A Rough Guide to IARC Carcinogen Classifications

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Today’s big news has been the story that the World Health Organisation (WHO) has classified processed meat (including bacon, ham, and salami) as a Group 1 carcinogen. This places it in the same group as smoking, which has led to a number of headlines claiming that it means the risk from the two is the same. It isn’t – and today’s post takes a close look at the International Agency for Research on Cancer’s classification system in order to explain why.

The IARC is a part of the WHO. The IARC’s system was developed to classify different chemical agents, mixtures, or exposures, into one of five groups depending on the evidence for their cancer-causing potential, or carcinogenicity. They began publishing their categorisations in 1971, and since then have assessed over 900 different agents.

The important thing to realise about the IARC classifications is that they don’t assess the level of risk that a particular agent poses with respect to cancer. They simply rank the quality of the evidence of it being cancer-causing. Group 1 is the highest in this regard – the placement of a substance into this classification means that there is sufficient evidence in humans for it causing cancer. Other example group 1 substances include alcohol and smoking.

Red meat, meanwhile, was placed into group 2A. This group is for substances defined as ‘probably carcinogenic to humans’; this means that the evidence in humans is still somewhat limited, but there is sufficient evidence in experimental animals of the substance’s carcinogenic nature. As the evidence decreases, so does the ranking. Group 2B ‘possibly’ causes cancer, group 3 is for substances for which the evidence remains inadequate to state either way, and group 4 is for those for which there is evidence that they are not carcinogenic.

So substances being in the same group tells us the evidence for their carcinogenicity is comparable, but tells us nothing about their relative risks. According to Cancer Research UK, smoking causes 19% of all cancers; by contrast only 3% of all cancers are thought to be caused by processed meat and red meat combined. To put this in a little more perspective, it’s estimated that 34,000 cancer deaths worldwide every year are caused by diets high in processed meat, compared to 1 million deaths per year due to smoking, and 600,000 due to alcohol consumption. It’s clear then that the headlines likening the risk of cancer from smoking to that of eating processed meat are well wide of the mark.

It’s also interesting to note the other substances found within the different IARC groups. Group 1, as we’ve mentioned, contains alcohol, which a large number of us drink on a regular basis. It also contains sun exposure – the DNA damage caused by UV radiation from the sun can increase the risk for developing skin cancers.

Red meat falls into the same category, group 2A, as the emissions from frying food at high temperatures. Additionally, exposure to various substances whilst working as a hairdresser or barber is also found in this category. Remember, this simply means the substances or exposures in this group all probably cause cancer, and doesn’t tell us the level of the risks.

When you get down to the other groups, it becomes clear that merely having an IARC classification doesn’t always pose a cause for concern. Substances like coffee are classified as ‘possibly carcinogenic’, simply because the evidence isn’t strong enough one way or the other. In fact, any substance or exposure tested by the IARC gets put into one of these five groups, and there’s actually only one substance that’s been placed into group 4 (probably not carcinogenic) in the history of all the substances that have been assessed.

After all this, you might be wondering what the news on processed meats and red meat actually means for you. Should you give up both and go fully vegetarian? Well, the IARC concluded that eating 50 grams of bacon per day would increase your risk of colorectal cancer by 18%. This sounds pretty significant, but when you look at the actual numbers behind the percentage increase, it makes it a bit clearer. On average, 64 out of 100,000 people develop colorectal cancer per year; eating 5o grams of bacon every day would raise your risk to 72 in 100,000.

In short, unless you go on regular bacon binges, today’s news isn’t something to be overly concerned about. Smoking is still a vastly bigger risk factor for cancer than the odd rasher of bacon every now and then. There are health benefits to eating meat, too, so it’s not necessary to cut it out of your diet entirely – simply enjoy it in moderation, as with most things.

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