Promoting and protecting the health and safety of all Idahoans

FAQs About Harmful Algal Blooms

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Harmful algal blooms are actually bacteria (not algae) that can produce toxins. When weather conditions are calm and there is an increase in water temperature and nutrients, they can rapidly increase in number producing a bloom. Blooms can occur at any time, but they most often occur in late summer or early fall.


HABs can be blue, bright green, brown, or red and may look like paint or anti-freeze floating on the water. As the bloom matures, it may look like foam, scum, or mats on the surface of freshwater lakes and ponds. If you or pets or livestock swim in or drink from water that contains a bloom, the effects can be dangerous. 


People and animals can be exposed to HABs by swallowing water and/or touching the water during recreational activities such as swimming, water skiing, and diving.


The most common health effects are skin and eye irritation. Other more severe health effects can include:

  • Difficulty breathing
  • Stomach pain
  • Nausea
  • Vomiting
  • Diarrhea
  • Dizziness
  • Numbness and tingling in lips, fingers, and toes

If you experience mild irritation, rinse with clean water immediately. If you experience severe symptoms, seek medical attention. 

Pets and livestock exposed to HABs may exhibit symptoms and conditions such as:

  • Weakness
  • Staggering
  • Foaming at the mouth
  • Difficulty breathing
  • Convulsions/seizures

If your pet comes in contact with a bloom, rinse them with clean, fresh water immediately. If severe symptoms occur, call a veterinarian. In severe cases, animals may die within tens of minutes to hours after they have been exposed to harmful algal blooms. When harmful algal blooms decompose, they can also kill fish that live in the water.  


  • Do not go in or near water that has an unusual color, and keep children, pets, and livestock out of the water.
  • Avoid any water sports such as swimming, diving, water skiing, or boating in areas with HABs.
  • Do not use untreated water for drinking, bathing, cleaning, or cooking. Boiling water that has been contaminated by a HAB  will not remove toxins, it may actually cause more toxins to be released.
  • Remember: "When in doubt, stay out."

Use an alternate source of drinking water such as bottled water while the bloom is present.


It is best to avoid direct contact with waters affected by a HAB. If you choose to fish in affected waters, wear protective clothing such as gloves or waders, and wash your hands thoroughly with clean water. 

There have been no reports of people becoming sick from eating fish caught during a bloom. Information about the risk of eating fish from affected waters is limited. However, fish fillets are less likely to accumulate toxins compared to other parts of the fish. If you decide to eat fish from affected waters: 

  • Remove the skin, organs, and fatty deposits from the fish
  • Avoid cutting into organs
  • Rinse the fillets with clean water before cooking
  • Remember to follow Idaho Fish Consumption Advisory recommendations when choosing to eat fish

One way you can report a harmful algal bloom is to download the bloomWatch app on your smartphone or tablet. This app will teach you what to look for and allows you to report photos of potential blooms that you find. 

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