Dr Aust discusses the attempt by Dr John Briffa to convince people to buy more bottled water (much of which is actually just tap water!) by emphasising the apparent risk from drinking tap water: drinking chlorinated water regularly over large periods of time may be associated with a small increase in the risk of bladder cancer:

SO… what is the snag with the statement as presented on John Briffa’s blog?

To my way of thinking, the problem is the lack of explanation, and particularly of context. The basic statement “there is quite a body of evidence linking the consumption of tap water with an increased risk of cancer” leaves out several crucial pieces of information that you need to put this information in context.


Anyway, without this kind of information, which you are not given by being told “There is scientific evidence that cholorinated water increase cancer risk” you cannot make a sensible judgement about whether to be worried.

Instead, you just worry. And then drive off to the supermarket in your 4×4 SUV to get a huge multi-pack of bottled mineral water. Ker-chhhinnng.

Context is vital. As it turns out, the studies are not even close to clear: they suggest, but do not prove, a small increased risk of bladder cancer. Dr Aust goes on to suggest a less misleading way to put the risk in perspective:

Risk factors are interesting. They are the meat and drink of medical and scientific papers about causes of disease, but are notoriously poorly understood. And peoples’ reaction to them is quite strange and unpredictable. The same “risk rate” means different things to people in different contexts.

An interesting way to probe how you view a “risk factor”, I find, is to “invert” it – to turn it back to front. This is because “increase” always sounds more scary, and hence more useful to someone trying to sales-pitch you, than “decrease”.
“Do you know you can reduce your risk of bladder cancer by about one-sixth if you never drink, or bathe in, or swim in, chlorinated water?”

I bet that most people, on reading something like that, would immediately ask “Okay, what’s my risk of bladder cancer now?” — a question that all too often gets forgotten if you describe risk in terms of an increase. (The answer is: not very high. Most bladder cancers are diagnosed in people in their 70s or older, and they are one of the more easily treated cancers.)