Spinning mobile phones

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understandinguncertainty.org was produced by the Winton programme for the public understanding of risk based in the Statistical Laboratory in the University of Cambridge. The aim was to help improve the way that uncertainty and risk are discussed in society, and show how probability and statistics can be both useful and entertaining.

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When it comes to causing cancer in humans, is using a mobile phone as risky as talcum powder, or as risky as coffee, or as risky as the notorious insecticide DDT? Actually we don't know, as I explained in my previous blog entry on last week's IARC announcement on mobile phones and brain cancer. That didn't stop the media comparing the risk of mobile phone use with all these things and more. But why did different newspapers make different comparisons?

What the IARC (International Agency for Research on Cancer, part of the WHO) did was to classify the radio waves that mobile phones use as "possibly carcinogenic to humans" - that is, they consider that it's possible that using a mobile phone may cause cancer, specifically, a certain form of brain cancer (glioma).

As I explained before, this classification is not saying anything direct about the level of risk, that is, about how much it might change your chance of getting a brain cancer if you use a mobile phone. It's just classifying how strong the evidence is that there's any risk at all.

IARC put mobile phones in their Category 2B. Putting it crudely, that means they consider there is some evidence that they might cause cancer in humans, but that evidence is not at all strong. But the meaning of the IARC's classification system is not straightforward, so it's not surprising that many of the press reports tried to explain it by giving examples.

The trouble is that the IARC currently lists no less than 266 different agents in Category 2B. You wouldn't expect a news report to list all 266. What's more, most of them are chemicals with long names. I certainly have never heard of most of these, and I suspect I'm not alone in that, so it wouldn't help most readers for a newspaper to list 4,4'-Diaminodiphenyl ether or 3-Monochloro-1,2-propanediol as things to compare mobile phones to.

But there are a few that would be meaningful to most people - some examples are coffee, talcum powder (applied to the nether regions), petrol (gasoline) exhaust fumes, and working as a firefighter.

Different newspapers and media chose different comparators from this list. For instance, the Sun and the Daily Express mentioned the same two - petrol exhaust fumes and DDT, the insecticide.

Does that make mobile phones sound scary?

The headline in the Times, on the other hand, was "Mobile phone cancer risk 'is at coffee level', WHO study finds". (The full story is behind a paywall.) I think that sounds more reassuring. Similarly, the Guardian went with talcum powder, working in a dry cleaner's, and "low-level magnetic fields" (by which I think they mean "extremely low frequency magnetic fields", such as come from domestic electricity supplies).

The overall tone of the stories in the Guardian and the Times, compared to that of the Express (particuarly) and the Sun, is different too. The Guardian and the Times report the IARC classification, but are generally not alarmist about the actual level of risk.

The Express story makes the risk of mobile use sound really pretty serious. Linking mobile use to exhaust fumes and to DDT helps the scary line that the Express is trying to get across. Nobody thinks exhaust fumes are good for you. They can kill, in high enough concentration - but not by giving you cancer.

Similarly, most people would know that the use DDT is now under tight legal restriction in most countries, and that it can have serious effects on wildlife. It must be nasty stuff, mustn't it? Well, perhaps, but its nastiness has nothing to do with cancer in humans. In fact the IARC regard the direct evidence in humans that DDT causes cancer to be "inadequate". They gave it its 2B evidence level only because there is evidence that it causes cancer in experimental animals.

But mud sticks, and putting mobile phones alongside exhaust fumes and DDT does make phones sound pretty dangerous.

On the other hand, putting phones alongside everyday things like coffee and talcum powder gives entirely the opposite impression, I'd say. There is some weak, limited evidence that coffee might have a causal link to bladder cancer in humans, and that talcum powder might have a causal link to ovarian cancer, but that isn't what would spring first to most people's minds when thinking of those substances. (Don't forget that the IARC classification system says nothing about the actual level of risk, only about the level of evidence for risk.)

So, in what appeared simply to be a useful way of helping readers to understand what the IARC was really saying, newspapers could get across their angle on the story more effectively just by choosing the examples that fitted. The media had to be selective, because they couldn't list all the possible comparators, but being selective involves choice, and you have to ask yourself why they made the choices they did.

The overall message here is that, when you're reading a story about risk (or indeed about anything else), you should always ask yourself, "Why are they telling me this?" and "What are they not telling me here?" That is, be suitably sceptical!

(Needless to say, you should ask these very same questions when you're reading my blog...)