Harmful algal blooms (HABs) are the rapid growth of algae that can cause harm to animals, people, or the local ecology. A HAB can look like foam, scum, or mats on the surface of water and can be different colors. HABs can produce toxins that have caused a variety of illnesses in people and animals. HABs can occur in warm fresh, marine, or brackish waters with abundant nutrients and are becoming more frequent with climate change.

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.

Freshwater

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.

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.

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.External 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.

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.

Harmful algal blooms (HABs) can produce toxins that cause illness in people, companion animals (dogs, cats), livestock (sheep, cattle), and wildlife (including birds and mammals). Exposures to the toxins can occur when people or animals have direct contact with contaminated water by:

• Swimming
• Breathing in aerosols (tiny airborne droplets or mist that contain toxins) from recreational activities or wind-blown sea spray
• Swallowing toxins by drinking contaminated water or eating contaminated fish or shellfish.

Human and animal illnesses and symptoms can vary depending on the how they were exposed, how long they were exposed, and the particular HAB toxin involved.

Freshwater Environments

In freshwater, a harmful algal bloom (HAB) is most commonly caused by small organisms called phytoplankton. The phytoplankton that commonly cause HABs are cyanobacteria, which use sunlight to create food. Some cyanobacteria produce toxins called cyanotoxins. Depending on the specific chemical structure, cyanotoxins can be neurotoxins that affect the nervous system, hepatotoxins that affect the liver, dermatoxins that affect the skin, or other toxins that affect the stomach or intestines. Some common cyanotoxins that are known to cause illnesses in humans and animals are microcystins, cylindrospermopsin, anatoxins, saxitoxins, nodularins, and lyngbyatoxins.

Human and animal illnesses and symptoms can vary depending on the how they were exposed, how long they were exposed, and the particular HAB toxin involved. No human deaths in the United States have been caused by cyanotoxins; however, companion animal, livestock, and wildlife deaths caused by cyanotoxins have been reported throughout the United States and the world.

Humans and Freshwater HAB-associated Illnesses

Skin Contact and Inhalation

People or animals can be directly exposed to cyanotoxins in freshwater during recreational activities or by breathing in aerosolized toxins (toxins in water turned into tiny airborne droplets or mist). People or animals exposed to cyanotoxins through direct skin contact or inhalation may experience the following symptoms:

• Skin irritation
• Eye irritation
• Nose irritation
• Throat irritation
• Respiratory irritation

Ingestion

People can be exposed to cyanotoxins by eating freshwater fish from cyanotoxin-contaminated lakes or ponds or by drinking cyanotoxin-contaminated water. Additionally, some toxins may be ingested when taking blue-green algae dietary supplements. People who ingest cyanotoxins may experience the following symptoms:

• Abdominal pain
• Headache
• Neurological symptoms
• Vomiting
• Diarrhea
• Liver damage
• Kidney damage

Animals and Freshwater HAB-associated Illnesses

Pets, Livestock, and Wildlife

Pets, livestock, and wildlife can be poisoned through direct contact by swimming in waters with a HAB or by drinking cyanotoxin-contaminated water. Coyote deaths have been reported after they were believed to have eaten fish that washed up on the beach from a part of the Gulf of Mexico that was experiencing a HAB. Dogs are especially at risk to cyanotoxin poisoning due to their behaviors, which include swimming in contaminated waters, drinking the water, and licking algae or scum from their fur after swimming. Domestic pets may also be poisoned if they eat dietary supplements that contain algae contaminated with cyanotoxins.

A HAB may also make birds sick. Side effects of cyanotoxin poisoning in birds are sometimes confused with, or occur at the same time with, avian botulism, a disease that paralyzes birds. Birds can get avian botulism when they eat toxins produced by the bacteria Clostridium botulinum directly or eat insects containing the toxins. Avian botulism is becoming more common in the Great Lakes region since it was first reported in 1963, and there is some evidence that Clostridium botulinum can grow in the presence of algae commonly found in the Great Lakes region.

Animals who are exposed to cyanotoxins may experience the following symptoms:

• Excessive salivation
• Vomiting
• Fatigue
• Staggered walking
• Difficulty breathing
• Convulsions
• Liver Failure
• Death

Death in animals can occur within hours to days of exposure.

