“Swimming too often in chlorinated water ‘could increase risk of developing bladder cancer’, claim scientists”.
But did they, or is this another example of misreporting cancer research by the media?
Unfortunately, it’s the latter. This is an example of poor reporting, which makes unwarranted claims about human health. The research itself says nothing new about bladder cancer, and didn’t even involve people with the disease.
Let’s have a look at what’s going on.
The story came from research carried out by a Spanish team, led by Dr Gemma Castaño-Vinyals, which is investigating the possible effects of man-made chemicals on cancer rates in the region. For this study, the researchers tried to find out whether people from different social classes and backgrounds might have different levels of exposure.
Their findings, which were published in the journal ‘Environmental Health’ (not, as the newspaper claimed, ‘BioMed Central’), is freely available to download – you can read it here (pdf) and here’s the accompanying press release (Word).
What does the research say?
The paper describes how the researchers re-analysed data from a study called the Spanish Bladder Cancer case-control study, which was carried out during 1998 – 2001. As part of this study, 1,271 people without bladder cancer were interviewed about a variety of their habits and behaviours – this was to compare them to people with bladder cancer as a ‘control’ group.
The new study looked at the information recorded about these ‘control’ subjects, in particular whether they had said they drank tap or bottled water, how often they said they showered or took baths, and how often they remembered going to the swimming pool. It also looked at the information on their social background and where they lived.
Next, the researchers asked water companies around Spain for data from the same period, about the levels of certain types of by-product from the disinfection process called trihalomethanes (THMs). They also measured levels of these chemicals themselves. This allowed them to make a rough estimate of how much THM people might have been exposed to over the study period.
Finally they did some stats – they looked at the relationship between estimated THM exposure and people’s social background. They concluded:
The most highly educated subjects were less exposed to chlorination by-products through ingestion, but more exposed through [skin] contact and inhalation in pools and showers/baths. Health risk perceptions and economic capacity may affect patterns of water consumption that can result in differences in exposure to water contaminants.
The results didn’t say anything about at difference in bladder cancer risk between the different social groups, it merely proposed that different people behave differently with respect to how they use water.
Do THMs affect human health?
Several studies and organisations have looked at whether substances we add to water to stop bacteria growing it might be inadvertently harming us. It’s fair to say that there’s some evidence of a possible risk, but other studies that don’t show an effect. More importantly there’s no hard evidence of a ‘smoking gun’ – if these chemicals do have any effect, it’s likely to be very small.
The World Health Organisation’s recommendations on drinking water conclude that:
The use of chemical disinfectants in water treatment usually results in the formation of chemical by-products. However, the risks to health from these by-products are extremely small in comparison with the risks associated with inadequate disinfection, and it is important that disinfection not be compromised in attempting to control such by-products.
The UK’s Drinking Water Inspectorate has also published a comprehensive analysis (pdf). It too concludes that there’s very little good quality data on the health effects of THMs, and that the guidelines that do exist are ‘highly precautionary’ – i.e. they are almost certainly sufficient to protect our health, given what we currently know.
So given the scientific consensus that these chemicals probably aren’t significantly affecting our health, it’s somewhat unfortunate that a national newspaper chose to highlight that they are, despite any new research to the contrary.
And if a thorough review of the evidence concludes that the science isn’t strong enough yet to assess any risks, it certainly isn’t strong enough to justify a scary headline.
So what does cause bladder cancer?
By far the biggest risk factor for bladder cancer – as for so many types of cancer – is tobacco, which causes about two thirds of cases.
There’s also evidence that people who work with high concentrations of certain chemicals such as dyes, for long periods of time may have an increased risk. There’s more detailed information about what is known about the causes of bladder cancer in the CancerStats section of our website.
And what about water?
In developed countries such as the UK, we have extremely strict standards and regulations governing what can and can’t be present in the water we drink and bathe in. It’s fair to say that our water is the safest it’s ever been – especially as far as cancer is concerned.
There have, of course, been persistent myths and rumours about possible contaminations from the plastic bottles many of us use to drink from – and headlines like the ones in the papers this week do nothing to dampen down these concerns.
At Cancer Research UK we do our best to provide you with the latest evidence on what causes cancer – and what doesn’t. You can read more about this in the Cancer Controversies section of our website. Next time you see an alarming headline in the media, it’s always worth popping by to find out what the science really says.
Castano-Vinyals, G., Cantor, K., Villanueva, C., Tardon, A., Garcia-Closas, R., Serra, C., Carrato, A., Malats, N., Rothman, N., Silverman, D., & Kogevinas, M. (2011). Socioeconomic status and exposure to disinfection by-products in drinking water in Spain Environmental Health, 10 (1) DOI: 10.1186/1476-069X-10-18
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