If swallowed, blue-green algae can cause serious gastrointestinal and respiratory problems. Here’s how to keep children and dogs safe when swimming in ponds and lakes, plus photos of toxic algae so you know what to look for.

By Nicole Harris
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In July 2019, three puppies died from blue-green algae poisoning after swimming in a North Carolina pond. Dog lovers everywhere showed support on a Facebook post by the owners, Melissa Martin and Denise Mintz. And parents began asking a very justifiable question: Is blue-green algae dangerous for my children?

Here’s everything you need to you know about blue-green algae and how to keep your kids safe from the summertime safety hazard.

What is Blue-Green Algae?

Blue-green algae is commonly found in lakes and ponds. The tiny organisms that make up blue-green algae—also called cyanobacteria—are billions of years old, says Hans W. Paerl, Kenan Professor of Marine and Environmental Sciences at the UNC-Chapel Hill Institute of Marine Sciences in Morehead City. 

Typically, algae only causes problems during a “bloom” situation characterized by rapid accumulation. Blooms occur most often during warmer weather—which means summertime in the Midwest and Northeast. But in regions with consistently hot temperatures, such as the Southwest and Deep South, blooms can pop up all year round. 

“Blooms occur because we’re polluting these systems with more nutrients (nitrogen and phosphorus) that come from fertilizers, waste water, urban stormwater runoff, and other sources,” says Paerl. The stagnant water of lakes and ponds worsens the problem—and so does warm water temperature. “When combined, these three factors create the ideal conditions to get one of these blooms,” says Paerl.

Is Blue-Green Algae Toxic for Kids?

Shortly after swimming in the pond, Martin and Mintz’ dogs began experiencing seizures, and one of them had liver failure. But while this probably raises alarm in parents, Paerl says that minimal contact with blue-green algae shouldn’t pose a health risk. Problems usually occur when a child or animal ingests large amounts of water containing toxin-producing cyanobacteria. 

The side effects of blue-green algae ingestion depend on the type of toxins—either microcystins or anatoxins—present in the water. Microcystins affect the liver and cause sore throat, diarrhea, upset stomach, and other gastrointestinal issues. Anatoxins target the nervous system, producing symptoms like tingling, respiration impairment, and muscular contractions. Pearl adds that swimming in contaminated water may also cause skin reactions

If your child experiences any worrisome symptoms after swimming in a lake or pond, take them to a doctor right away. Fortunately, though, “there have been very few human deaths from algae,” says Paerl. “Most occur in domestic pets and animals, as well as wildlife, because they naturally drink more water in the lake or pond” 

How to Identify Blue-Green Algae

You can sometimes spot blue-green algae in a lake or pond: the water usually has a olive-greenish tint or a paint-like shiny bright green scum on it. Paerl says it might also have an unpleasant musky odor. If you notice these signs of algae, it's not advisable to go into the water. Even playing near the water’s edge can pose problems, since airborne cyanotoxins can potentially lead to coughing and skin rashes. 

Your local lake might post warning signs about algae blooms, and you should always pay attention to them. Some organizations also monitor algal conditions online. For example, check out this interactive map from the Environmental Working Group (EWG). Paerl says it’s important to note, however, that these lists aren’t comprehensive, since undocumented algal blooms can pop up anytime. 

The following images from the UNC Institute of Marine Sciences depict toxic algae in lakes and ponds:

UNC Institute of Marine Sciences
UNC Institute of Marine Sciences
UNC Institute of Marine Sciences
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