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Your Environment. Your Health.

Algal Blooms


red tide in water

What are algae?

Algae are microscopic organisms that live in aquatic environments and use photosynthesis to produce energy from sunlight, just like plants. Algae can be found in all types of natural waters, including salt water, fresh water, and brackish water (a mix of salt and fresh water).

A few types of algae produce toxins. In these algae, toxin production can be stimulated by environmental factors such as light, temperature, salinity, pH, and nutrient levels. Algal toxins released into the surrounding water or air can seriously harm people, animals, fish, and other parts of the ecosystem.

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. 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.

Some blooms are easy to spot, but others are hard to see because they grow near the bottom of water bodies. You cannot tell if a water body has a harmful bloom just by looking at it.

Why do HABs occur?

Many factors may contribute to HABs but understanding how these factors come together to create a harmful bloom of algae is a topic of ongoing research. Scientists know that certain environmental conditions, such as warmer water temperatures in the summer and excessive nutrients from fertilizers or sewage waste brought by runoff, trigger HABs, but they are still learning more.

As climate change gradually warms the earth’s climate, scientists expect HABs to become more frequent, prolonged, and severe, and in different geographic areas.

What are impacts from HABs?

HABs can damage the environment by depleting oxygen in the water, which can kill fish and other living creatures. HABs that bloom near the water surface can also block sunlight from reaching organisms deeper in the water.

The economic impacts of HABs to fisheries and recreational areas can be extensive. Businesses lose significant revenue if they are forced to close or reduce operations due to HABs.

Some HABs are associated with serious health effects, as explained below.

How are people exposed?

"closed to shellfishing" sign

During an 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. NIEHS research has helped us better understand how toxins accumulate in seafood and are later absorbed by our bodies.

People can get sick from eating seafood contaminated with HAB-related toxins. But it rarely happens from eating commercial seafood because state regulators monitor fisheries for HABs and close them during blooms.

Illness from HABs can be prevented by following local health advisories on 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.

Organism Water Type Color Toxin Health effects
Alexandrium sp. Salt Red or brown Saxitoxins Gastrointestinal (nausea, vomiting), and neurological (a floating sensation, headache, or muscle weakness)
Cyanobacteria Fresh Blue-green Cylindrospermopsin Nausea, vomiting, diarrhea, abdominal tenderness, pain, or acute liver failure
Gambierdiscus Salt Orange Ciguatoxins Nausea, vomiting, diarrhea, or stomach pain
Karenia brevis Salt Red Brevetoxins
  1. Gastrointestinal illness, muscle cramps, seizures, paralysis
  2. Respiratory problems, especially for asthmatics
Pseudo-nitzschia Salt Red or brown Domoic acid Vomiting, head weaving, nausea, seizures, diarrhea and abdominal cramps, bulging eyes, or headache
Microcystis Fresh Blue-green Microcystin Gastrointestinal illness, liver damage

What Is NIEHS Doing?

Environmental Sample Processor

The Environmental Sample Processor can be left in a body of water to continually test for HABs. It was developed with NIEHS support at the Woods Hole Center for Oceans and Human Health. (Illustration by E. Paul Oberlander, Woods Hole Oceanographic Institution)

Studying health effects from low doses over time

Since the immediate effects of harmful algal blooms are well known, NIEHS-funded scientists are now investigating potential long term effects of HABs. For example, researchers are studying whether consuming trace amounts of neurotoxic domoic acid over time damages brain function, especially among children or the elderly. In animals, scientists have observed that chronic, low level exposure to domoic acid altered the expression of genes controlling the nervous system and impaired cell function.

Developing better detection

Rapid detection of HABs can help state regulators protect public health with minimal economic impacts to fisheries and recreational areas. The Woods Hole Center for Oceans and Human Health (WHCOHH) in Massachusetts designed a robotic system called the Environmental Sample Processor that can be anchored in a body of water to remotely test seawater samples for HABs as they begin to bloom, and send the results to scientists. Other NIEHS-supported researchers are developing instruments that can detect HABs by counting algal cells in water samples or use satellite images to monitor HABs from afar.

Scientists also are studying the ability of water treatment plants to detect HABs and are developing new methods to aid in detection.

Improving prediction

With the frequency and severity of HABs increasing worldwide, improved prediction enables public officials to be one step ahead of blooms. NIEHS-funded researchers from the Woods Hole Center for Oceans and Human Health found that in deep waters, the algae Alexandrium fundyense may bloom based on an internal annual clock, while in shallow waters, conditions such as temperature may trigger blooms. These researchers have also shown that the number of dormant cysts formed after a summer Alexandrium fundyense bloom can predict the extent of the next year’s bloom.

NIEHS Research Efforts


Further Reading

Additional Resources

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