People who swim, bathe or take part in water sports in the sea are substantially more likely to experience stomach bugs, ear aches and other types of illness than those who do not.

The large-scale research analysis was led by the University of Exeter Medical School in collaboration with the Centre for Ecology and Hydrology. It is the first systematic review to examine the evidence on whether spending time in the sea is associated with increased risk of reporting a variety of ailments.

The results demonstrated that sea bathing doubled the odds of reporting general ear ailments, and the odds of reporting earache specifically rose by 77%. For gastrointestinal illnesses, the odds increased by 29%.

Dr Anne Leonard, of the University of Exeter Medical School, said: "In high-income countries like the UK, there is a perception that there is little risk to health of spending time in the sea. However, 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 diarrhea. We think that this indicates that pollution is still an issue affecting swimmers in some of the world's richest countries."

Despite significant investment resulting in an improvement water quality in recent years, seawater is still polluted from sources including industrial waste, sewage and run-off from farmland.

The researchers whittled down more than 6,000 studies to 19 studies which met the strict criteria for inclusion in the meta-analysis, designed to establish robust research evidence. Many of the studies included recruited 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, UK, Australia, New Zealand, Denmark and Norway.

Dr Will Gaze, of the University of Exeter Medical School, supervised the research. He 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. 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. We have come a long way in terms of cleaning up our waters, but our evidence shows there is still work to be done. We hope this research will contribute to further efforts to clean up our coastal waters."

Dr Ruth Garside, of the University of Exeter Medical School, is an expert on systematic reviews. She said "Systematic reviews use rigorous methods to identify and assess all the relevant research on a topic in an unbiased way. The study findings are then combined statistically -- this allows us to have more confidence in the results."

The work was funded by the European Regional Development Fund.