In the early 1770s London doctor Percival Pott noticed something strange about the chimneysweeps in his town: an unusually high percentage of them had a rare form of cancer.

Most chimneysweeps at that time were boys under the age of fifteen because kids were better at squeezing into narrow chimneys. Orphans and street kids, most of them, with not much family to care what happened to them either way. Pott found that by the time they hit their twenties and thirties they were unusually likely to get cancer of the scrotum. Outside of chimneysweeps scrotal cancer was pretty rare. But among chimneysweeps — much more common. He remembered one patient in particular, a young man aged 28 with a tumor the size of your hand growing out of his balls.

The sore [occupied] the whole of the left side of the scrotum and the inner angle of the thigh…

Unfortunately for this poor guy, anesthesia and chemotherapy had yet to be invented, so there wasn’t a whole lot Pott could actually do. He could do surgery without anesthesia to remove the tumor, and doctors back then sometimes did, but that was never very popular with their patients. (Getting tied down on a table, fully conscious, while a doctor hacks at your balls with a knife? and then hope you don’t get an infection or bleed to death? I don’t know but I’m guessing most patients would rather take their chances with the cancer, thanks much.)

What was spreading this bizarre epidemic of cancer among London chimneysweeps? In a brilliant flash of intuition, Pott correctly identified the cause. He realized there must be something carcinogenic in fireplace soot and smoke. Young boys climbing up chimneys wound up covered in soot, of course, and being about as clean as kids typically are, they’d never bother to clean it off. Maybe their faces and hair, but the rest, probably not. The soot left sticking to their skin contained something dangerous — something that caused cancer.

At that time chemistry was still in its infancy, and even though he’d figured out soot was carcinogenic, Pott didn’t know which chemical in the soot was the problem. It wasn’t until the 20th century that modern analytical chemists nailed the culprit: a potent natural carcinogen called benzo[a]pyrene found in smoke and ash from burning logs and cigarettes.

Yes, you read that right: the smoke from your fireplace contains carcinogens, just like cigarette smoke. The main difference is you don’t suck fireplace smoke into your lungs and hold it there, because unlike fireplace smoke, tobacco smoke is addictive. But the really interesting thing about this benzo[a]pyrene is that you find it not only in fireplace smoke but in smoked meats and meats cooked over charcoal grills as well. So when you barbecue a hot dog or a hamburger on the grill, you may be adding a potent natural carcinogen to your food, depending on how you cook it. (For an article discussing this see here.)

I’m not telling you this to scare you away from the grill necessarily. Is benzo[a]pyrene a carcinogen in lab tests? You bet, and a potent one too. But does that mean the dose in your smoked turkey sandwich is enough to increase your cancer risk? I don’t know — the jury’s still out. Among doctors this is still a matter for debate.

How can there be a debate about this? It’s a carcinogen, we’ve known that ever since Dr. Pott first noticed this weird anomaly among London’s chimneysweeps. So don’t put it in my sandwich. What’s there to argue about? And yet, not so simple. You see, the cells in your body that serve as the first line of defense — the cells in the lining of your intestine — if anybody’s getting DNA damage from benzo[a]pyrene, they’re first in the line of fire. They get turned over pretty rapidly, on the order of 4-5 days, and replaced with fresh cells by cell division. That’s why it’s at least possible to have a situation where benzo[a]pyrene caused DNA damage but the dose you got from your bacon sandwich was too small to actually increase your risk of cancer; the cells that got damaged got replaced shortly thereafter. Moreover, how much or how little benzo[a]pyrene is added to the meat depends on how it’s smoked or grilled exactly. So the question is, as always: how much is a dangerous dose? That’s what we don’t know. That’s why the jury is still out. Researchers have tried to look for statistical correlations but it’s tricky; you can’t usually get the kind of information you’d need from a survey, and most of us probably can’t remember everything we ate last month or last year and exactly how it was cooked.

