Skip to main content

Algae Blooms

What are they?

Algae are tiny rootless plants that grow in water and are an important part of the marine food web. Most algae species are harmless. Harmful algae blooms, or HABs, occur when toxic algae species grow quickly and form clusters that make the ocean look red or brown.

Why are they a concern?

Some algae species can produce toxins that are harmful to humans, even causing death. People who eat shellfish contaminated by algae blooms may get food poisoning, sometimes very severe, and have respiratory problems. The most severe reactions occur when large amounts of contaminated water are swallowed.

Who is at risk?

People who come in contact with algae blooms are at risk for illness. It may be dangerous to drink affected water, cook with it, or eat seafood caught in it. 

Local health departments issue warnings when HABs occur. In places where HAB monitoring programs do not exist, these blooms may go unnoticed until they cause illnesses or death in humans who eat products from the sea. 

What pollutants are of greatest concern?

Some algae produce dangerous toxins that can build up in seafood. When people eat contaminated seafood, it can cause:

  • Domoic acid poisoning – Exposure to this compound affects the brain, causing seizures and possibly death.
  • Paralytic shellfish poisoning – Biotoxins produced by algae, such as saxitonin, affect the nervous system and paralyze muscle, sometimes causing death.
  • Diarrhetic shellfish poisoning – This illness is marked by intense diarrhea and severe abdominal pains.

Reduce your risk

  • Do you eat fish and shellfish?
  • Do you fish in waters that may be contaminated by algae blooms?
  • Do you swim or play in waters that may be contaminated by algae blooms?
  • Do not drink affected water, cook with it, or eat seafood caught in waters with harmful algae blooms.
  • Local health departments issue warnings when harmful algae blooms occur. Follow their advice on shore and water activities, and drinking water and shellfish safety. Cooking contaminated seafood or boiling contaminated water does not destroy the toxins.
  • If you come across areas of thick algae, take precautions by avoiding water contact and keeping pets out of the water.
  • Report suspected algal blooms to your state and local health department.
National Library of Medicine Resources and Databases
Additional Resources
Back to top