Researchers have found that people who swim, bathe or take part in water sports in the sea are substantially more likely to experience stomach flu, ear aches and other types of illness than those who do not.

This could be due to seawater pollution, suggests the study published in the journal International Journal of Epidemiology. The results showed that sea bathing doubled the odds of reporting general ear ailments, and the odds of reporting earache specifically rose by 77 per cent. For gastrointestinal illnesses, the odds increased by 29 per cent.

"Our paper shows that spending time in the sea does increase the probability of developing illnesses, such as ear ailments and problems involving the digestive system, such as stomach ache and diarrhoea," said Anne Leonard of the University of Exeter Medical School in Britain. Despite significant investment resulting in an improvement in water quality in recent years, seawater is still polluted from sources including industrial waste, sewage and run-off from farmland.

For their review of studies, the researchers whittled down more than 6,000 studies to 19 studies which met the strict criteria for inclusion in the analysis. Many of the studies included thousands of participants. The number of people analysed in total exceeded 120,000. All the studies were conducted in high-income countries since 1961. The studies looked at the links between sea bathing and the incidence of illness in countries including the US, Britain, Australia, New Zealand, Denmark and Norway. "We think that this indicates that pollution is still an issue affecting swimmers in some of the world's richest countries," Leonard said.

"We don't want to deter people from going into the sea, which has many health benefits such as improving physical fitness, wellbeing and connecting with nature. However, it is important that people are aware of the risks so they can make informed decisions," Will Gaze of the University of Exeter Medical School, who supervised the research, said. "Although most people will recover from infections with no medical treatment, they can prove more serious for vulnerable people, such as the very old or very young, or those with pre-existing health conditions," Gaze added.