You probably don’t spend a lot of time worrying about the green film of algae on your local pond, stream or lake. But though it may look innocent, that algae can make you and your pets very sick—even if you don’t get in the water. Here’s what you need to know…

Blue-green algae or cyanobacteria (scientists argue whether it is truly a plant or a bacterium) grow naturally in waterways (fresh and salt) from summer through early fall.  Most algae on the surface of fresh water is harmless. However, under certain conditions the algae get overfed with nutrients and grow out of control, creating a “bloom” that fills the water and causes the algae to start producing toxins that are harmful to people and animals. This happens when hot weather combines with periods of rain that wash heavy concentrations of phosphorus and nitrogen from fertilizers on lawns and farmlands into lakes, streams and ponds.

Toxic algae blooms occur yearly in the US, affecting every state. These blooms are mostly a problem when they occur in water that is used recreationally, such as for swimming and boating where people are directly exposed to the water…but in any body of water, they can kill fish, and in some cases they can introduce toxins into the drinking- water supply. Public health officials and the US Army Corps of Engineers are working to contain or reduce the smelly toxic algae bloom that covers large portions of Lake Okeechobee, Florida’s largest freshwater lake. The Finger Lakes in New York and Utah Lake in Utah also have had large blooms. In 2014, 500,000 people near Lake Erie, Ohio, were without drinking water because of a toxic algae bloom that now occurs yearly with this year’s bloom twice the size of last year’s.

Not every algae-covered body of water is toxic. Generally, algae growths are harmless if they look like hair or wet fabric clinging to rocks or plants or floating on the surface of the water. But it isn’t possible to tell for sure by sight (or smell) whether a particular algae bloom is harmful—it requires testing.

However, algae are likely to be toxic if the water…

  • is green or brown
  • is cloudy or thick (“pea soup”)
  • looks like it has streaks of blue, green or white paint spilled on the surface
  • looks scummy or foamy on its surface
  • smells bad.

If water contaminated with toxic algae gets on your skin, it can cause a rash or blisters. Swallowing the water or getting it in your mouth, such as from swimming, can cause mouth ulcers, headaches, sore throat, muscle pains, diarrhea and vomiting. Small children, who are more likely to swallow greater amounts of water relative to body weight, and adults with liver disease, kidney damage or weakened immune systems are at greatest risk.

Even if you don’t touch the water, you can inhale the toxins from tiny water droplets in mist or spray.


If public health officials have identified an area that has a toxic algae bloom, warnings to stay out of the water are usually posted. If a pond or lake isn’t posted but looks suspicious, it’s better to err on the side of caution—don’t swim, ski, paddle, sail, canoe, kayak or fish in the water. The toxins can be deadly to dogs and cats, so keep pets away from the water, too.

Don’t eat fish caught in water that has a toxic algae bloom, either. Fish that live in such water can build up toxins in their muscle tissue and internal organs.

And if you suspect that a body of water that isn’t posted has a toxic algae bloom, report it to your state or local department of public health or department of environmental protection. You may be able to do it online. If possible, attach photos—including a close-up of the bloom and a landscape view to show the scope and location.

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