Skip directly to search Skip directly to A to Z list Skip directly to navigation Skip directly to page options Skip directly to site content

General Information

General Information

What are algae?

The term algae refers to plant-like organisms that are multi-celled or single-celled and photosynthetic (use sunlight to create food). Algae are vitally important to oceans, lakes, and rivers because they are the building blocks of the food chain and ecosystem.  Algae are also vital to water bodies because they produce oxygen to sustain life.

Multi-celled algae can include seaweed, and single-celled algae include microscopic organisms called phytoplankton. Phytoplankton can be divided into two categories, cyanobacteria and microalgae.

What are cyanobacteria?

Cyanobacteria are photosynthetic, single-celled aquatic organisms. Although they are not true algae, they are often referred to as blue-green algae.

What are microalgae?

Microalgae or microscopic algae are aquatic single-celled algae that are photosynthetic. Two major groups of microalgae are diatoms and dinoflagellates.

What is an algal bloom?

An algal bloom is a visible colony of cyanobacteria or microalgae. An algal bloom can be visible when cyanobacteria or microalgae grow quickly.

What does an algal bloom look like?

An algal bloom can look like foam, scum, or mats on the surface of water or it may not be visible on the water surface. An algal bloom may look like paint floating on the water and can change the water different colors, such as blue, green, brown, yellow, orange, or red. When organisms in a bloom die and decompose, they can release unpleasant odors (like the smell of rotting plants).

Where can an algal bloom occur?

An algal bloom can occur in marine (salt) water, brackish water (mixture of fresh and salt water like in estuaries), and freshwater, including waters that people may use for drinking and/or recreation. Algal blooms more often occur in water with increased levels of nutrients, such as nitrogen and phosphorus, which may come from fertilizer.

What is a Harmful Algal Bloom?

A harmful algal bloom (HAB) refers to the fast growth of any phytoplankton (cyanobacteria and microalgae) that can cause harm to animals, people, or the local ecology. HABs are a One Health issue because they affect human, animal, and environmental health.

A HAB can harm people, animals, or the local ecosystem. A HAB can cause harm by producing toxins that can poison humans, fish, seabirds, aquatic animals, livestock, wildlife, and household pets (such as dogs) that are near the water, consume the water, or swim in the water. A HAB can also cause harm through the buildup of toxins in both fresh and marine water fish and shellfish, which can cause illness when a person or animal eats the contaminated fish or shellfish.

A HAB can also cause harm by growing densely and blocking the sunlight from reaching the lower depths of the water. A HAB can also starve fish and plants of oxygen in the water when they decompose, resulting in fish kills and damage to the local ecology.

Not all algal blooms are harmful. Algal blooms known as “nuisance blooms” can discolor water, smell bad, and/or cause the water or fish to taste bad. Nuisance blooms are not dangerous as they do not produce toxins that can harm people or animals. However, they can discourage people from visiting beaches, drinking tap water, or eating fish from the water that is experiencing the bloom.

It is not possible to tell if a bloom is harmful just by looking at it.

What causes a Harmful Algal Bloom?

In freshwater, a HAB is most likely to form in warm, still waters with abundant nutrients such as phosphorus and nitrogen. In marine waters and brackish waters, a HAB forms under certain environmental conditions, such as when:

  • Existing nutrients accumulate near the surface of the ocean.
  • There are increases in sea surface temperature.
  • Sea currents change.

Climate change also might increase the frequency and occurrence of HABs or cause them to be more severe in both freshwater and marine waters. For example, warming temperatures in Lake Erie have contributed to extensive blooms that have lasted long into the early winter months. These blooms were caused by Microcystis aeruginosa, a toxin-producing cyanobacteria. In the past, blooms of Microcystis aeruginosa in Lake Erie were of smaller size and did not last until winter.

Where are Harmful Algal Blooms found?

HABs are found in bodies of fresh water, marine (salt) water, and brackish (mixture of fresh and salt water) waters around the world. In the United States, HABs occur in waters across the country.


In 2014, a national survey found that over 50% of responding states in the United States (38 states and the District of Columbia) reported that HABs are present every year in some of their lakes or other freshwater bodies.

Marine water

All coastal states in the United States have experienced HABs, including Hawaii and Alaska. HABs in marine waters have been associated with food poisoning and have occurred in coastal areas in the United States and throughout the world. Imported seafood has been implicated in a number of HAB-associated poisonings in inland areas.

Brackish water

HABs have been reported in brackish waters including estuaries and coastal waters in the United States.

How are people and animals commonly exposed to a Harmful Algal Bloom and Harmful Algal Bloom toxins?

