), which, in a sufficiently acidic environment, reduce over time into nitrites) come in. The curing process for sausages meant to be shelf-stable involves complex and lengthy chemical and biological reactions including dessication and lactic fermentation, and a sausage is a very, very hospitable substrate for bacteria in general. Worry all you want about chemicals in your food, but nitrites (usually used by professionals in the form of a mixture with salt called "pink salt" or "Prague powder #1") are absolutely necessary to prevent C. botulinum from taking advantage of this.
The sneaky bit here: celery juice. Look on any label of "uncured" meat, and there's a good chance you'll find some kind of celery derivative on there. That's not just because some people are enamored of the flavor of celery salt; celery, like many green vegetables, is rather high in nitrates, significantly more so than most cured meats. As a result, producers such as Applegate Farms and Niman Ranch can slip nitrates through the back door using the "all natural" loophole, which in turn reduce to nitrites just like the pure nitrate salts like saltpeter do, thereby performing the same function as the traditional pink salt. As a side benefit, it allows things like hot dogs to be labeled as organic when they normally couldn't be with the isolated salts.
Is it a scam? Not exactly. Calling a piece of cured meat "uncured" is clearly false, but you don't wind up paying much more than you would for any other boutique brand of meats. It does, however, foster ignorance in the buying public.
 Balancing it out with mustard on light rye
At the end of the day, nitrite fear doesn't come out of nowhere, but it's a classic example of the dose making the poison. The science connecting the typical level of nitrosamines in lunch meats to cancer is a little dodgy and not completely resolved, but any danger is far outweighed by the danger of food poisoning. Food science author Harold McGee doesn't dismiss the danger, but has said at worst you need only moderate your intake of cured meats, which is a wise idea based on sodium and fat consumption alone. In addition, the toxicity of sodium nitrite in particular isn't really an issue when spread across an entire batch of ham or sausage unless something went horribly wrong during the prep process, in which case the use of pink salt instead of pure nitrite would make the food a whole lot saltier as well.
The take-home: chemicals are an inherent part of food production, and it really helps to know what's what because it's entirely too easy to fall into a state of irrational fear over food additives. (Alternately, everything can kill you anyway, so do your homework and enjoy with prudence.)
- ↑ In other words, treated with salt(s) to reduce or eliminate most microbial activity, thereby theoretically increasing the storage life of the meat.
- ↑ Note that nitrates and nitrites are not the same – nitrates are salts of nitric acid (HNO3), while nitrites are salts of the weaker nitrous acid (HNO2) and as such have different chemical properties.
- ↑ A damn sight nastier than acetaminophen, but nothing compared to, say, cyanide.
- ↑ See also Eleven Blue Men
- ↑ http://permanent.access.gpo.gov/lps1609/www.fda.gov/fdac/features/1998/398_pain.html
- ↑ National Toxicology Program summary of nitrosamines
- ↑ In the United States, pink salt is legally defined as 6.25% sodium nitrate and 93.75% sodium chloride, and it's literally dyed pink to prevent it getting mixed up with regular salt.
- ↑ The 5th incarnation of The Doctor of Doctor Who, played by Peter Davison, wore a celery stalk on his lapel.
- ↑ http://ruhlman.com/2007/04/mcgee_on_nitrit.html
- ↑ It's also kind of an annoyance to the producers themselves, which would rather use a somewhat more consistent curing agent like, oh, I don't know, pink salt.
- ↑ To be clear, some nitrosamines are in fact known animal carcinogens; what's in question here is whether the dosage in, say, an order of bacon or three is really enough to worry about.