Swim healthy

Updated 24 June 2019

Open water swimming refers to swimming in lakes, rivers and the sea, rather than at a swimming pool. Without the confines of ropes and lanes it can be an invigorating experience. However, swimming in open water carries different risks to swimming in a pool.

Open water swimming can increase the risk of gastrointestinal illnesses, or stomach bugs, which may cause diarrhoea and/or vomiting, as well as respiratory, skin, ear and eye infections. Most symptoms of these illnesses caused by micro-organisms such as norovirus, giardia and cryptosporidium, will generally be mild. However, there is also a risk of more severe infections caused by micro-organisms such as E.coli O157 which may cause severe gastrointestinal illness and leptospirosis (Weil’s disease), which can cause liver and kidney problems.

Bathing water quality and how it’s monitored

Many coastal waters and a few inland lakes known to be popular swimming locations are designated as bathing waters. Bathing waters are monitored and protected from sources of pollution known to be a risk to bathers’ health. The Environment Agency monitors over 400 designated bathing waters in England between May and September, and each is given an annual classification of Excellent, Good, Sufficient or Poor. As well as the annual classification, the Environment Agency makes daily pollution risk forecasts for a number of bathing waters where water quality may be temporarily reduced due to factors such as heavy rainfall. Bathers may be at a higher risk of becoming ill when using a bathing water at this time.

Rivers and other open water locations that are not designated as bathing waters are managed for the purpose of protecting fish and wildlife, not people, so health risks from using these locations may be higher than at designated bathing waters. These waters are not monitored for the 2 types of bacteria; intestinal enterococci and E. coli, which are used to assess bathing water quality. They can contain levels of sewage, faeces from livestock and pollution from farming or industry which are harmless to wildlife but would not be acceptable in designated bathing waters.

Health risks

Anyone can become unwell from swimming in any open water as there will always be micro-organisms present. The risk of becoming ill depends on various factors:

  • children and novice swimmers are more likely to swallow water accidentally
  • those with an impaired immune system are more susceptible to infection
  • those swimming in rivers and estuaries are more likely to become unwell
  • heavy rainfall can wash harmful bacteria from agricultural land, urban areas and sewage to rivers, seas and bathing waters and affect water quality

Risks from freshwater and marine algae

There are various types of algae that occur naturally in all natural waters, but evidence suggests the one that poses the greatest risk to health is blue-green algae (cyanobacteria) in freshwaters.

Contact with freshwater blue-green algae in the form of blooms and scum can lead to symptoms such as skin rashes, eye irritation, vomiting, diarrhoea, fever, muscle and joint pain. Toxins produced by freshwater blue-green algal blooms are poisonous to wild and domestic animals and can cause severe illness and death.

Blue-green algal blooms and scum are not always toxic. However, it is not possible to tell from their appearance whether they are harmful. At freshwater locations, the risk from blue-green algae is assessed and warning signs are displayed if a toxic bloom is confirmed. Children, farmers and pet owners should treat all blue-green algae with caution.

The risks to human health from contact with, ingestion or inhalation of marine algae that currently occur in UK coastal waters is considered to be low. However, some individuals may be more sensitive and display some reactions. A common marine algae found in UK coastal waters is Phaeocystis, which is often mistaken for sewage as it forms foam and a brown scum, but it is non-toxic.

Reducing the risk of getting ill

There are a number of things you can do to reduce the risk of illness when swimming in open waters.

Choose where and when to swim

Up to date bathing water quality information is available online during the bathing season between May and September. Other considerations to help you choose where to go include:

Before you swim

There are a number of things you should consider including:

  • choosing the location carefully and avoid swimming in water with blue-green algal blooms or scums in freshwaters

  • covering cuts, scratches or sores with a waterproof plaster before swimming

  • wearing appropriate protective clothing such as a wetsuit, gloves or protective footwear

While you are swimming

Remember to:

  • avoid stream water running across the beach

  • try to avoid swallowing or splashing water into your mouth

  • observe local safety advice

After swimming

Following the swim, you can minimise the risk of becoming ill by:

  • cleaning your hands thoroughly with soap and water ensuring that all wet sand is removed from hands before eating or handling food

  • thoroughly cleaning cuts or abrasions using soap and water

  • handle your wetsuit with care after use. Rinse it with clean water as soon as is practicable after swimming. Clean with detergent and rinse as advised by the manufacturer. Always wash your hands with soap and water after handling or cleaning your wetsuit. Allow the suit to dry thoroughly before reuse.

What to do if you become unwell

If you do become unwell with diarrhoea or any other symptoms, seek medical help and let them know you have been open water swimming. Do not swim again until you have had no diarrhoeal symptoms for at least 48 hours, or for a longer period if advised by a doctor.

Information on bathing water and beach risks