General Information

Harmful Algal Bloom Basics

What are algae?

Algae are a large group of plant-like organisms usually found in water. They can vary in size from very small (microalgae) to very large (macroalgae) and can be found in all types of water, including marine (salt) water and fresh water. Like plants, algae are essential to the earth because they produce the oxygen needed to sustain life.

What is an algal bloom?

Algal blooms occur when algae multiply very quickly. Blooms can form in waters that are rich in the nutrients the algae need to grow, such as nitrogen, phosphorous, and iron. Warmer waters may also help algae grow quickly to form blooms. Blooms may become more frequent as the earth warms and the levels of nutrients in our waters increase.

What causes an algal bloom?

In fresh water, algal blooms are most likely to form in warm, still waters that have a lot of nutrients such as phosphorus and nitrogen.

In marine and brackish (mixture of fresh and salt water) waters, a harmful algal bloom 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 number of blooms or cause them to be more severe in both fresh water and marine waters.

What does an algal bloom look or smell like?

An algal bloom can look like foam, scum, mats, or paint floating on the surface of water. Some blooms are not visible at the water surface. Water bodies with an ongoing bloom may look 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).

What is a harmful algal bloom?

Harmful algal blooms (also known as HABs) result from the rapid growth, or bloom, of algae that can cause harm to animals, people, or the local environment. They can look like foam, scum, or mats on the surface of water and can be different colors. Harmful algal blooms can occur in warm fresh, marine, or brackish waters with abundant nutrients. There are many organisms that produce harmful algal blooms and harmful algal blooms may have many have different names.

A harmful algal bloom 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, drink the water, or swim in the water.
  • Causing illness when a person or animal eats fish or shellfish contaminated with algal toxins.
  • Becoming dense enough to keep sunlight from reaching the lower depths of the water.
  • Removing the oxygen from the water as it decomposes, starving fish and plants of oxygen, and damaging the local ecology.

What types of algae can become harmful algal blooms?

In salt water, harmful algal blooms may be called “tides” based on the color of the bloom. One example is Karenia brevis, an organism that forms the red tides sometimes seen in ocean waters along the Gulf Coast of Florida. Breathing in toxins from red tides can cause irritation of the throat and lungs.

In fresh water, such as lakes and rivers, harmful algal blooms are most commonly formed from cyanobacteria. Because of their color, they are often called blue-green algae. Although they are called bacteria, they do not cause infections in people. Instead they can harm people by creating toxic substances that people might breathe in or accidentally eat.

Where are harmful algal blooms found?

Harmful algal blooms are found in bodies of fresh water, marine (salt) water, and brackish water around the world. They occur in waters across the United States.

  • Fresh water —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 harmful algal blooms are present every year in some of their lakes or other fresh water bodies.
  • Marine water—All coastal states in the United States have experienced harmful algal blooms, including Hawaii and Alaska. Harmful algal blooms in the U.S. and around the world have been associated with food poisoning. Imported seafood has been implicated in a number of HAB-associated poisonings in inland areas.
  • Brackish water—Harmful algal blooms have been reported in brackish waters, including estuaries and coastal waters, in the United States.
Exposure, Illness, and Prevention

How can people or animals get sick?

People or animals can get sick when they have contact with water or food contaminated with algal toxins by:

  • Participating in recreational activities such as swimming, kayaking, fishing, or wading through water.
  • Breathing in contaminated tiny water droplets or mist from recreational activities or wind-blown sea spray.
  • Drinking contaminated water.
  • Eating contaminated seafood (fish or shellfish).

How can I prevent illness?

If you see an algal bloom, stay out of the water and keep your pets out of the water. You cannot tell if an algal bloom is harmful by looking at it, so it is best to be careful 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. Do not let your pets lick their fur until you wash them off with tap water.

When visiting lakes, rivers, or beaches, check your local water conditions, and follow any advice posted by your state or local environmental health department online or near the water. If you have concerns about the appearance, smell, or taste of the water, stay out of the water and contact your local park authority, environmental authority, or health department.

If you have been notified of a harmful algal bloom in a local water body or in your public drinking water supply, follow local or state guidance to reduce your risk. The U.S. Environmental Protection Agency (EPA) maintains a list of many state monitoring programsexternal icon. These programs may help you find information about official advisories in your state.