Fish and Aquatic Animals

When a HAB decomposes, it can use up the oxygen in a body of water. When this happens, fish may not have enough oxygen to breathe and may die. Fish and other aquatic animals (vertebrates or invertebrates that live in the water) may also eat cyanobacteria,storing the cyanotoxins in their bodies. When other animals eat these animals (for example, when small fish are eaten by larger fish), the toxins can build up and move up the food web. This process is called bioaccumulation. Some cyanotoxins can also kill fish by affecting their gills and preventing fish from breathing.

Marine Environments

Marine or saltwater harmful algal bloom (HAB) toxins can cause a variety of illnesses in humans and animals. Exposure to marine HAB toxins can occur through direct contact by swimming, breathing in aerosolized toxins (toxins in water turned into tiny airborne droplets or mist), or eating toxin-contaminated shellfish or finfish. In marine mammals, fish, and other aquatic marine life, exposure to HAB toxins can cause widespread illness or death. However, most states at risk for marine HABs have excellent monitoring programs in place to close harvesting when toxins are present in shellfish. Birds can also get sick by eating algae, drinking contaminated water, or eating contaminated marine fish or shellfish. For example, pelicans and cormorants have been poisoned by exposure to these toxins, and, in some cases, thousands of birds have died.

Marine HABs have occurred in the Gulf of Mexico, and along the Atlantic and Pacific coasts of the United States. Two major groups of marine phytoplankton, diatoms and dinoflagellates, produce HAB toxins. Some common marine HAB toxins include brevetoxins, azaspiracid, ciguatoxins, domoic acid, okadic acid, saxitoxin, and dinophysistoxins.

Humans and Marine Water-associated Illnesses

Skin Contact and Inhalation

Marine HABs can cause a variety of illnesses in people. Florida red tides, the most well-known marine HABs in the United States, occur frequently in the Gulf of Mexico. Florida red tides are caused by the dinoflagellate Karenia brevis,which can produce toxins called brevetoxins. Karenia brevis breaks up easily in ocean waves. When this happens, toxins inside the algae can become incorporated into aerosols that winds blow across the water and inland. People can then be exposed by breathing in these aerosols. People can also be exposed to brevetoxins through skin contact. Human exposure to brevetoxins via inhalation or skin contact can cause various symptoms, including the following:

• Respiratory irritation (coughing, sneezing)
• Shortness of breath
• Throat irritation
• Eye irritation
• Skin irritation

Ingestion: Eating Contaminated Seafood and Marine Toxin Poisoning

Marine HAB toxins can build up in seafood when fish or shellfish eat toxin-producing algae. Humans and animals that eat these contaminated fish or shellfish can become poisoned from HAB toxins, making them sick. Most human illnesses from HABs occur when people eat contaminated seafood. Symptoms of HAB toxin poisoning can vary depending on the type of toxin. Marine toxins and toxin poisoning information is listed below.

Ciguatera Fish Poisoning (CFP)

The most commonly reported illness caused by a HAB toxin in food is ciguatera fish poisoning (CFP). CFP is caused by eating fish with ciguatera toxins or ciguatoxins produced by a dinoflagellate species, Gambierdiscus toxicus. The dinoflagellates are eaten by plant-eating fish that are then eaten by fish-eating fish. As the toxins move through the food web, they change and become poisonous. The toxins can build up in in both fish- and plant-eating reef fish in tropical and subtropical waters, such as those found around Hawaii, Puerto Rico, South Florida, and the Gulf of Mexico. When these fish are eaten, the ciguatoxins can cause stomach and intestinal symptoms, including the following:

• Diarrhea
• Abdominal pain
• Nausea
• Vomiting

These symptoms often start within 12–24 hours of eating the contaminated fish and might last for up to 4 days. Stomach and intestinal symptoms might be followed by or accompanied by symptoms related to the heart, blood vessels, and nerves, including:

• Numbness and tingling in the extremities
• Dizziness
• Muscle aches
• Decreased heart rate
• Low blood pressure
• Weakness
• Heightened response to hot or cold temperatures

Symptoms have been reported to last anywhere from a few weeks to years. However, newer information suggests that symptoms from CFP typically go away within months and may be confused with symptoms of other chronic conditions.

Neurotoxic Shellfish Poisoning (NSP)

Neurotoxic Shellfish Poisoning (NSP) is caused by eating shellfish contaminated with brevetoxins, a toxin produced by a dinoflagellate species Karenia brevis. These toxins can be spread throughout the marine food web and have been found in shellfish, including oysters, clams, and mussels. This toxin is most commonly found in shellfish from the Gulf of Mexico but has also been found in shellfish from the in Mid-Atlantic waters.

