Harmful Algal Blooms
A harmful algal bloom (HAB) occurs when certain types of microscopic algae grow quickly in water, typically forming visible patches that may harm the health of the environment, plants, or animals. HABs can deplete the oxygen and block the sunlight that other organisms need to live, and some HABs produce toxins that are dangerous to animals, including people.
HABs can occur in marine, estuarine, and fresh waters and can impair drinking and recreational waters. In addition, HAB-associated toxins can contaminate seafood. HABs appear to be increasing in frequency along the coastlines and in the surface waters of the United States according to the National Oceanic and Atmospheric Administration (NOAA). These increases are likely responses to an overabundance of nutrients, such as nitrogen from fertilizers, and warmer temperatures associated with climate change.
Are you a writer or producer working on a current TV or film project? Contact the program for technical assistance.
Although scientists do not yet understand fully how many HABs might affect human health, health agencies in the United States and abroad are monitoring HABs and developing guidelines for HAB-related public health action. CDC works with public health agencies, universities, and federal partners to investigate how the following algae, which can cause HABs, may affect public health:
- Cyanobacteria, also known as blue-green algae, can produce toxins that may taint drinking water and recreational water. Humans who drink or swim in water that contains high concentrations of cyanobacteria or cyanobacterial toxins may experience gastroenteritis, skin irritation, and allergic responses. People exposed to cyanobacterial toxins through dialysis water contamination may experience life-threatening liver damage.
- Harmful marine algae occur in the ocean and produce toxins that may harm or kill fish and marine animals. Humans who eat shellfish contaminated with HAB toxins may become sick with shellfish poisoning, such as neurotoxic shellfish poisoning, paralytic shellfish poisoning, or ciguatera fish poisoning. Inhaling marine aerosols containing toxins from Florida red tides may induce symptoms in people with asthma.
- Pfiesteria piscicida, a single-celled organism that lives in estuaries, has been found near large quantities of dead fish. However, no toxin has been isolated from this organism, and studies supported by CDC have not identified any lasting symptoms associated with exposure to this organism.
To reduce your risk of adverse effects from HABs, you can:
Protect yourself, your family, and your pets from exposure to cyanobacterial HABs (CyanoHABs):
- Don't swim, water ski, or boat in areas where the water is discolored or where you see foam, scum, or mats of algae on the water. If you do swim in water that might have a CyanoHAB, rinse off with fresh water as soon as possible.
- Don't let pets or livestock swim in or drink from areas where the water is discolored or where you see foam, scum, or mats of algae on the water. If pets (especially dogs) swim in scummy water, rinse them off immediately—do not let them lick the algae (and toxins) off their fur.
- Don't irrigate lawns or golf courses with pond water that looks scummy or smells bad.
- Report any "musty" smell or taste in your drinking water to your local water utility. Respect any water-body closures announced by local public health authorities.
Reduce the occurrence of cyanoHABs:
- Reduce the nutrient loading of local ponds and lakes by using only the recommended amounts of fertilizers and pesticides in the yard, properly maintaining a household septic system, and maintaining a buffer of natural vegetation around ponds and lakes to filter incoming water.
Stay informed about HABs:
- Red tide events are tracked by the Florida Marine Institute and information is posted online to indicate which beaches and recreational areas should be avoided. (www.floridamarine.org)
- Information about HABs is also available from other federal agencies, including the Environmental Protection Agency (EPA) and the Food and Drug Administration (FDA).
Additionally, the U.S. Environmental Protection Agency (EPA) has added certain algae associated with HABs to its Drinking Water Contaminant Candidate List. This list identifies organisms and toxins that EPA believes are priorities for investigation. The public should be aware of the potential threat that algal blooms pose so they can prevent becoming a victim to their effects.
- Harmful algal blooms can produce potent toxins resulting in skin irritation, allergies and gastroenteritis. They are typically associated with warm periods and overabundance of nutrients, for example, the excessive use of fertilizers on lawns and golf courses. They also produce toxins that can be ingested by eating contaminated shellfish.
- There are various types of algal blooms that pose a potential risk to humans and animals:
- Cyanobacteria, also known as blue-green algae, can produce toxins that may taint drinking water and recreational water.
- Harmful marine algae, such as those associated with red tides, occur in the ocean and can produce toxins that may harm or kill fish and marine animals. Humans who eat shellfish containing toxins produced by these algae may experience neurologic symptoms (such as tingling fingers or toes) and gastrointestinal symptoms.
- Pfiesteria piscicida, a single-celled organism that lives in estuaries, has been found near large quantities of dead fish.
- Protecting against HABs includes avoiding high risk areas and times of the year when there are a larger number of HABs present and rinsing thoroughly with clean water if contact is made with HABs.
- Page last reviewed: July 19, 2016
- Page last updated: July 20, 2016
- Content source:
- Centers for Disease Control and Prevention
- Page maintained by: Division of Public Affairs (DPA), Office of the Associate Director for Communication (OADC)