Saturday 4 June 2011
Mobile phones “possibly” cause brain cancer, according to a report this week from the IARC (International Agency for Research on Cancer), part of the WHO (World Health Organization). The report has triggered over 3,000 news articles around the world. Like you, I’m not interested in marginal changes around small lifestyle risks for the risks themselves; but I am interested in the methodological issues they throw up.
First, transparency: science isn’t about authoritative utterances from men in white coats, it’s about showing your working. What does this report say? How do they reason around contradictory data? Nobody can answer those questions, because the report isn’t available. Nobody you see writing confidently about it has read it. There is only a press release. Nobody at IARC even replied to my emails requesting more information.
This isn’t just irritating. Phones are a potential risk exposure where people can make a personal choice. People want information. It’s in the news right now. The word “possibly” informs nobody. How can we put flesh on that with the research that is already published, and what are the limits of the research?
The crudest data you could look at is the overall rate of different brain cancers: this hasn’t changed much over time, despite an increase in mobile phone use, but it’s a crude measure, affected by lots of stuff.
Ideally, we’d look at individuals, to see if greater mobile use is correlated with brain cancer, but that can be tricky. These tumours are rare – about 10 cases in every 100,000 people each year – and that affects how you research them.
For common things, such as heart disease, you take a few thousand people and measure factors you think are relevant – smoking, diet, some blood tests – then wait a few years until they get the disease. This is a “prospective cohort study”, but they’re less useful for studying rare tumours because you won’t get enough cases appearing in your study group to spot an association with your potential cause.
For rare diseases, you do a “retrospective case-control study”: gather lots of cases; get a control group of people who don’t have the rare disease, but are otherwise similar; then finally, see if your cases are more or less likely to report being exposed to mobile phones.
This sounds fine, but such studies are vulnerable to the frailties of memory. If someone has a tumour on the left of their head, say, and you ask, “Which side did you mostly use your phone on 10 years ago?”, they might unintentionally be more likely to remember “the left”. In one study, 10 cases (but no controls) reported phone usage figures that overall worked out as more than 12 hours a day, which might be an exaggeration.
Then there are other problems, such as time course: it’s possible that mobile phones might cause brain cancer but through exposure over 30 years, while we’ve only got data for 10 or 20 years, so the future risk may be unknowable right now (though, to be fair, exposures that cause peak problems after decades, such as asbestos, do still have measurable effects after only 10 years). And then, of course, phones change over time: 20 years ago phones had more powerful transmitters, for example. So we might get a false alarm, or false reassurance, by measuring the impact of irrelevant technology.
But lastly, as so often, there’s the issue of a large increase in a small baseline risk. The absolute worse case scenario, from the Interphone study, is this: it found phone use overall was associated with fewer tumours, which is odd. But very, very high phone use was associated with a 40% increase in tumours. If everyone used their phones that much – an extreme assumption – and the effect is true, this would still only take you from 10 cases in 100,000 people to 14 cases in 100,000 people.
So that’s what “possible” looks like. As I said, the risk is less interesting than the science behind it.
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