Symptoms of NSP are often related to the stomach, intestines, and nervous system. Symptoms begin 1–3 hours after eating the contaminated shellfish and can include the following:

• Numbness
• Tingling in the mouth, arms and legs
• Loss of coordination
• Vomiting
• Diarrhea
• Heightened response to hot or cold temperatures

Symptoms usually resolve in 2–3 days. There have been no reports of long-term effects from NSP, but there have been no follow-up studies of patients to confirm this.

Paralytic Shellfish Poisoning (PSP)

Paralytic Shellfish Poisoning (PSP) is caused by eating shellfish contaminated with saxitoxins, a toxin produced by dinoflagellates of the genus Alexandrium. Saxitoxins, also known as PSP toxins, cause symptoms related to the nervous system. PSP toxins can be found in shellfish (such as mussels, cockles, clams, scallops, oysters, crabs, and lobsters) that usually live in the colder coastal waters near the Pacific states and New England. A species of puffer fish found off the east coast of Florida was recently discovered that also contained saxitoxins.

Symptoms usually begin within 2 hours of eating contaminated shellfish, but can start anywhere from 15 minutes–10 hours after the meal. Symptoms are generally mild and can include the following:

• Numbness or tingling of the face, arms, and legs
• Headache
• Dizziness
• Nausea
• Loss of coordination
• A floating sensation
• Muscle paralysis and respiratory failure can occur in severe cases

In cases of severe poisoning, muscle paralysis and respiratory failure can lead to death in 2–25 hours. The risk of death from PSP is reduced if healthcare professionals have access to machines to help people breathe (ventilators) if the ill person becomes paralyzed.

There are no reports of long-term effects, but there have not been any long-term follow-up studies of those affected.

Domoic Acid Poisoning and Amnesiac Shellfish Poisoning (ASP)

Domoic acid poisoning is caused by eating shellfish contaminated with domoic acid, a toxin produced by the diatoms Psuedo-nitzschia, Nitzschia, and Amphora. These diatoms have been found in the United States along the Pacific coast, northeast coast, and the western coast of Florida.
Domoic acid poisoning has caused a variety of symptoms ranging from memory loss to death. The first reported human domoic acid poisoning event occurred in Canada in 1987 when 143 people became ill and 3 died from eating domoic acid-contaminated mussels. Reported signs of the poisoning were stomach and intestinal symptoms, confusion, disorientation, memory loss, coma, and death. The illness was named Amnesic Shellfish Poisoning (ASP).

Most of what we know about domoic acid poisoning comes from studies of marine mammals, particularly sea lions. Domoic acid-poisoned animals, including marine mammals (seals, walruses, and sea lions), may exhibit neurotoxic effects, and the poisonings can be fatal.

Shellfish, such as mussels, can accumulate these toxins, making people who eat them sick with various symptoms, including the following:

• Vomiting and diarrhea within 24 hours of eating
• Dizziness
• Headache
• Disorientation
• Short-term memory loss
• Seizures, weakness, paralysis, and death can occur in severe cases

Diarrhetic Shellfish Poisoning (DSP)

Diarrhetic Shellfish Poisoning (DSP) is caused by eating shellfish contaminated with okadic acid and dinophysistoxins, toxins produced by the dinoflagellates Dinophysis and Procentrum. In the United States, these dinoflagellates have recently been found along the Gulf Coast of Texas.

DSP produces stomach and intestinal symptoms that usually begin 30 minutes to a few hours after eating contaminated shellfish and include:

• Vomiting
• Severe diarrhea
• Nausea
• Abdominal cramps
• Chills

Recovery occurs within about 3 days, with or without medical treatment. DSP is generally not life-threatening.

Azaspiracid Shellfish Poisoning (AZP)

Azaspiracid Shellfish Poisoning (AZP) is the most recently discovered human illness related to shellfish contaminated with a HAB toxin. AZP is believed to be caused by a dinoflagellate that produces toxins that have been found in Ireland, the Netherlands, Belgium, Morocco, and eastern Canada.

Eating contaminated shellfish can result in symptoms including:

• Nausea
• Vomiting
• Diarrhea
• Stomach cramps

What can I do to prevent poisoning by marine HAB toxins?

