Alcohol: Well known to be carcinogenic to humans. Despite this, a large proportion of the population drink it regularly. More surprisingly, whenever the International Agency for Research on Cancer updates its carcinogen classifications for other substances with a lower cancer risk, there’s often media fanfare. In recent years, the IARC has upgraded classifications for red meat and aspartame, leading to a spate of panicked articles. This republished and updated post takes a look at what the classification groups actually mean, and how worried we should be about a substance’s classification.
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 examples of group 1 substances include alcohol and smoking.
Red meat, on the other hand, is 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 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 of 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 pickled vegetables 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.
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. This was caprolactam, a compound primarily used to manufacture nylon. As of 2019, however, group 4 in IARC’s classification stands empty: caprolactam was upgraded to group 3, ‘carcinogenicity not classifiable’, after a review of evidence.
After all this, you might be wondering what the frequent news reports on IARC classifications actually mean for you. Should you give up anything classified above group 3? It all comes back to the fact that the IARC’s system tells us nothing about the relative increases in the risk of cancer from the substances it classifies. A harsher criticism would be that it’s a system which is more frequently misleading than helpful, at least in terms of how it’s often reported in the media. And since you’re probably having some exposure to IARC’s group 1 carcinogens anyway, it’s probably not worth sweating the small stuff!
References & Further Reading
- Agents classified by the IARC monographs – IARC
- IARC monographs evaluate red meat & processed meat – IARC Press Release
- Meat & tobacco – the difference between risk & strength of evidence – S Gage, The Guardian
- Agents classified by the IARC monographs – IARC
- Carcinogenicity of consumption of red meat & processed meat (£) – IARC Working Group