You could always say why take a risk? Why eat food that contains a known carcinogen? I’ll pass on that hot dog, thanks much. But then you’d have to skip grilling hamburgers and cooking bacon and Fourth of July hot dogs and that’s something most folks won’t do. Just imagine if the FDA came out next month and told everyone, hey, kids, ummm….we’ve decided you can’t eat bacon anymore because…well, it contains something that might be bad for you, and we’re the FDA and we’re here to take care of you, so no more bacon for you. I can’t think of many issues that would unite most Republicans and Democrats, but I’m guessing that right there is one of them.

Besides, it’s easy to forget about benzo[a]pyrene and other natural carcinogens like heterocyclic amines because you don’t see them on ingredient lists; they weren’t added by the manufacturer, they were put there by none other than “Mother Nature” herself (with the aid of the chef). The irony here is rich. Folks get all riled up about GMOs and food coloring and potassium sorbate and so forth; and all the while, they’re adding a natural carcinogen to their food that’s probably more dangerous than any of the things they’re protesting could ever be.

Trouble is “Mother Nature” doesn’t actually desire our happiness (as I never tire of pointing out). Nor are our bodies or our natural environment as well-crafted as we like to believe. And benzo[a]pyrene is a nice illustration of that fact, because it’s not actually carcinogenic in and of itself. Like many carcinogens, benzo[a]pyrene isn’t a carcinogen until enzymes in your liver have turned it into one.

But why would your liver do that? The simple answer is because your body wasn’t designed; it was cobbled together by a few billion years of messy, brutal evolution. There’s a reason why your liver works the way it does, and it has to do with plants: the ones that are trying to kill you.

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It’s kind of interesting to think about how many plants you can’t eat. There’s a gazillion different species of plants in our world, and the ones we can are a tiny fraction of a fraction of the plant species we know. The others are poisonous or distasteful or too tough (ever try chewing on wood?) or too fibrous or too difficult for our digestive system to break down. Even out of the handful we do eat, many are plants we’ve cooked or bred over time to make them less poisonous.

Take almonds, for example. Like peach and cherry pits, wild almonds contain a compound that breaks down to release cyanide in your digestive tract. Stone Age farmers somehow found a mutant tree that didn’t have the cyanide and bred it to make the almonds we eat today. Or take soybeans; we eat them, but we never eat them raw, because raw soybeans make you sick. They contain a protein that interferes with your digestive system. Cooking them denatures that protein so you can eat the bean with no ill effects. Same goes for raw kidney beans. The ancestors of the modern potato and tomato were rich in a deadly poison called solanine, and even the potatoes we eat today are poisonous if they turn green; when they are exposed to light over an extended period of time, the tubers ramp up production of both chlorophyll (to utilize the light they’re getting) and solanine (to stop anyone from eating them). And did you know that nutmeg and cinnamon naturally contain traces of a chemical called safrole which is classified as a weak carcinogen by the FDA? No lie. You can’t add safrole to anything you make because FDA found evidence to show carcinogenicity at high doses, so they banned it. But you can add nutmeg and cinnamon which contain safrole, because — well, I don’t know why, really.

I could go on, but I think I’ve made my point. There’s a lot of plants out there that try to poison animals who eat them. Even some of the plants we eat today are descended from poisonous wild relatives and were only made safe by breeding. So back in our hunter-gatherer days, plants that contained either weak or potent poisons were a constant hazard. That’s why humans (and most other animals) have been equipped by evolution with ways to get rid of possible poisons (natural or otherwise) that find their way into our system. The three key tools your body uses are the kidneys, the bile, and the metabolic machinery of your liver.

Your kidneys filter your blood continually, hanging onto useful chemicals like sugars and amino acids and letting everything else go into the waste stream. This works really well for chemicals that are highly water-soluble, and your body can usually get rid of them pretty quickly that way. Your body also excretes chemicals for which it has no use through your bile. Sometimes your liver chemically alters them as well.