• Discuss your risk with your health care provider if you eat marine fish or shellfish and you have a chronic illness. Although any person eating fish or shellfish containing HAB toxins may become ill, persons with some chronic diseases, such as liver disease, could potentially have more severe illnesses. Discuss your risk with your health care provider if you eat marine fish or shellfish and you have a chronic illness.
• You cannot get rid of a toxin once it’s in a marine fish or shellfish. Unlike some other causes of foodborne illness, HAB toxins are not destroyed by storage, such as freezing or salting, or by cooking, such as grilling or frying.
• Check with local health officials before collecting shellfish, and look for advisories about harmful algal blooms or water conditions that may be posted at fishing supply stores, by beach managers, or local health authorities. HAB advisories are posted online by many states.
• Do not eat finfish or shellfish sold as bait. Bait products do not need to meet the same food safety regulations as seafood for human consumption.

Animals and Marine Water-associated Illnesses

Contact exposures to marine HABs have been fatal for aquatic animals. During November-December 2007, a widespread die-off of seabirds was caused by a massive HAB produced by the dinoflagellate Akashiwo sanguinea in Monterey Bay, California. Affected birds that came into direct contact with the bloom were covered in a slimy material produced by the algae. This material coated their feathers, affecting their natural water repellency. As the birds’ feathers became soaked with water, their body temperatures dropped dangerously low. There were no confirmed reports of human illness related to this HAB, although there were anecdotal reports of illness from local surfers.

Marine HABs have also caused neurologic effects in animals including aquatic animals and birds. Over 400 sea lions, seals, and birds died or were affected by a HAB produced by the diatom Pseudonitzschia australis near Monterey Bay, California. The HAB produced domoic acid, a neurotoxin, which was also detected in mussels, anchovies, and sardines that were likely eaten by the sea lions.

A harmful algal bloom (HAB) can occur in water bodies around the world and can affect those who use these water bodies for recreation, agricultural, or drinking. People can be exposed to a HAB or HAB toxins when they swim, wade, or play in or near contaminated water; eat contaminated fish or shellfish; or use contaminated drinking water. The severity of illness and symptoms can vary depending on the type of exposure and the type of HAB toxin.

The main routes of exposure to HAB toxins are:

• Skin contact (through activities like swimming)
• Inhalation (by breathing in tiny airborne droplets or mist contaminated with HAB toxins)
• Ingestion (by eating or drinking food or water contaminated with HAB toxins)

Skin contact

Anyone who visits a contaminated water body during a HAB event can be exposed through direct contact with the contaminated water. Skin irritation and reactions in humans and animals can vary depending on the length of contact with the contaminated water and the type of HAB toxin present in the water.

Inhalation

People can be exposed to a HAB or HAB toxins by inhaling (breathing in) tiny water droplets, mist, or sea spray from a contaminated body of water. This can occur even if a person does not go into the water. Further research is needed to better understand the effects of long-term inhalation exposure to HAB toxins, especially for those who regularly work on or near water, such as boaters or lifeguards.

Individuals who have been on the beach or on a boat during a marine HAB event have reported breathing difficulties after inhaling air or water particles contaminated with HAB toxins. A study conducted during a Florida red tide found that marine HAB toxins could be transported in the air almost 4 miles inland from the water source. A marine HAB may cover hundreds of square miles of ocean and affect boaters across the entire area.

Ingestion

Recreational Activities

People can swallow contaminated water while they are swimming or playing in the water. Active water sports (like water-skiing) pose a higher risk of accidental ingestion. Swimmers may swallow up to 16–200 mL of freshwater (the equivalent of 0.5 – 6.8 ounces of water) during one swim.

Animal Exposures to Natural/Ambient Waters

Animals can become sick when they drink contaminated water, groom themselves after swimming in contaminated water, or eat toxic algae or contaminated fish and shellfish in the water. In fact, animals are at a higher risk of ingesting contaminated water because they do not avoid water that is discolored or smells bad.

Food

Freshwater

People and animals can be exposed to HAB toxins by eating fish or shellfish from a freshwater body with a HAB. Freshwater fish can become contaminated with cyanotoxins when they eat toxin-producing cyanobacteria.

Marine water

People and animals can be exposed to HAB toxins by eating seafood from contaminated marine (salt) water bodies. Shellfish can become contaminated when they filter and concentrate water with a HAB, and reef fish can become contaminated through the food chain (eating other animals that have accumulated the toxin).

