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Preventing health problems associated with blue-green algae blooms


Blue-green algae, also known as "cyanobacteria", are naturally in Quebec bodies of water. They spread especially in the summer. When they become too abundant, blue-green algae form blooms, which can spread over a part or the entire body of water. The water can turn green (sometimes red, brownish or turquoise) and its texture can resemble that of paint or broccoli soup. When there is a significant accumulation of blue-green algae, scum can also appear on the water’s surface.

Health effects

Blue-green algae blooms may pose a health risk. Some species produce toxins that can result in health problems. For instance, a person may experience symptoms of gastroenteritis or experience irritation of the skin or throat after drinking water affected by blue-green algae blooms or after coming into contact with it.

If you have been in direct contact with blooms or scum, or have swallowed water in areas affected by blue-green algae blooms, watch for the following symptoms:

  • Stomach ache
  • Diarrhea
  • Vomiting
  • Headache
  • Fever
  • Skin irritation
  • Throat irritation

If you one or several symptoms are manifested 24 to 48 hours following contact with contaminated water, contact Info-Santé 811 or see your doctor. State that you have been in direct contact with blue-green algae or that you have swallowed water in areas affected by it.

Precautions for blue-green algae blooms

If you’re around a body of water that has blue-green algae blooms, take the necessary precautions before using that water or engaging in water activities.

Water usage

Water from rivers and lakes contains a lot of bacteria and contaminants that generally cannot be treated with domestic water purifiers. Always avoid drinking river and lake water, especially when blooms and scum are present. Also avoid such water for the following:

  • Preparing drinks or ice
  • Preparing or cooking food
  • Brushing teeth
  • Drinking water for animals.

In addition, it is not recommended to boil such water: this may increase the effects of toxins produced by the blue-green algae blooms.

If the water does not have an unusual colour or smell, you may use it for the following:

  • Bathing
  • Laundry
  • Dishwashing

Only water from an artesian well or from a water supply system can be used as usual, unless otherwise forbidden by the municipality or a regional public health authority.

Swimming and water activities

You can swim and do water activities in areas of the water where there are no blooms or scum. In areas where these are present:

  • Keep at least 3 meters away from the blooms and scum
  • Avoid activities that may lead to contact with blooms and scum

You may continue swimming and doing water activities in areas where blooms and scum have disappeared, but only 24 hours after their disappearance.

Other precautions

If you have touched blue-green algae blooms or an object or animal that has been in contact with it:

  • Avoid putting your hands in your mouth
  • Wash your hands as soon as possible

If you find yourself in an area affected by blue-green algae blooms or close to one, keep an eye on children so that they do not play with the blooms or scum.

Also avoid consuming the internal parts (ex., eggs) of fish caught in bodies of water affected by blue-green algae blooms.

Lastly, if you think there are blue-green algae blooms in a body of water, please contact the ministère du Développement durable, de l'Environnement et de la Lutte contre les changements climatiques This hyperlink will open in a new window..


Health effects

Mammals, such as livestock or pets, can also be affected by blue-green algae blooms. Dogs are particularly vulnerable because they can swallow water from an area affected by blooms while swimming and then lick the particles off their coat while grooming.

The following symptoms have been observed in animals:

  • excessive salivating;
  • coughing or respiratory problems;
  • diarrhea;
  • vomiting;
  • skin irritation;
  • lack of coordination or ataxia;
  • convulsions.

Some symptoms can result in death.

If your animal shows one or more of these symptoms within hours or days following exposure to areas affected by algae blooms, contact your veterinarian immediately. Inform your veterinarian that your animal has been in contact with blue-green algae blooms.


Watch your domestic animals to make sure that they don’t swim in areas affected by blue-green algae blooms. If you were not able to avoid it, wash their coat immediately with clean water. Make sure that they don’t drink water affected by algae blooms or eat small dead animals nearby.

Last update: June 27, 2023


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