Tap water may be the cause of one in 20 cases of bladder cancer in Europe each year, a major study suggests.
Researchers have linked drinking, showering and bathing in the water to more than 6,500 cases of the disease in 26 countries across the continent.
They estimate 1,356 bladder cancer diagnoses in Britain since 2005 were caused by contaminated water, making up a fifth of all cases in the EU.
Long-term exposure to a group of chemicals called trihalomethanes (THMs) is thought to be the cause.
The chemicals, shown to be carcinogenic in animal studies, are formed as an unintended byproduct when water is disinfected with chlorine at supply plants.
As well as being drunk, steam given off in the shower can allow tap water to seep into people’s pores, and so too can prolonged periods in the bath.
Earlier research has found an association between THMs and bladder cancer, but this is the first to estimate the scale of the problem.
Some 10,000 people are diagnosed with bladder cancer each year in the UK, making it the tenth most common form of the disease.
The latest study estimates 9 per cent of them are caused by exposure to water contaminated with THMs.
Scientists from the Barcelona Institute for Global Health (ISGlobal) set out to analyse the presence of the chemicals in drinking water in all 28 EU states between 2005 and 2018.
They did this by sending questionnaires to bodies responsible for the national water quality. Data was obtained for 26 countries – all except Bulgaria and Romania.
The scientists probed levels of THMs in water at the tap, in the distribution network and at water treatment plants.
To make estimates as accurate as possible, the researchers also used other sources including open data online, reports and scientific literature.
The findings revealed considerable differences between countries.
Average THM levels were above legal levels in nine countries – Britain (24.2ug/l) , Spain, Portugal, Poland, Cyprus, Estonia, Hungary, Ireland and Italy.
The European Union has said the maximum limit is 11.7 micrograms per litre (ug/L).
Tap water may be behind more than one in 20 cases of bladder cancer in Europe each year. The cause is long-term exposure to a group of chemicals known as trihalomethanes (stock)
Researchers estimated the number of attributable bladder cancer cases using incidence rates and levels of THMs.
Analysis suggested Cyprus had highest percentage, with a quarter of diagnoses linked to the chemicals.
Joint second-wort were Ireland and Malta, where one in six bladder cancer patients (17 per cent) are thought to have developed it from exposure to tap water.
Spain (11 per cent) and Greece (10 per cent) were not much better. At the opposite extreme was Denmark, where less than 0.1 per cent of cases were caused by THM water contamination.
In Holland it was just 0.1 per cent of cases and Germany 0.2 per cent. THMs could be blamed for 0.4 per cent of cases in Austria and Lithuania.
In total, the researchers estimated that 6,561 bladder cancer cases per year are attributable to THM exposure in the European Union.
The study authors say if the 13 countries with the highest averages were to reduce their THM levels to the EU average, 2,868 annual bladder cancer cases could be avoided.
Chloroform is the most infamous type of THM, which the World Health Organisation (WHO) describes as ‘probably carcinogenic to humans’.
Co-study author Manolis Kogevinas, a researcher from ISGlobal, said: ‘Over the past 20 years, major efforts have been made to reduce trihalomethanes levels in several countries of the European Union, including Spain.
‘However, the current levels in certain countries could still lead to considerable bladder cancer burden, which could be prevented by optimising water treatment, disinfection and distribution practices and other measures.’
The findings are published in the journal Environmental Health Perspectives.
A spokesperson for Britain’s Department for Environment, Food and Rural Affairs said: ‘This Government takes the safety of our drinking water extremely seriously, and have a robust monitoring regime in place which tests for a wide range of chemicals, including trihalomethanes, to ensure the public can have absolute faith in the water coming from their taps.
‘The latest figures from 2018 found only four samples out of 11,900 exceeded the legal limits, and firm action was taken in each of these four cases.’