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Frequently asked questions: Blue-green algae

Published on: 6/9/2016 3:17 PM
What are blue green algae?
Blue green algae (or cyanobacteria) are a common native algal species often found in water or wet areas.
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 and may have an unpleasant odor.
What causes blooms?
​Warm, slow-moving waters that are rich in nutrients can cause algae growth. Blooms can occur at any time, but are most common in late summer or early fall.
How do I know if a bloom is toxic?
​Only a few types of blue green algae are known to produce poisons. The presence of blue-green algae in a water body does not necessarily mean toxins are always present. However, identifying the presence of toxins is a difficult process and one that may involve many days to weeks before results are available. Therefore, it is prudent for recreational users to adhere to the following precautions with regard to blue-green algae blooms in Santa Clara County water bodies.

Where water contact is permitted, 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?
​If toxic algae touches your skin, or you accidentally inhale or swallow water containing the toxin during recreation, you could get a rash or an allergic reaction, or develop gastrointestinal problems. The long-term effects of these exposures are not well known, but children and pets are at greatest risk. Dogs can be exposed to particularly high levels of toxins by licking blue green algae off their fur after a swim.
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 and pets out of such areas at all times. If you or your pets swim or wade in water with algae, rinse off with fresh water as soon as possible.
  • If no algal scums or mats are visible, you should still carefully watch young children and warn them not to swallow the water.
  • Do not drink or cook with this water. Even if you boil or filter it, the toxins can persist.
  • Do not let dogs swim in or drink from areas where you see foam, scum, or mats.
  • Do not let your horse drink from areas where you see foam, scum, or mats.
  • Get medical treatment right away if you think that you or your pet might have been poisoned by blue green algae toxins.
  • If you accidentally swallow water from an algae bloom and experience one or more of the following symptoms: stomach cramps, vomiting, diarrhea, and fever, you should contact your physician or seek medical treatment.
What about fishing and other activities?
​Eating fish caught during a heavy bloom can pose a health risk. Always remove the guts and liver, and rinse fillets in tap water before eating the fish. Other activities near the water such as camping, picnicking, biking and hiking are safe.
Find more information at the California Department of Heath Public Health’s website.
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