What are the symptoms in people?

Symptoms can vary depending on the type of exposure and whether the harmful algal bloom was from fresh or marine water. The symptoms also depend on the amount and type of toxin and how long the person was exposed to the contaminated water or bloom.

Symptoms can include:

  • Skin, eye, nose, or throat irritation
  • Abdominal pain
  • Headache
  • Neurological symptoms
  • Vomiting
  • Diarrhea

Symptoms from exposure to red tide blooms, such as coughing and eye irritation, start as soon as people arrive at the beach and typically end when they leave.

Symptoms from exposure to other harmful algal bloom toxins may begin within hours of exposure and can last for a few days. In some cases, people can also experience liver or kidney damage.

What are the long-term health effects of harmful algal blooms?

The long-term health effects of harmful algal blooms in people and animals remain unclear. Because we are still learning about the long-term health effects, it’s important to take steps to keep people and pets safe, such as not going into water containing harmful algal blooms.

Talk with your health care provider if you have questions about your health and exposure to a harmful algal bloom.

What is BMAA?

Most species of cyanobacteria—a type of bacteria that commonly produces harmful algal blooms in fresh water—can produce a substance called Beta-N-methylamino-L-alanine, or BMAA.

Is BMAA harmful to humans?

Some research involving animals showed that BMAA has harmful effects on the brain. However, more research is needed to understand the link between BMAA and brain diseases in people and animals. In 2017, EPA published a critical reviewexternal icon that looked at the suspected role of BMAA in brain diseases in people. The review concluded that existing data do not show that BMAA causes brain disease.

What are the symptoms in pets?

In animals, harmful algal bloom toxins can cause serious disease such as too much salivation, weakness, staggered walking, difficulty breathing, convulsions, or even death. Animals can die within hours to days of exposure.

What should I do if I have been exposed?

If you had direct or indirect contact with contaminated water, or if you may have eaten contaminated fish or shellfish, see your healthcare provider for advice about how to relieve your symptoms.

There are currently no tests or treatments for HAB-associated illnesses, but information about the suspected cause of your illness might help your healthcare provider manage your symptoms.

What should I do if my pet has been exposed?

Animals are often the first affected because they are more likely to swim in or drink from bodies of water that contain algal blooms. Dogs are also likely to lick algae off their fur after swimming in water with a harmful algal bloom. They may also eat fish or other creatures killed by harmful algal blooms.

If you think your pet may be sick because of a harmful algal bloom, consult your veterinarian right away. Animals can get very sick quickly, so don’t delay contacting your veterinarian.

How do I report an illness?

Report any HAB-associated illness to your local, state, or territorial health department. In some states/territories, you can report this information through an online form or a hotline.

If you have immediate questions about your symptoms, call your local or state poison center.external icon

Who should I contact if I think I see a harmful algal bloom?

Report suspected harmful algal blooms or associated illnesses in humans and animals to your local, state, or territorial health department. This can help them understand and prevent harmful algal blooms and any illnesses.

In some states, the public can help the state track when and where blooms are occurring through tools like the BloomWatchexternal icon app or the National Phytoplankton Monitoring Networkexternal icon. EPA maintains a list of many state monitoring programsexternal icon. These programs may help you find information about how to report harmful algal blooms or HAB-associated illnesses in your state. Examples of state-specific citizen science programs include:

How do I know if fish or shellfish are safe to eat?

Harmful algal bloom toxins in fish or shellfish are tasteless and odorless and are not removed by cooking. State officials monitor and close shellfish beds and send out fish advisories when there is a known harmful algal bloom. Look for state or local advisories about harmful algal blooms or water conditions that may be posted at fishing supply stores or online.

Check EPA’s Fish and Shellfish Advisories websiteexternal icon to know what types of fish or shellfish may be more likely to have algal toxins.

Although some harmful algal blooms may not be detected, seafood monitoring programs regularly test commercially caught marine fish and shellfish for toxins to make sure they are safe to eat. For more information on HAB-associated foodborne illness, visit CDC’s Marine Environments page.

Can a harmful algal bloom contaminate tap water?