Drinking Water

People and pets might be exposed to HAB toxins through a contaminated tap water supply, though this is still an uncommon occurrence. Exposure to HAB toxins can occur in a healthcare setting, although this is also rare. This type of exposure can occur if patients receive a medical treatment, such as dialysis, that uses water that has not had HAB toxins removed. In 1974, 23 dialysis patients in Washington, DC, became ill when the local water source became contaminated with cyanotoxins, which are toxins from a type of phytoplankton called cyanobacteria. In 1996, 52 dialysis patients died and another 64 became ill in Brazil when the water used for dialysis treatment was contaminated with cyanotoxins.

The occurrence of cyanotoxins in drinking water can depend on the level of cyanotoxins in untreated or raw source water and the effectiveness of water treatment methods to remove them. Some public drinking water systems use surface water from lakes that could contain a HAB. A survey of 24 source water and treatment facilities between 1996-1998 in the United States and Canada found 80% of their 677 water samples contained detectable levels of microcystin, a type of cyanotoxin. Of the 80% of water samples with detectable microcystin levels, 4% of the samples exceeded the World Health Organization (WHO) drinking water guideline limit of 1µg L^-1 for microcystin in drinking water. However, of the 4% of samples that exceeded the WHO drinking water guidelines, only two water samples were finished or treated water samples (water ready for consumption). This means that the majority of sample water contained microcystin, but the facilities could adequately remove microcystin to levels below the WHO life-time drinking water guideline.

There are currently no federal regulations in the United States on the acceptable levels of cyanobacteria or cyanotoxins allowed in public drinking water. However, as of June 29, 2015, the US EPA issued new health advisories for algal toxins in drinking water. The EPA 10-day health advisory recommends limits for two types of cyanotoxins, microcystin and cylindrospermopsin. The advisory recommendation is 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 exceed these levels, the public should be notified that no one should drink or boil the water.

The Environmental Protection Agency (EPA) suggests the risk of a HAB contaminating drinking water is increasing due to nutrient pollution of source waters such as agricultural runoff of fertilizer containing phosphorus and nitrogen, which are nutrients that promote algae growth.

Water treatment facilities have options to remove cyanobacteria and their toxins from water during treatment. Coagulation, sedimentation, and filtration have been shown to remove between 90 and 99.9% of algae. However, when algae begin to die, toxins contained in the algae can be released from the algal cells. These toxins may not necessarily be removed through coagulation, sedimentation, or filtration of the water. However, the toxins released by algae can be treated through other processes, such as activated carbon filtration and oxidation. These methods are not always a part of water utility treatment processes You can find out more about your local drinking water on EPA’s website.

Nutritional supplements

Nutritional supplements that contain algae can also pose a risk for exposure to HAB toxins. When algae are harvested to produce supplements, a toxin-producing cyanobacteria (such as Microcystis) might accidentally be collected as well.

In 1996 in Oregon, a HAB produced large amounts of the HAB toxin microcystin in Upper Klamath Lake. Oregon officials issued a precautionary advisory for supplements that used algae collected from the lake. The state established an allowable limit of 1 microgram of microcystin per gram of algal supplement. As a result of the advisory issued by state officials and consumer concern, 87 samples of algae supplements sourced from Upper Klamath Lake were tested, and 72% had levels of microcystin that exceeded this limit. New technologies have since been used for collecting algae to reduce the risk of taking toxic cyanobacteria from this lake.

How to Reduce Exposures and Prevent Illness

It is not possible to know if an algal bloom is harmful just by looking at it. Additionally, toxins can be present even when an algal bloom is not visible.

Protect yourself and your pets from harmful algal blooms (HABs):

• Avoid entering or playing in bodies of water that:
  • smell bad
  • look discolored
  • have foam, scum, or algal mats on the surface
  • contain or are near dead fish or other dead animals (for example, do not enter a body of water if dead fish have washed up on its shore or beach)
• Follow local or state guidance if you are notified that your tap water contains algal toxins. Boiling water does not remove algal toxins and can increase the amount of toxin in the water by concentrating it. Be aware of advisories and health risks related to consuming contaminated fish and shellfish. For more information, see the Environmental Protection Agency’s (EPA) Choose Fish and Shellfish Wisely web pages.

