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Risks of swimming

The risks and health effects of outdoor swimming


When temperatures are rising, many people are cooling down in and around the water. During hot summer periods we can go to the sea for a swim. But not only the sea is a place for summer recreation. One can also go for a swim in inland lakes and pools. Lots of people will either go surfing, sailing, or swimming.

Risks of swimming in seas and lakes

Unfortunately, there are certain risks to swimming in seas and lakes.
Firstly, the water may contain pathogens. This cannot always be detected visually.
Secondly, turbid water may be a risk. Turbid water does not necessarily contain harmful microrganisms. The water may be turbid due to suspended solids, such as sand particles. But there are risks to turbid water, such as the disability to guess the depth of the water and the disability to see obstacles in the water.

Thirdly, water currents may be a risk. When currents change during an afternoon of swimming, people in the water may be surprised and may find it hard to return to the beach. Fourthly, the transition from un-deep to deep water may occur very sudden. In deep water the risk of obtaining swimmers cramp is much higher and there are ships, surfers and sailors that may overlook swimmers. It is not wise to go swimming in harbours, in waterways, or near floodgates.
Fifthly, there is a risk to swimming in polluted water. One should not swim near industrial points of discharge or near sewer water treatment plants. Make sure that there is no dirt or dead plant and animal material in the water on the location where you are swimming.
For your own safety it is always better to go swimming on supervised locations and to not let your kids enter the water alone.

Health effects

When one takes a swim in water that contains pathogens, one might end up being ill. Pathogens enter the body during swimming activities and when one accidentally swallows swimming water.
This may cause people to experience health problems after swimming, such as allergic reactions, skin rashes, inches and headaches. The causes of these symptoms may be algae, plants, caterpillars or bacteria that are located on the beach or in the water.
When you experience health problems after swimming, do not hesitate to call your doctor. When you report your complaints to your local authorities, this may lead to water research or measures to prevent other people from becoming sick. You may want to consider that.
Furthermore, before you go out swimming somewhere you may want to call your local authorities to inform about the quality of the water. They will be able to provide you with all the necessary information, because water quality is checked annually and the authorities will have gained knowledge of the circumstances, such as the presence of harmful bacteria.

When water temperatures are above twenty degrees Celsius, it becomes a substrate for botulism. Another disturbing factor for water quality are algae. Waterborne algae may contain blue seaweeds, which are toxic for humans.
When we discharge waste into the water, it may become a habitat for rats. Rats can transport diseases, which can infect humans.

Below, examples of a few serious diseases, caused by infection during swimming, are summed up.


Botulism is a disease that occurs mainly with water birds and fish, when water temperatures rise above twenty degrees Celsius. The disease causes muscular paralysis with infected animals. The animals that suffer from botulism will eventually die.
Botulism is caused by a bacterium called Clostridium botulimium. The bacterium produces a nervous toxin, which causes paralyses. There are several kinds of botulism, which cannot all cause health effects. When the bacterium is truly a danger to humans, authorities will forbid people to swim in the infected water.

When the presence of botulism has been proved, dead animals need to be removed from the water as soon as possible, to prevent contagion from spreading further. When dead animals remain in the water for a longer period of time, the disease will become a danger to human health. The health effects caused by botulism are lack of vision and disability to speak, listlessness and muscular paralysis.
When you find dead animals in the water, you should immediately warn your local authorities. Do not try to remove the animals from the water single-handedly.

Botulism cautions:
- Avoid contact with dead animal materials
- Contact your local authorities or a water supervisor
- Do not go swimming when dead animals are found in the water and keep your kids from playing in the water

Blueweeds (Echium vulgare)

Blueweeds are bacteria that resemble algae. They float in the water as strings or slimy accumulations. When the strings or accumulations close in as temperatures rise, blueweeds die off and float to the surface, where they form a smelling layer of dead organic matter. While dying off, blueweeds produce toxins that are harmful for human and animal health.
Swimming in water that contains blueweeds may cause skin irritations. We recommend that you take a shower immediately after. When you have been in touch with blueweeds toxins for too long, you may suffer from headaches, skin rashes, stomach cramps, nausea, vomiting, diarrhea, fevers, throat pains, earaches and nose and lip swellings, twelve hours after contagion.
These symptoms will be noticeable for about five days, after which they will eventually vanish on their own.

Not only botulism can cause these symptoms. There are other bacteria and viruses that can cause them. Because of this we recommend that you contact your doctor when you experience them.
Also, be careful with your children. Especially small children are more likely to get sick after these toxins have entered their body, and they are more likely to swallow water while swimming.

For more information on health effects of microrganisms, move to the water health FAQ

More information on water quality and water quality assessment is available here

For water terminology, check out our water glossary, or move to the water FAQ index

If you have any other questions, please feel free to contact us

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