Blue-green Algae

Blue-green algae, also called cyanobacteria, are a type of microscopic bacteria that occur naturally in freshwater lakes, bays, ponds, rivers and streams usually beginning in late summer or early fall.

Blue-green algae are not normally visible in the water, but as the algae multiply they can quickly increase to form a large mass or scum called a ‘bloom’. Algal blooms usually occur in the hot summer months and early fall. They tend to occur repeatedly in the same water bodies.

Blue-green algae thrive in areas where the water is shallow, slow moving and warm, but they may be present in deeper, cooler water.

Some, but not all, blue-green algae blooms are considered harmful as they may produce toxins that can be harmful to the health of people and pets. There’s no way to tell by looking at a bloom whether it contains toxins or not.

You can be exposed to blue-green algae by:

  • Direct skin contact (including swimming or recreational activities such as boating, waterskiing or diving)
  • Drinking water
  • Accidental swallowing of water
  • Inhaling mist in the air containing blue-green algae cells or toxins (e.g., waterskiing, showering)
  • Eating fish caught in water where blue-green algae occurs

How to recognize blue-green algae

Dense blue-green algae blooms may make the water look like bluish-green, like green pea soup or turquoise paint. When the blooms are very dense, they may form solid-looking clumps. Fresh blooms often smell like newly mown grass; older blooms may smell like rotting garbage.

Blue-green algae 'bloom' looks like solid greenish clumps on the water
Photo credit: Haliburton, Kawartha, Pine Ridge District Health Unit

What to do if you suspect blue-green algae

If you suspect there could be blue-green algae in a body of water you should take the following measures:

  • Assume toxins are present
  • Avoid using the water:
    • Don’t drink the water
    • Don’t swim or bathe in the water
    • Keep animals out of the water
    • Don’t touch or handle the algae
  • If swimming near or in the harmful blue-green algal bloom immediately take a shower.
  • Consult a healthcare provider if symptoms such as skin, eye or throat irritation, allergic reactions or breathing difficulties; nausea, vomiting or diarrhea develop.
  • Report the suspected algae to:
  • Learn how to identify blue-green algae

Health effects of blue-green algae

Some blue-green algae produce toxins that can pose a health risk to people and animals.

Symptoms include:

  • Itchy, irritated eyes and skin
  • Headaches, fever, diarrhea, stomach pain, nausea and vomiting
  • More serious effects such as liver damage may occur when large amounts are ingested

Please note that infants and young children (under age 6) are most at risk of developing health problems (e.g., liver damage) from exposure to blue-green algae.

What to do if you’re exposed to blue-green algae blooms

  • Rinse off with clean water immediately
  • Consider speaking with your healthcare provider for symptoms such as skin, eye or throat irritation, allergic reactions or breathing difficulties; nausea, vomiting or diarrhea
  • Report the suspected algae and your symptoms as soon as possible to:

Where to get more information