Preventing Freshwater Exposures

• Do not drink directly from lakes, rivers, or ponds. Even if the water looks “safe,” it might contain toxins or germs that can cause other types of illness.
• Check your state or local environmental health website for beach or lake closures in your area before visiting.
• Do not fish, swim, boat, or play water sports in areas that are experiencing a HAB. Do not let pets eat algae, get in the water, or go on the beach or shoreline.
• Rinse pets off with tap water after they have been in a lake, river, or pond; do not let them lick their fur until they have been rinsed.
• Do not fill pools with water directly from lakes, rivers, or ponds,as the water might contain toxins or germs that can cause other types of illness.

Preventing Marine (Salt) Water Exposures

• Check your state or local environmental health website for beach closures in your area before visiting.
• Do not fish, swim, boat, or play water sports in marine waters that are experiencing a HAB. Do not let pets eat algae, get in the water, or go on the beach or shoreline during a HAB.
• Rinse pets off with tap water after they have been in the ocean; do not let them lick their fur until they have been rinsed.
• Avoid eating very large reef fish (especially the head, gut, liver, or roe). You can reference the FDA Reef Fish Guidance for more information on reef fish associated with unsafe levels of toxins.
• Follow local guidance if you plan to eat any fish or shellfish that you harvest yourself.

What should I do if I have been exposed?

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 that you have recently consumed fresh or marine water fish or shellfish that might have be contaminated with toxins. 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.

What should I do if my pet has been exposed?

If your pet has come into contact water that has an algal bloom, rinse them with tap water as soon as possible.

Seek veterinary care immediately if your pet has consumed or licked algae on its fur after swimming or playing in water that has an algal bloom.

What is a harmful algal bloom?

A harmful algal bloom (HAB) occurs when toxin-producing algae grow excessively in a body of water. Algae are microscopic organisms that live in aquatic environments and use photosynthesis to produce energy from sunlight, just like plants. The excessive algal growth, or algal bloom, becomes visible to the naked eye and can be green, blue-green, red, or brown, depending on the type of algae.

Algae are always present in natural bodies of water like oceans, lakes, and rivers, but only a few types can produce toxins. In these algae, toxin production can be stimulated by environmental factors such as light, temperature, and nutrient levels. Algal toxins are released into the surrounding water or air can seriously harm people, animals, fish, and other parts of the ecosystem.

Why do HABs occur?

Scientists know that environmental conditions trigger HABs, such as warmer water temperatures in the summer and excessive nutrients from fertilizers or sewage waste brought by runoff, but are still learning more about why HABs occur. As climate change gradually warms the earth’s climate, scientists expect HABs to become more frequent, wide-ranging, and severe.

How are people exposed?

During a HAB, people can get exposed to toxins from fish they catch and eat, from swimming in or drinking the water, and from the air they breathe. In recent years, there have been numerous instances of HABs in lakes that provide drinking water, like Lake Erie. Importantly, cooking contaminated seafood or boiling contaminated water does not destroy the toxins.

People rarely get sick from HAB-related toxins in commercial seafood, however, because state regulators closely monitored fisheries for HABs and close them during blooms.

People can prevent exposure to HABs by following local health advisories regarding the safety of recreationally caught seafood and drinking water sources.

What are the health effects of harmful algal blooms?

Depending on the type of algae, HABs can cause serious health effects and even death. For example, eating seafood contaminated by toxins from algae called Alexandrium can lead to paralytic shellfish poisoning, which can cause paralysis and even death. The algae Pseudo-nitzschia produces a toxin called domoic acid that can cause vomiting, diarrhea, confusion, seizures, permanent short term memory loss, or death, when consumed at high levels.

HABs that occur in freshwater, like the Great Lakes and other drinking water sources, are dominated by the cyanobacteria Microcystis. This organism produces a liver toxin that can cause gastrointestinal illness as well as liver damage.

As with many environmental exposures, children and the elderly may be especially sensitive to HAB toxins. Populations that rely heavily on seafood are also at risk of long term health effects from potentially frequent, low level exposures to HAB toxins.

The table below summarizes common HABs and their health effects.

Other impacts from HABs

In addition to health concerns, HABs can damage the environment by depleting oxygen in the water, which can cause fish kills, or simply by blocking sunlight from reaching organisms deeper in the water. The economic impacts of HABs to fisheries and recreational areas can also be extensive. Closed fisheries can lose millions of dollars in revenue each week.