Now the liver is just a biochemical machine. It doesn’t sit there and think about this and go, “gee, this chemical might be a poison because it’s highly reactive, but it WOULD be a lot less reactive if we stuck an oxygen on it right here, so….” Nothing like that. It has a bunch of enzymes that latch onto molecules with certain kinds of structural features and shapes and then alters those in a specific way. Sometimes this makes a chemical less toxic; other times it actually makes the chemical much worse and more dangerous. Methanol isn’t very dangerous, for example, until your liver turns it into formaldehyde. The ethylene glycol in your antifreeze isn’t very dangerous until your liver turns it into oxalic acid. Benzene isn’t a carcinogen until your liver turns it into one. Same goes for benzo[a]pyrene, the carcinogen you’re adding to your hamburgers when you charcoal-grill them.

How do we know what the liver does to stuff? Turns out it’s simpler than you’d think. If you take homogenized liver (basically liver that’s been ground up, like if you put it in a blender) from a human donor, you can put it or a fraction (S9, microsomes) from it in a test tube at 37C with some buffer to maintain the pH and a couple other additives, then add the drug or chemical you want to test. Next you go back and do some chemical analysis to see whether the liver enzymes in the homogenate altered the chemical and if so what they converted it into and at what rate. This kind of experiment is very common in the pharma industry, because drugmakers need to know what the liver may do to their drug. It’s also how toxicologists know the liver and the lining of your intestines take benzo[a]pyrene from wood smoke and bacon and hot dogs and convert it into a carcinogen that can react with DNA.

It might seem odd the liver would do something so bizarrely stupid, but remember the biochemical machinery of the liver wasn’t designed, it was cobbled together by evolution to deal with poisonous plants. Our mammalian ancestors got bombarded by chemicals from plants as part of their diet. Sure, the kidneys will filter these chemicals from the bloodstream, but if the chemical in question is a deadly poison, that may not be fast enough. They needed biochemical machinery that could take non-useful chemicals and chemically alter them quick, just in case they were poisons — if they are you need to alter them before they can do their job. And if that biochemical machinery occasionally happened to take the odd chemical and make it into a carcinogen…well, that was less important on the whole than rapidly altering possible poisons, because some of the plants you eat are trying to kill you. If you aren’t equipped to deal with that, some of them will succeed. Evolution doesn’t usually come up with perfect solutions; just solutions that work. It’s not an artist; it’s a tinkerer. Which is why benzo[a]pyrene is a problem.

The odd thing about this whole story to me is this. Benzo[a]pyrene is a fairly potent carcinogen; toxicologists have shown it causes cancer in animals, and there’s plenty of evidence going all the way back to Dr. Pott to show it can cause cancer in humans. And yet here we are adding it to our meat by smoking it or grilling it over charcoal and so forth.

OK, yeah, so as I said before, the jury’s still out on exactly how dangerous the doses in cooked meat may be, but…stop and think about this a minute. Just imagine that a major corporation was adding a known carcinogen to their food products. How upset would we be? Just imagine the kinds of blog posts and editorials you’d see from The Food Babe and the NYT. A lot of people would be outraged, and rightly so. But here we keep putting benzo[a]pyrene in things, and…no outrage. No outcry. Silence. All kinds of people are angry about food coloring, apparently, because Kraft recently responded to a petition by pulling food coloring from their mac and cheese. But food coloring has been studied extensively and there is no reason to think it is carcinogenic. The benzo[a]pyrene in your smoked meat, on the other hand, is almost certainly far more dangerous. But it’s natural, so no one is angry about it.

That’s why I often feel so confused about the way our society feels about chemical additives (or perhaps food in general). I feel like sometimes we worry a great deal about things that probably aren’t very important for our health or environment or the future of our species, and don’t worry very much about things that could easily be just as dangerous if not more so. We care about our health and the environment, and so we eat free-range organic smoked turkey packaged in plastic wrap. The turkey has been smoked and therefore has benzo[a]pyrene added; the plastic wrap was made from non-renewable crude oil derivatives and will wind up in a landfill as soon as we’re done with it. But it’s all natural, the turkey is free range and organic and no major corporations were involved, so we are One With Nature and everything is all good.

Right?