Be Aware of Harmful Algal Blooms

Beach polluted with blue-green algae bloom

Harmful algal blooms can produce toxins (poisons) that can make people and animals sick and affect the environment. Learn more about them to keep you and your pets safe.

Warm weather is a time when tiny plant-like organisms—algae and cyanobacteria—are more likely to overgrow in rivers, lakes, and oceans. These overgrowths, called algal blooms, can sometimes have foam, scum, or thick layers on the surface, or can look and smell bad. Algal blooms can also make the water appear green, red, brown, or blue. When they contain toxins that affect the health of people, animals, and the environment, they are known as harmful algal blooms.

Blooms are becoming more frequent as temperatures warm and the levels of nutrients in our waters increase.

How People Get Sick

You can’t tell if a bloom is harmful just by looking at it, and not all blooms are easy to see. Health hazards can be present even when you can’t see a harmful bloom. People or pets can get sick when they have contact with contaminated water by:

  • Doing recreational activities such as swimming, kayaking, fishing, or wading through water
  • Breathing in tiny water droplets or mist that contains toxins from recreational activities or wind-blown sea spray
  • Drinking contaminated water
  • Eating contaminated seafood (fish or shellfish)

Illnesses and symptoms from harmful algal blooms can vary depending on how people and animals are exposed, how long the exposures last, and the toxins involved.

Symptoms can include:

  • Skin, eye, nose or throat irritation
  • Abdominal pain
  • Headache
  • Neurological symptoms
  • Vomiting
  • Diarrhea

Exposure to a HAB can also cause liver and kidney damage. If you think you or your pet have become sick because of a harmful algal bloom, see your healthcare provider or veterinarian. Animals can get very sick quickly, so don’t delay contacting your veterinarian.

Reporting an Illness

Report any illness associated with a harmful algal bloom to your local, state, or territorial health department. In some states/territories, this can be done through an online form or a hotline.

If you have immediate questions about your symptoms, call your local or state poison center.external icon

When in Doubt, Stay Out

When visiting lakes, rivers, or beaches, check your local water conditions. Follow any advice your state or local environmental health department posts online or near the water.

If a health notice is posted or if you suspect a harmful bloom may be in the water, follow these recommendations:

  • Don’t swim or come in direct contact with water that smells bad, looks discolored, or has scum on the surface
  • Don’t let pets drink water, eat algae, or play or swim in it
  • Stay out of the water if it has dead fish or other dead animals in or near it
  • Know the health risks of eating contaminated fish and shellfish and follow warnings

Follow guidance of your local health officials if you learn about a harmful algal bloom in your area or if you are notified that your tap water contains algal toxins.

Animals are often the first affected because they are more likely to swim in or drink from bodies of water that contain algal blooms.

How CDC Collects Data to Protect Health

One Health Harmful Algal Bloom System (OHHABS) logo

Because harmful algal blooms affect people, animals, and the environment, it’s important to track when they happen, where they happen, and whether they cause illness. CDC’s One Health Harmful Algal Bloom System (OHHABS) is a web-based national tracking system for harmful algal blooms. Health departments and their designated environmental health or animal health partners can use this voluntary system to report harmful algal bloom events and associated illnesses in people and animals. The data collected help CDC and its partners learn more about these blooms and understand how to best prevent them and the illnesses they cause.

Harmful algal blooms are an example of a One Health issue. One Health is based on the fact that human, animal, and environmental health are connected. Using this approach to track harmful blooms highlights the opportunity for human health, animal health, and environmental health experts to work together to effectively address the problems that harmful algal blooms can cause. Learn more about harmful algal blooms and One Health in action from a story about poisoned sea otters in California.

Page last reviewed: July 5, 2019