Why do some people contract cancer and others not? Some cancers are genetic but the vast majority occur when something goes awry in the normal process of cell division, usually due to damage to DNA caused by exposure to some form of radiation or certain chemicals. But the devil is in the details and the extent and type of exposure matters. Sunlight can cause skin cancer but there is a big difference between going for a lunchtime walk in Montreal in the winter and baking in the Australian summer sun. Furthermore, not every Australian sun worshipper gets skin cancer. Not everyone who smokes gets lung cancer and not everyone exposed to benzene develops leukemia. Why is this?

The body has defense mechanisms to repair DNA damage. There is a limit to the amount of repair that can be done. These repair mechanisms are individualized, determined by genetics, physical fitness and intake of dietary components, mostly those found in fruits and vegetables. This is why discussions of carcinogenicity are always peppered with terms like  “possibly” and “probably.”

Various organizations classify carcinogens differently, with the most widely referenced one being the International Agency for Research on Cancer (IARC). Its Group 1 includes substances that are definitely known to cause cancer based on human epidemiological evidence. These include substances like tobacco products, wood dust, diesel exhaust, salted fish, asbestos, benzene, formaldehyde, dioxin and ultraviolet light. But aflatoxins from moulds, the cancer drug tamoxifen, infection by Helicobacter pylori, herbal remedies containing Aristolochia species, X-rays, alcoholic beverages and occupations such as painter, cabinetmaker or chimney sweep are also on the list. Exposure is key. The vast majority of people can consume alcohol without it triggering cancer.

Group 2A lists “probable carcinogens” as determined by strong evidence from animal studies but limited evidence from human studies. These include lead compounds, acrylamide in baked goods, emissions from frying foods, hairdressing as an occupation, shift work and insecticides such as diazinon and malathion. Group 2B substances are “possibly carcinogenic to humans” based on limited animal or human evidence. These include coffee, pickled vegetables, radiofrequency electromagnetic fields, titanium dioxide and DDT, along with occupational exposure through carpentry, firefighting and textile manufacturing. In all, there are more than 450 substances or processes that are classified as known, probable or possible carcinogens.

Public concern reached new heights in 2015 when IARC listed glyphosate, the widely used herbicide, as a “probable human carcinogen.” Just a few months later, the European Food Safety Authority (EFSA) concluded that it was very unlikely that this chemical causes cancer. How can two groups of experts come to such different conclusions? Which one is right? Actually, both are. IARC’s evaluation is based on hazard, EFSA’s on risk. If a substance can cause cancer in some animal in some situation, no matter how extreme, IARC labels it a carcinogen. But a risk analysis, as performed by EFSA as well as Health Canada, evaluates the likelihood of contracting cancer under realistic exposures. This is the evaluation that is relevant for the public. EFSA concluded that there is no risk at a daily exposure of 0.5 milligrams of glyphosate per kilogram of body weight, which for an 80 kilogram person would equal residues found on 400 kilograms of fruits or vegetables.

The IARC later created panic by declaring that processed meats belong in Group 1. After poring through some 800 peer-reviewed publications, IARC estimates that eating 50 grams of processed meat every day over a lifetime increases the cancer risk by about 18 percent. In other words, if 100 people followed such a regimen over a lifetime, there will be seven cases of colorectal cancer instead of six. For an individual, the chance of getting colon cancer because of eating processed meats is about one in 100. 
What do we do with all this information? There is no way to avoid all the known and suspect carcinogens but we can limit alcohol intake, sun exposure and avoid smoking and gorging on processed meats. Fruits and vegetables may help boost repair mechanisms. And if you are offered a job as a chimney sweep, don’t take it.   

Joe Schwarcz is the director of McGill­ University­’s Office for Science and Society. Read his blog at www.mcgill.ca/oss.