Danger in the Water: When Algae Becomes Toxic

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Ever wondered what’s causing the water in your favorite lake to turn red?  Or were the family photos from your river rafting trip spoiled by brown water in the background?

You may be looking at an algal bloom. Summer is upon us and warm weather is the perfect environment for these algal blooms, which can cause a range of problems, from simply being an eyesore to becoming a harmful algal bloom (HAB) that can make people and animals sick or damage local environments.

So, what is an algal bloom?

Red algal bloomAlgae are plant-like organisms that come in a variety of shapes and sizes – ranging from microscopic to large seaweed that may be over 100 feet long. Algae are found all over the planet, and can live in sea water, fresh water, and brackish water (a combination of fresh and sea water). Algae are vitally important building blocks of the food chain and ecosystem.

Algal blooms occur when there are overgrowths of algae, including green, brown, or red microalgae, or cyanobacteria that are commonly referred to as blue-green algae.

Not all algal blooms are harmful; however, when there is fast growth of algae and cyanobacteria that can harm people, animals and the environment, they are referred to as harmful algal blooms (HABs). HABs can produce toxins that are harmful to people and animals. The algae and cyanobacteria also reduce the levels of oxygen in the water when they decompose, and these lower oxygen levels may kill other plants and animals in the water.

Can HABs make you sick?

If people and animals are exposed to the toxins produced by HABs through water, food, or air they may experience symptoms that can range from mild to severe. These symptoms may affect the skin, stomach and intestines, lungs, and nervous system.

You may be exposed to HABs while enjoying outdoor recreational activities, while working near a body of water with a HAB, or from drinking water or food that has been contaminated.

  • Between 2009 and 2010, three states reported 11 outbreaks associated with HABs after people were exposed to freshwater in a recreational setting. These accounted for nearly half of all reported outbreaks associated with untreated recreation water that year.
  • Between 2007 and 2011, 273 people became sick after eating food that was contaminated as a result of a HAB. These illnesses were reported after people ate fish or shellfish contaminated with HAB toxins.

Are HABs increasing?

There is evidence that HABs are occurring more often, and that they are becoming more severe due to climate change, farming practices, and storm and wastewater runoff.  It is important to identify when and where HABs occur in order to protect water and food supplies, and to let people know when there may be a problem in their community.

CDC and partners have created the One Health Harmful Algal Bloom System for state and territorial public health partners to report cases of human and animal illness after being exposed to a HAB and environmental data about HABs. Data about the number of people and animals who get sick from HABs, the symptoms they experience after exposure, and where HABs occur is important to understand and prevent HABs and HAB-associated illnesses.

Learn more about HABs from CDC’s Harmful Algal Bloom-Associated Illness website.

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10 comments on “Danger in the Water: When Algae Becomes Toxic”

Comments listed below are posted by individuals not associated with CDC, unless otherwise stated. These comments do not represent the official views of CDC, and CDC does not guarantee that any information posted by individuals on this site is correct, and disclaims any liability for any loss or damage resulting from reliance on any such information. Read more about our comment policy ».

    Algal blooms are very common problem in surface water reserviors like canal, pond,lakes etc especially where water is stagnated. This may be removed by chlorination or these alga may be used as biofertilizers. For sewage treatment these microalgae may be used because these algae absorb heavy metals, Nitrogen, Phosphorus and othernutrients.

    A man made pond by the city of Rockville Maryland has been covered by Algae.
    Contacting the city official brought no result since the man in charge believes he and his department can do nothing.
    I am afraid of the consequence of so many Algae in my backyard for the health of my family.
    Whom should I contact for this issue?

    If someone is dumping smelly algae from ponds in piles behind a campsite and just allowing it to sit there and smell and attract bugs until the point you can’t go outside your camper… can you get sick from it? Either by breathing the rotting mess in or by the flies and mosquitoes that are attacking you?
    I am very concerned because I am literally right beside it. Should they just cover it up and pretend like it isn’t there? What am I supposed to do?
    Is this safe and if not what could me or my dog get sick from?
    Please help!

    We currently have a blue green algae bloom in our pond despite our pond management service using twice the amount of chemicals usually needed to control it in a pond of this size. We have dogs and grandchildren and we have the beach roped off but we are still concerned. Not to mention the wildlife…
    The neighbor across the road uses surry water to irrigate his crops…and that probably is providing extra nutrition for the algae.
    We need to know if this algae contains cyanobacteria or any neurotoxins. Who do we contact? Any help is much appreciated!!!

    Some comments from visitors to the Caribbean indicate that one area of the beach appears discoloured by algae bloom but further down the beach another area appears clear.. Does that mean that it would likely be safe to swim in the clear-looking area of the ocean, or is it likely to be dangerous as well?

    I am a veterinarian and am trying to determine if blue green algae illnesses are reportable

    I would like to know whether the material remains toxic when dry. It occurs to me that the algae could be used as fertilizer for plants.

    Recently one of my indoor/outdoor cats appeared sickly. My neighbors said she saw him drinking from an algae pond it looks like another neighbor is growing. Why is he growing algae?. We live in a city park with wildlife everywhere. I’m very worried as to what to do. Thank you.

    I have a water but and when it’s full to the top with water I empty it into empty plastic milk bottles so not to waste the water and save it for my food plants . On the top of some off the milk containers towards the top they have gone red round the top of the containers will this harm my food plants

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Page last reviewed: June 28, 2016
Page last updated: June 28, 2016