Algal blooms, which develop in rivers, lakes, and oceans throughout the summer, produce toxins that can be harmful to people, animals, and the environment, according to the CDC.

Algal bloom toxins have sickened hundreds of people with a variety of skin, breathing, neurologic, and stomach symptoms, caused water shutdowns in major US cities, and have killed wildlife and pets.

According to the CDC, people can be exposed to harmful algal toxins in a number of ways, including:

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  • Swimming or coming in direct contact with the toxins
  • Breathing in the toxins
  • Swallowing contaminated water that contains toxins
  • Eating seafood contaminated with toxins

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

In 2014, 38 states responded to a survey about harmful algal blooms, and more than half reported that they occur every year in a freshwater body. All coastal states and the Great Lakes have reported harmful algal blooms in marine waters. Evidence suggests that harmful algal blooms are increasing in number and severity because of changing temperatures, farming practices, storm water runoff, and wastewater overflows.

The CDC created the One Health Harmful Algal Bloom System (OHHABS), a web-based national reporting system for harmful algal blooms, to document when and where algal blooms occur and whether they cause illness.

To determine if a body of water is contaminated with harmful algae, the CDC recommends that people check to see if the water smells bad, looks discolored, has foam, scum, or algae mats on the surface, or contains dead fish or animals. The CDC also recommends to check for beach advisories in your state that may be posted online or on signs near the water and follow local or state guidance if you learn about a harmful algal bloom in a local water body or if you are notified that your tap water contains algal toxins.


  1. Harmful algal blooms [press release]. Centers for Disease Control and Prevention. July 12, 2017.