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Blue Green Algae Health Concerns
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Blue Green Algae Health Concerns

Blue Green Algae Frequently Asked Questions

What are blue green algae?

Blue green algae, also known as cyanobacteria, are simple plants found naturally in water and wet environments. 

What is a blue green algae bloom?

When conditions are right, algae can rapidly build up or “bloom” on the surface of reservoirs, rivers, creeks, lagoons, lakes and ponds. The bloom can be green, blue green, white or brown, and may look like a floating layer of scum or paint. 

What causes blooms?

Warm temperatures, calm water and nutrients such as fertilizer, manure runoff or septic tank discharges, contribute to the rapid growth of algae. Blooms can occur at any time, but are most common between June and October. 

How do I know if a bloom is toxic?

Only a few types of blue green algae are known to produce toxins. Most blooms of algae in our region are made up of harmless green algae. Often, the first sign that a bloom is toxic is a dog that has gotten sick after swimming in stagnant water. Always look for the signs of an algae bloom before you enter the water, or before you let your children or pets enter the water. 

How dangerous is toxic algae?

Exposure to toxic algae during recreational activities, by touching the skin or accidentally inhaling or swallowing the water, can cause rashes, skin and eye irritation, sore throat, asthma-like symptoms, allergic reactions, gastrointestinal upset, numbness, tingling, dizziness and other effects. The long-term effects of these exposures are not well known, but children and pets are at greatest risk. Dogs can be exposed to high levels of toxins because they more readily drink water that smells or tastes bad and lick their fur after swimming. 

What should I do if I see a bloom?

  • Stay out of areas where the water has foam, scum, or mats of algae. Keep children, pets and livestock out of such areas at all times. If you or your pets swim or wade in water with algae, rinse off with a clean source of water as soon as possible. Always warn young children not to swallow any water, whether or not you see signs of algae.
  • Do not drink or cook with this water. Even if you boil, filter or treat it, the toxins can persist.
  • Get medical treatment right away if you think that you, your family member, pet or livestock might have been poisoned by blue green algae toxins. 

What about fishing and other activities?

Eating fish caught during a bloom may pose a health risk. Always remove the guts and liver, and rinse fillets in tap or bottled water before eating the fish. Other activities near the water such as camping, picnicking, biking and hiking are safe. 

Additional Resources:

California Harmful Algal Bloom Portal

Report a potential harmful algal bloom at:

California State Water Resources Control Board:

U.S. Centers for Disease Control & Prevention (CDC)

U.S. Environmental Protection Agency (EPA)

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