Harmful Algal Blooms: How to Spot Them and Keep Kids and Pets Out of the Water
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How we wrote this: A plain-English summary of the CDC's harmful algal bloom pages, checked October 29, 2026. Follow local advisories, and call a doctor or veterinarian if anyone gets sick.
A green, scummy lake or a red-tinged bay may be more than unpleasant. The CDC says harmful algal blooms (HABs) can make people and animals sick, and can be deadly for animals.
What a harmful algal bloom is
Per the CDC, HABs are the rapid growth of algae or cyanobacteria (blue-green algae) in water that can harm people, animals or the environment. Some produce toxins. Others harm by becoming too dense, using up oxygen or releasing harmful gases.
- Fresh water (lakes, rivers): cyanobacteria are the most common cause.
- Salt water (oceans, bays): dinoflagellates or diatoms are most common.
- They've been reported in all 50 states, Puerto Rico and the U.S. Virgin Islands.
Blooms are more likely when water is warm, slow-moving and full of nutrients such as nitrogen or phosphorus, which can wash in from fertilizer, sewage or runoff, for example during rainstorms.
What they look like
The CDC says blooms can look like foam, scum, mats or paint on the surface, and can turn water green, blue, brown or red. Some grow under the surface, and toxins can be present even when you can't see a bloom.
How people and pets get sick
You or your animals can get sick by going in or near contaminated water, swallowing it, eating contaminated fish or shellfish, or using contaminated blue-green algae supplements.
Pets: the CDC says animals can get very sick or die within hours. Contact a veterinarian right away if a pet seems sick after being in or near water.
CDC prevention steps
- Check for swimming, fishing and shellfish advisories before you go. Local or state governments post them online, near the water, or both.
- Stay out of water that smells bad, looks discolored, has foam, scum, algae mats or paint-like streaks, or has dead fish or animals on shore. Keep kids, pets and livestock out too.
- Rinse off right away with tap water if you go in or touch water that may have a bloom.
- Don't trust your senses with seafood. Toxins in fish and shellfish have no taste or smell, and cooking doesn't remove them. Follow local shellfish advisories.
For bacteria advisories at ocean and Great Lakes beaches, see our beach water quality guide; also see drowning prevention.