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Health Unit urges caution around blue-green algae blooms

August 21, 2026   ·   0 Comments

By Brian Lockhart

The Simcoe Muskoka District Health Unit is reminding residents to avoid contact with water where blue-green algae (cyanobacteria) blooms are present.

Blue-green algae occur naturally in Ontario lakes, ponds, rivers and streams. Blooms are most common during the summer and early fall when water is warm and calm.

While not all blue-green algae blooms are harmful, some produce toxins that can make people, pets, and wildlife ill.

Exposure to toxins produced by blue-green algae can irritate the eyes, nose, throat, and lungs, as well as cause stomach pain, vomiting, diarrhea, headaches, muscle weakness, fever, and dizziness.

In severe cases, swallowing affected water can damage the liver or kidneys.

Anyone who has come in contact with blue-green algae and develops symptoms after exposure should contact a health-care provider.

Pets can become seriously ill if they drink affected water or ingest blue-green algae while grooming after swimming. Contact a veterinarian if your pet may have been exposed to a bloom and shows unusual behaviour or signs of illness.

A blue-green algae bloom can cause water to:

• Appear blue-green, bright green, or turquoise

• Look like pea soup or as though paint has been spilled on the surface of the water

• Develop floating scum, streaks or thick mats

• Produce a must, garbage-like or rotten-egg odour

Never use untreated water from a lake or river for drinking, cooking or preparing infant formula.

Boiling water does not destroy blue-green algae toxins and may increase their concentration when water evaporates. Private water supplies and residential water treatment systems may not remove the toxins effectively.

If water appears to contain a bloom, assume toxins are present and avoid contact.

• Do not swim, wade or participate in water sports where a bloom is visible

• Do not allow pets or livestock to swim in or drink affected water

• Be cautious about eating fish caught from water where a bloom is present

• Do not apply herbicides, copper sulphate or other algaecides, as these can cause algae cells to break open and release additional toxins into the water

If you suspect a blue-green algae bloom, avoid the area and report it to Ontario’s Ministry of the Environment, Conservation and Parks through the Spills Action Centre.

Public health, municipalities, and environmental agencies work together to investigate confirmed blooms and inform the public. 

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