Danger in the Water: When Algae Becomes Toxic

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?

Algae 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.