Hot coffee causes cancer? Here's what you really need to know

With the latest news from WHO, people are confused about the cancer risk of coffee and hot drinks in general. So here's what you need to understand about those rankings.

What is the IARC classification?

You’ve probably heard the latest news this week - according to World Health Organisation, coffee ‘does not cause cancer’ any more, while any drink enjoyed too hot (including coffee) is now a probable carcinogen.

But what does ‘probable carcinogen’ actually mean? And why does this information keep changing?

The International Agency for Research on Cancer (IARC), WHO’s authoritative ‘cancer branch’, has been compiling the evidence on cancer causes since the 1970s. To date, they have reviewed more than 900 agents, such as chemicals, occupational exposures, personal habits, and others.

These are categorised into five groupings, and changed according to what new evidence brings to light.

Level of evidence, not level of risk

All these agents investigated by the IARC have been classified into five categories based on the level of evidence - that is, how certain we are this particular thing causes cancer.

But these categories do not reflect the level of risk - that is, how likely you are to get cancer if you go anywhere near the substance, how many people it is likely to affect, what kinds of cancer it could cause, or how many cancers.

This difference is crucial.

It means that two substances included in the same category must not be likened to one another based on their category alone. They do not present the same level of danger.

Let's take a look at these categories below.

Group 1: Carcinogenic to humans

What it means: this thing really does cause cancer and we know it. In other words, the evidence is convincing and sufficient.

Right now this group includes 118 agents, including such well-known examples as asbestos and tobacco smoke. But the list also contains furniture making (wood dust), painting (with benzene-containing dyes), and, famously, processed meat.

Just having the evidence that something is carcinogenic says nothing about the risk it presents, as demonstrated in this infographic by Cancer Research UK.

Group 2A: Probably carcinogenic to humans

What it means: this thing could cause cancer but we are not entirely sure. In other words, there is some evidence, but other explanations can’t be ruled out.

Right now this group includes 80 agents, including acrylamide, red meat, indoor emissions from firewood, and, as of this week, very hot beverages (over 65°C).

Again, this says nothing about how much cancer any of these things are likely to cause.

Group 2B: Possibly carcinogenic to humans

What it means: this thing can’t be dismissed as not causing cancer, usually because there is some evidence from animal studies.

Right now this group includes a whopping 289 agents, from lead to ‘coconut oil diethanolamine condensate’, as well as welding fumes and occupational exposure to dry cleaning.

Until this week, coffee was in this group, but has been bumped thanks to a new review - a great example of evidence assessment in action: acquiring new knowledge leads to updates.

Group 3: Not classifiable as to its carcinogenity to humans

What it means: literally, we don’t know whether this thing causes cancer or not, because the evidence is inadequate. We looked into it, but there’s not enough to go on.

Right now this group includes 502 agents. It’s a big grab bag of multisyllabic compounds along with more recogniseable things like fluorescent lighting, talc, several food colourings and, as of this week, coffee.

As you can see now, in terms of certainty, the difference between groups 2B and 3 really isn’t much.

Group 4: Probably not carcinogenic to humans

What it means: this thing does not cause cancer, we’re pretty sure about that. In other words, we looked into it and evidence says it’s okay.

You might think this is where we find all the “natural”, “healthy”, “good-for-you” stuff? Nope.

Right now, this group includes one agent. It’s called caprolactam, an organic compound used in the production of nylon. It might not cause cancer, but it’s a slightly toxic irritant.

So how dangerous is drinking of hot beverages again?

Here’s what the IARC wants you to know:

“We don′t know how dangerous it is. The IARC evaluation tells us only that there is a probable link between drinking very hot beverages and oesophageal cancer. In most countries, coffee and tea are not drunk at such high temperatures, but in parts of Central Asia, South America, and East Africa, it is common for people to regularly drink beverages such as tea and maté at very high temperatures."

Next time you hear that something causes cancer, remember that it says nothing about how dangerous it actually is.

Read these next
As you were, coffee drinkers. WHO says coffee is not a cancer risk.
Coffee consumers across the globe can continue to sip their favourite beverage without fear. The World Health Organisation says there’s not enough evidence to rule that coffee causes cancer.
What's the deal with nanoparticles in sunscreen?
Some people worry about nanoparticles in their sunscreen, but experts say these products are safe and effective.
How the chemistry of water can affect the taste of your coffee
A computational chemist is changing the way artisan coffee makers think about water - and science as well.