Sources of Exposure

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Harmful algal blooms caused by certain types of algae and cyanobacteria (also called blue-green algae) happen in bodies of water around the world. You can be exposed and get sick if you swim, wade, or play in or near them; eat contaminated fish or shellfish; or use contaminated drinking water.

You can be exposed to harmful algae and cyanobacteria and their toxins through:

Your symptoms and how sick you get can vary depending on the type of exposure, the type of harmful algae or cyanobacteria that are present, and the type of toxin (poison) involved. In some cases, more than one toxin may be present. People are mainly exposed through:

  • Skin contact through activities like swimming
  • Breathing in tiny airborne droplets or mist that contain toxins
  • Swallowing water that contains toxins
  • Eating food or supplements containing toxins

Skin contact

Skin contact icon

Anyone who visits a body of water that has harmful algae, cyanobacteria, or their toxins can be exposed through skin contact with the water.

Skin irritation and other reactions in people and animals can vary depending on how long they were in contact with the contaminated water. It can also depend on the type and amount of toxins in the water.

Breathing in toxins

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People can be exposed to algal or cyanobacterial toxins by breathing in tiny water droplets, mist, or sea spray from a contaminated body of water. You can breathe in toxins even if you do not go into the water. More research is needed to better understand the effects of breathing in toxins over a long period of time, especially for those who regularly work on or near water, such as boaters or lifeguards.

People who have been on the beach or on a boat in salt water have reported breathing difficulties after inhaling air or water particles contaminated with toxins.

Did you know?

A study conducted during a Karenia brevis red tide (a type of harmful algal bloom) in Florida found that algal toxins could be transported in the air almost 4 miles inland from the water source. Harmful algal blooms may cover hundreds of square miles of ocean and affect boaters across the entire area.

Swallowing contaminated water

People and animals can be exposed to algal or cyanobacterial toxins when they drink contaminated water. This can happen during recreational activities (such as accidentally drinking water while swimming) and by drinking contaminated tap water.

Person swimming in deep water
Recreational activities

People can swallow water contaminated with algae, cyanobacteria, or their toxins while they are swimming or playing in the water.

  • Active water sports (like water-skiing) pose a higher risk of accidentally swallowing water.
  • Swimmers may swallow up to 16–200 mL of water (the equivalent of 0.5 – 6.8 ounces of water) during one swim.
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Drinking water

Though uncommon, people and pets might be exposed to cyanobacterial toxins if the tap water supply contains cyanobacteria. The marine (saltwater) algae that form harmful algal blooms are not found in fresh water, so their toxins would not be in drinking water. Whether there are cyanobacterial toxins in drinking water can depend on the level of toxins in untreated or raw source water. It can also depend on how effective the water treatment methods are in removing the toxins.

Some public drinking water systems use surface water from lakes. Water treatment facilities have options to remove cyanobacteria and their toxins from water during treatment; however, these methods are not always a part of a water utility’s standard treatment processes. In June 2015, the U.S. Environmental Protection Agency (EPA) issued drinking water health advisory levelsexternal icon for toxins made by cyanobacteria. Health advisories are not regulations, but guidance for health officials and the public that help to protect people’s health.

You can find out more about your local drinking water on EPA’s website.external icon


Rarely, people have been exposed to cyanobacterial toxins during dialysis. This can happen if the source of the dialysis water contains toxins which are not removed by the water treatment system. In 1974, 23 dialysis patients in Washington, DC became ill. In 1996, 116 dialysis patients became ill or died in Brazil.

Eating contaminated food

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People and animals can be exposed to algal or cyanobacterial toxins when they eat contaminated seafood or take contaminated nutritional supplements.


People and animals can be exposed to toxins when they eat seafood.

Fresh Water

Freshwater fish can become contaminated with cyanobacterial toxins by eating other animals that already have toxins in their bodies. More research is needed to better understand how often people come in contact with toxins by eating freshwater fish.

Marine (Salt) Water

Shellfish can become contaminated with algal toxins when they filter and concentrate water that contains toxins. Reef fish can become contaminated by eating other animals that already have toxins in their bodies. This is called bioaccumulation.

For more information on illnesses caused by eating seafood contaminated with marine toxins, visit Illness and Symptoms: Marine (Saltwater) Algal Blooms or the CDC’s Yellow Book, Chapter 2: Food Poisoning from Marine Toxins.

Bioaccumulation: Fish and other aquatic animals may eat algae or cyanobacteria, building up the toxins in their bodies. When other animals eat these animals (for example, when small fish are eaten by larger fish), the toxins can build up, or bioaccumulate. Top predators, including large fish and people, can be poisoned when they eat fish that have accumulated toxins.

Nutritional supplements
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Nutritional supplements that have blue-green algae (cyanobacteria) can also pose a risk for exposure to cyanobacterial toxins. When algae are harvested to produce supplements, a toxin-producing cyanobacteria (such as Microcystis) might accidentally be collected as well.

Many supplements have good safety records, but federal law does not require companies that make nutritional supplements to prove they are safe to FDA’s standards before they are marketed. Find more information about supplement safety on FDA’s website.external icon

Animals can be exposed to harmful algal and cyanobacterial toxins

Illustration of dog swimming

Animals, including pets and livestock, can become sick when they:

  • Drink water containing algal or cyanobacterial toxins.
  • Swim or play in water containing algal or cyanobacterial toxins.
  • Eat or lick toxic algae or cyanobacteria that is in the water, on the shore, on their fur, or in supplements.
  • Eat fish, shellfish, or dead animals on the shore that contain algal or cyanobacterial toxins.

In fact, animals are more likely than people to swallow water containing algal or cyanobacterial toxins because they do not avoid water that is discolored or smells bad.

Learn how to protect your pets and livestock.

Page last reviewed: May 2, 2022