The International Agency for Research on Cancer (IARC) considered more than 800 studies investigating the potential links between cancer and eating certain types of meat. This year-long, deliberative process of evaluating the accumulated scientific literature led 22 experts to classify processed meats in the same category of cancer risk as cigarettes and asbestos.

Why Can Eating Processed Meat Increase Cancer Risk?

IARC’s findings received major attention, but the verdict is hardly new. Scientists have scrutinized these maligned foods for decades. Since 2002, the American Cancer Society has cautioned that the methods by which red and processed meat are prepared for human consumption and broken down in the gut can be problematic. “Meat processing, such as curing and smoking, can result in formation of carcinogenic chemicals, including N-nitroso-compounds (NOC) and polycyclic aromatic hydrocarbons (PAH).” High-temperature cooking by various methods such as pan-frying, grilling, or barbecuing can also produce PAH and heterocyclic aromatic amines (HAA).

Risky Business: How Scientists Estimate Dose-Response Relationships

Risk measures the probability that cancer will occur, taking into account the level of exposure to the agent. So, how many hot dogs are safe to eat? We don’t know. The IARC does not provide guidelines on risk, nor do they recommend regulations, legislation, or public health interventions.

We do know that eating a 50-gram portion of processed meat daily (about one hot dog) can increase a person’s relative risk of colorectal cancer by 18 percent. This means risk is multiplied by 1.18. But lifetime smoking increases risk of lung cancer fiftyfold.

Relative Risks are calculated by dividing the likelihood of developing cancer for people exposed to a particular risk factor, by the likelihood of developing cancer for people not exposed to this risk factor. Source: Cancer Research UK

Scientists are still arguing about the relationship between food and cancer because the human body is inconceivably complex. To study the entire physiological system in a randomized control trial involves feeding real food to real human subjects for months or years, which is both prohibitively expensive and ethically questionable (if trying to measure if foods might cause cancer). Instead, researchers rely heavily on observational data. Studies can control for many possible “confounding factors” — or variables that may influence a particular outcome —but it’s impossible to account for everything that may matter.

While the IARC acknowledged the many important proteins and nutrients in red meat, including vitamin B and iron, its report could hurt the American meat industry, which argues vigorously against linking their products with cancer. “They tortured the data to ensure a specific outcome,” said Betsy Booren, vice president of scientific affairs at the North American Meat Institute. The IARC defended its research. “On the basis of the large amount of data and the consistent associations of colorectal cancer with consumption of processed meat across studies in different populations, which make chance, bias, and confounding unlikely as explanations,” they wrote in The Lancet Oncology.

The IARC classifications describe the strength of the scientific evidence about what causes cancer. Placing processed meat in the same category as tobacco doesn’t mean smoking cigarettes and eating bacon every day have the same cancer risks.