People or animals (household pets, livestock, and wildlife) can be exposed to a HAB or HAB toxins in fresh, brackish, or marine water bodies through direct contact, ingestion, or inhalation.

  • Direct contact with HAB toxins can occur when people or animals swim in water containing a HAB.
  • Ingestion of HAB toxins can occur when people or animals swallow HAB-contaminated water or eat HAB-contaminated fish or shellfish.
  • Inhalation of HAB toxins can occur when people or animals breathe in HAB toxins that have been aerosolized (turned into tiny airborne droplets or mist) from the water. These toxins may be present in the immediate area and, in some cases, may be blown by the wind to surrounding areas.

For more information on HABs exposure, visit Sources of Exposure and Risk Factors.

What are the symptoms of Harmful Algal Bloom-associated illnesses?

In humans, exposure to waters contaminated with HAB toxins can cause a variety of signs and symptoms of illness, depending on the amount of toxins, the type of toxin, and the length of time that the person was exposed to the contaminated water or bloom. Symptoms usually begin within hours of exposure and can last for a few days.

People whose skin comes into contact with contaminated water  can experience skin and eye irritation, and those who breathe in toxins can experience respiratory irritation or illness. Swallowing the toxin can cause abdominal pain, vomiting, and neurologic effects.

In animals, HAB toxins can cause severe disease such as excessive salivation, weakness, staggered walking, difficulty breathing, convulsions, or even death. Death in animals can occur within hours to days of exposure.

For more information, visit Illness and Symptoms.

What should I do if I have been exposed to a Harmful Algal Bloom or Harmful Algal Bloom toxins?

Contact your local or state health department if you suspect there is a HAB or if you think you may be experiencing a HAB-associated illness. Some local and state health departments have web forms or hotlines for reporting suspected HAB-associated illnesses directly to the health department.

If you have any questions about symptoms that you are experiencing, call your local or state poison information center. The specialists might be able to provide information about HAB-associated illnesses.

Consult a healthcare provider for advice about how to relieve your symptoms. If you do consult a healthcare provider, please let them know that you might have been exposed to a HAB or may have been recently exposed to HAB toxins when consuming fish or shellfish. There are currently no available tests or special treatments for HAB-associated illnesses, but information about the suspected cause of your illness might help your healthcare provider to manage symptoms.

If you have a question about the health of your pets, consult your veterinarian.

How can I prevent illness from a Harmful Algal Bloom?

If you see an algal bloom, stay out of the water and keep your pets out of the water. It may be difficult to determine if an algal bloom is harmful by looking at it, so it is best to use caution and avoid contact. If you or your pets do go in water that has an algal bloom, wash yourself or your pets off immediately afterwards with tap water. When you visit a body of water, check for current local and/or state swimming or fishing advisories. If you have concerns about the appearance, smell, or taste of the water, stop using the water and contact your local park authority, environmental authority, or health department.

If you have been notified of a HAB in a local water body or in your public drinking water supply, follow local or state guidance to reduce your risk and report any illnesses that you believe resulted from exposure to a HAB or HAB toxins.

How do I know if my fresh or marine water fish or shellfish is free from Harmful Algal Bloom toxins?

HAB toxins in fresh and marine water fish or shellfish are tasteless, odorless, and are not removed by cooking. State management programs monitor and close shellfish beds and send out fish advisories in the event of a known HAB event.

Consumers should be aware of their fresh and marine water fish or shellfish sources by checking safety advisories from the Environmental Protection Agency’s (EPA) Agriculture or Fish Consumption Advisories web pages and understanding which fish or shellfish may be more likely to accumulate algal toxins.

Although some HABs may not be detected, seafood monitoring programs test marine fish and shellfish for toxins regularly, and commercially caught fish should be safe to eat. For more information on HAB-associated foodborne illness, visit the Eating Contaminated Seafood and Marine Toxin Poisoning page or CDC’s Food Safety website.

Can a Harmful Algal Bloom contaminate tap water?

HABs in freshwater bodies are a growing concern due to the potential for them to contaminate water bodies used for tap water. In freshwater, HABs are most commonly formed from a type of phytoplankton called cyanobacteria. Cyanobacteria can produce cyanotoxins (toxins produced by cyanobacteria). Harmful cyanotoxins do not cause changes in tap water taste or odor.

Cyanobacteria can also produce non-toxic compounds (mixtures of elements) that are not harmful but can change the taste or smell of water.