Harmful algal blooms in fresh water are a growing concern because they may appear in water bodies that supply tap water. In certain situations, toxins can contaminate tap water. Many water treatment plants have the ability to remove these toxins during the treatment process. Cyanobacteria can also produce nontoxic compounds (mixtures of elements) that are not harmful but can change the taste or smell of water.

How many people become sick from harmful algal blooms each year?

We do not know how many people become sick from harmful algal blooms each year in the United States because individual cases of illness are not tracked nationally. In 2016, CDC launched the One Health Harmful Algal Bloom System (OHHABS). Local, state, and territorial health agencies may use OHHABS to report human and animal cases of illnesses from HAB-associated exposures, as well as environmental data about harmful algal blooms.

Health departments can report 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 harmful algal blooms through CDC’s National Outbreak Reporting System (NORS).

What federal guidance exists to protect the public from harmful algal blooms in water?

EPA has issued drinking and recreational water health advisories and water quality criteria based on peer-reviewed, published science and methods. State and tribal governments may reference this information when setting their own water quality standards to protect human health.

Tap water:
Cyanotoxins have been on the EPA Contaminant Candidate List (CCL1)external icon since 1998. The Contaminant Candidate List is a list of contaminants that are not included in EPA drinking water regulations but potentially occur in drinking water systems.

In 2015, EPA issued new health advisories pdf icon[PDF – 3 pages]external icon for algal toxins in drinking water. The advisory recommends limits for two types of cyanotoxins: microcystin and cylindrospermopsin. The advisory states that algal toxin levels in drinking water should not exceed 0.3 micrograms per liter for microcystin and 0.7 micrograms per liter for cylindrospermopsin for children younger than 5 years old. 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 of a do not drink or a boil water advisory. For more information on exposure to harmful algal blooms and its toxins through swallowing drinking water, visit Sources of Exposure and Risk Factors.

Recreational water:
The World Health Organization (WHO) has recommended limits on the level of exposure to cyanotoxins that a person should experience in his or her lifetime. pdf icon[PDF – 22 pages]external icon

In 2019, EPA issued final recommended recreational ambient water quality criteria or swimming advisoriesexternal icon for two types of cyanotoxins: microcystins and cylindrospermopsin. The criteria recommend that a swimming advisory be considered if the water is determined to have microcystin concentrations that are more than 8 micrograms per liter or cylindrospermopsin concentrations that are more than 15 micrograms per liter.

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 efforts to prevent harmful algal blooms 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 harmful algal blooms events
  • Collaborating among agencies to better coordinate activities within and across states and nationally
  • Providing guidance for safe levels of harmful algal bloom toxins in waters used for drinking and bathing
  • Engaging with citizen scientists and other partners to monitor for harmful algal blooms in marineexternal icon and fresh waterexternal icon

In 1972, the United States 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 harmful algal bloom 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 Initiativeexternal icon (GLRI) and the Harmful Algal Bloom and Hypoxia Research and Control Amendments Act of 2014 (HABHRCA).external icon The GLRI was launched in 2010 to promote and speed up efforts to protect and restore the Great Lakes. The HABHRCA was enacted in 1998, amended in 2014, and reauthorized in 2018. This act helps direct resources, research, and policies regarding harmful algal blooms and hypoxia (events of severe oxygen depletion) that damage the ecology for coastal and inland waters.

Economic Impacts

What is the economic impact of harmful algal blooms?

Economic effects of harmful algal blooms 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 fully known, harmful algal blooms potentially cost the economy millions of dollars each year. For example:

  • The fishing industry loses as much as $34 million a year (adjusted from year 2000 estimate) in sales when seafood is contaminated with toxins from harmful algal blooms.1
  • HAB-associated human and animal illness costs millions of dollars in direct healthcare costs. For example, respiratory illnesses associated with blooms of Karenia brevis from Sarasota County on the Gulf Coast of Florida resulted in $0.5 to $4 million in hospital emergency department costs.2
  • The recreation and tourism industries loses millions of dollars each year. For example, a harmful algal bloom event in a single lake in Ohio cost $37 to $47 million in lost tourism revenue over two years.2
  • Monitoring and managing harmful algal blooms  costs states millions of dollars each year.
Page last reviewed: June 3, 2020