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This is a plain English summary of an original research article. The views expressed are those of the author(s) and reviewer(s) at the time of publication.
People who swim in seawater have almost double the odds of experiencing illness than people who avoid it. The specific illnesses linked to seawater exposure are ear and gastrointestinal illnesses, but the exact or absolute rates of infection are not available.
Many people enjoy coastal waters for sport and recreation, and it's important that they can access relevant risk information. This is the first systematic review to look at infection risk from swimming in seawater. It gathered data from 40, mostly observational, studies. Studies were conducted in high-income countries; eight came from the UK.
Although increased illness was seen consistently across analyses, it’s not possible to imply direct causation from seawater. There was insufficient information on the background characteristics or level of illness among bathers to understand if other factors were in play.
But given these caveats, the study does provide useful data to support public health information on the potential infection risk from swimming in seawater. For individuals, it will be important to put these small risks into context of the wider health benefits and enjoyment they may get from swimming.
Why was this study needed?
Swimming in sewage-contaminated water is thought to be associated with infections of the ear, nose and throat, and gastrointestinal system. In the UK, choosing a Blue Flag Beach is thought to be a safe option for swimming and other water-based activity (defined by these researchers as bathing). This means the area complies with a set of environmental and safety criteria.
Water quality regulation in the UK focuses on measuring levels of the bacterium Escherichia coli (E. coli), which originates from faeces. However, levels of these bacteria fluctuate with the weather, for example after storms and flooding because of overflow from agricultural and urban sources.
There is less information on other illnesses. This systematic review aimed to measure the risk of any illness in people who bathe in seawater. It also aimed to assess whether this risk differed depending on the amount of time people spent with their head immersed in water.
What did this study do?
The authors searched six literature databases, a specialist journal, and other information sources for studies that measured the risk of symptoms or markers of infection in people who bathe in seawater compared with people not bathing in seawater. It did not include studies of health risks from swimming in freshwater rivers and lakes.
Forty studies were included, totalling 147,583 participants. Half of the studies came from the US, eight from the UK, and the remainder came from a selection of other economically developed countries. Two were randomised controlled trials, randomising to bather and non-bather groups; the rest were observational studies. Study sample size varied widely from 25 to over 25,000 people. Most studies approached people at the beach, including a mix of adults and children. They gathered self-reported information on seawater exposure and symptoms of illness.
Most studies were assessed to be of moderate to good quality.
What did it find?
- Bathing was associated with an increased risk of any illness compared with no bathing (odds ratio [OR] 1.86, 95% confidence interval [CI] 1.31 to 2.64; six studies). This result should be treated with some caution because there was considerable variability in the results of individual studies.
- Bathing was associated with an increased risk of ear illnesses compared with no bathing (OR 2.05, 95% CI 1.49 to 2.82; six studies), this time with reasonably consistent effect across studies.
- Bathing was also associated with an increased risk of gastrointestinal illnesses (OR 1.29, 95% CI 1.12 to 1.49; 16 studies). This result should be treated with caution due to the variability between studies.
- Bathing with head immersion was associated with an increased risk of any illness compared with no bathing (OR 1.91, 95% CI 1.40 to 2.60; four studies). Similar results were obtained for the link with gastrointestinal illness or specific symptoms like diarrhoea or stomach ache. However, there was no analysis comparing bathing with and without head immersion. So there is no evidence that, when bathing, going underwater carries any greater risk.
- Results did not appear to differ by the continent in which the study was conducted.
What does current guidance say on this issue?
No relevant guidance is available.
What are the implications?
The findings are consistent with previous research linking seawater exposure with risk of illness. However, there are uncertainties.
Many studies have collected self-reports on bathing and illness history at the beach. We can’t be sure that there is a clear temporal relationship between seawater exposure and subsequent illness. We know little about participants’ history, such as whether they are regular swimmers, how long they have experienced symptoms or their severity. It is unknown whether there is any connection with the water quality of the areas studied.
Because of the nature of these studies, it’s difficult to be certain of the level of risk faced by an individual and balance this against the benefits expected. Absolute rates of infection were not available in this study. Providing location-specific information would be most helpful from a public health perspective.
Citation and Funding
Leonard AFC, Singer A, Ukoumunne OC, et al. Is it safe to go back into the water? A systematic review and meta-analysis of the risk of acquiring infections from recreational exposure to seawater. Int J Epidemiol. 2018. [Epub ahead of print].
This study was funded by the European Regional Development Fund (grant number 500020) and two researchers are funded by the NIHR CLAHRC South West Peninsula.
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