There are currently no federal regulations that determine the maximum number of cyanobacteria or the maximum concentration of cyanotoxins that are safe for public drinking water. The World Health Organization (WHO) has a recommended limit on the level of exposure to cyanotoxins that a person should experience in his or her lifetime [PDF – 22 pages]. Cyanotoxins have also been on the Environmental Protection Agency (EPA) Contaminant Candidate List (CCL1) since 1998. The Contaminant Candidate List is a list of contaminants that are not included in EPA drinking water regulations, but that have been known to or might occur in drinking water systems.

In 2015, EPA issued new health advisories for algal toxins in drinking water. The EPA health advisory recommends limits for two types of cyanotoxins, microcystin and cylindrospermopsin. The advisory states that algal toxin levels in drinking water do not exceed 0.3 micrograms per liter for microcystin and 0.7 micrograms per liter for cylindrospermopsin for children younger than school age (<5 years of age). For all other ages, the EPA recommends that algal toxins levels do not exceed 1.6 micrograms per liter for microcystin and 3.0 micrograms per liter for cylindrospermopsin. If algal toxins are over these levels, the public should be notified that no one should drink or boil the water. For more information on exposure to HABs and HAB toxins through swallowing drinking water, visit Sources of Exposure and Risk Factors.

How many people become ill from Harmful Algal Blooms annually?

It is unknown how many people become ill from HABs annually in the United States because individual cases of illness have not been tracked nationally. In 2016, CDC launched the One Health Harmful Algal Bloom System (OHHABS), which local, state, and territorial health agencies may use to report individual human and animal cases of illnesses from HAB-associated exposures, as well as environmental data about HABs.

Waterborne and foodborne disease outbreaks (defined as similar illness occurring in two or more people after exposure to the same water or food item) related to HABs may be reported by health departments to CDC’s National Outbreak Reporting System (NORS); however, reporting is limited to outbreaks, so individual cases are not recorded in this system.

What is the economic impact of Harmful Algal Blooms?

Economic effects of HABs may arise from public health costs associated with illness, commercial fishery closures and fish kills, declines in coastal and marine recreation and tourism, and the costs of monitoring and management.

While the economic cost in the United States is not well characterized, HABs potentially cost the economy millions of dollars each year. Examples of these costs include:

  • The fishing industry loses as much as $34 million a year (adjusted from Year 2000 estimate) from HABs in the form of lost sales when seafood is contaminated with toxins 1.
  • HAB-associated human and animal illness costing millions of dollars in direct healthcare costs. For example, respiratory illnesses associated with blooms of Karenia brevis from Sarasota County inon the Gulf Coast of Florida resulted in $0.5 to $4 million in hospital emergency department costs 2.
  • The recreation and tourism industries losing millions of dollars each year. For example, a HAB event in a single lake in Ohio cost $37 to $47 million in lost tourism revenue over two years 2.
  • The monitoring and management of HABs costs states millions of dollars each year.

What is currently being done to control Harmful Algal Blooms in the United States?

Local, state, tribal, and federal agencies and researchers across the United States are involved in various efforts to prevent HABs and HAB-associated illnesses, including:

  • Monitoring for algal blooms and associated toxins in recreational water, drinking water, and fresh and marine water fish or shellfish
  • Reducing nutrient loads in water
  • Conducting health surveillance for HAB-associated illnesses in humans and animals and HAB events
  • Collaborating among agencies to better coordinate activities within and across states and nationally
  • Engaging with citizen scientists and other partners to monitor for HABs in marine and fresh water

In 1972, the U.S. and Canada signed the Great Lakes Water Quality Agreement, as part of an effort to improve water quality. This agreement outlines work to control phosphorus levels that contribute to HAB development, among other issues. It also emphasizes the need to protect sources of drinking and recreational water in an effort to reduce human exposure to environmental pollutants.

Two U.S. federal legislative initiatives that support this work are the Great Lakes Restoration Initiative (GLRI) and the Harmful Algal Bloom and Hypoxia Research and Control Amendments Act of 2014 (HABHRCA). The GLRI was launched in 2010 to promote and speed up efforts to protect and restore the Great Lakes. The HABHRCA was originally enacted in 1998 and amended in 2014. This act helps direct resources, research, and policies regarding HABs and hypoxia (events of severe oxygen depletion) that damage the water body ecology for coastal and inland waters.

  1. Anderson DM, Hoagland P, Kaoru Y, White AW. Estimated annual economic impacts from harmful algal blooms (HABs) in the United States. [PDF – 96 pages] 2000;WHOI-2000-11.
  2. Florida Department of Health. Harmful Algal Blooms – Economic Impacts. [PDF – 1 page] 2008.