The bacon freak-out: Why the WHO's cancer warnings cause so much confusion
On Monday, the World Health Organization's International Agency for Research on Cancer (IARC) announced
Sadly, that ushered in a lot of sloppy journalism and needless panic. Some news outlets are suggesting
The main thing the IARC established was a casual link
The trouble is that the IARC uses a very confusing scale for classifying carcinogens. The group first examines various substances — from sunlight to alcohol to various chemicals — and then reviews all scientific evidence to see whether these substances can be linked to any type of cancer in humans. The group then classifies these substances based on the answer to this question. Here's a chart
state of research
Then comes Group 2B, substances that are "possibly carcinogenic," which means there's limited or insufficient evidence for a cancer link in both animals and humans. Coffee falls into this category. So does gasoline. Scientists have found suggestive hints these things might be able to cause cancer, but much more research is needed. That's all. It'd be absurd to conclude that coffee and gasoline fumes are equally good (or bad) for you based on this classification.
Further down, there's Group 3, substances that are "not classifiable," which includes stuff like tea where we just have no idea one way or the other about cancer risks. And, finally, there's Group 4 where scientists have conclusively ruled out any link to cancer. Amusingly, only one chemical, caprolactam, has made it into Group 4 so far. (Caprolactam is mildly toxic and an irritant, but scientists are quite confident it doesn't cause cancer.)
The WHO's scale doesn't tell us about the size of the cancer risks involved
Ultimately, though, it's not terribly interesting if a substance can cause cancer. What's important is what the actual risks to people are. But to understand that, we have to ditch the IARC's classifications and dig into the numbers. So let's compare processed meat with smoking.
The IARC estimated that eating 50 grams of processed meat daily — one hot dog, say, or two bacon slices — can increase your relative risk of colorectal cancer by 18 percent. That's way less scary than it sounds. In the United States, a person's lifetime risk of colorectal cancer is about 4.5 percent. So eating a hot dog every single day would bump that lifetime risk up to 5.3 percent. Eating more processed meat would nudge those numbers up further. But the overall risk is still fairly low.
Now consider cigarettes. One 1994 study of Canadians found that the lifetime risk of lung cancer for male non-smokers was about 1.3 percent. But regular smoking increased that risk to 17.2 percent. The CDC likewise estimates that smokers are 15 to 30 times more likely to develop lung cancer than non-smokers, depending on the situation. This is clearly a much bigger worry than having a hot dog now and then.
Or look at it this way: The Global Burden of Disease Project attributes about 34,000 cancer deaths each year to diets that are high in processed meat. That sounds like a lot, but it pales beside the 1 million annual cancer deaths attributable to smoking. Both are real public health issues, but one problem is orders of magnitude deadlier than the other. (By the way, these numbers are solely focused on cancer risk, not overall health risks or environmental impacts.)
Keep this in mind whenever you hear that the WHO has classified some new substance as "carcinogenic" or "probably carcinogenic." It's one thing to establish causality. It's another thing entirely to tell us what the risks actually entail.
Further reading: The WHO's new warnings about bacon and cancer, explained