Skip to main content Homepage

Frequently Asked Questions

Cyanobacterial (Harmful Algae) Blooms and Recreational Advisories

What are cyanobacteria (harmful algae)?

Cyanobacteria are not algae at all, but primitive photosynthetic, single-celled bacteria found naturally in fresh and salt water all over the world. In warm weather, when nutrients are plentiful, and water flow is low, they may grow quickly or “bloom,” sometimes producing toxins. Although blooms that produce cyanotoxins can be harmful, these bacteria are very beneficial to the environment and helped to create our oxygen atmosphere.

Why is it called cyanobacteria?

The word "cyan" means blue-green, which is an appropriate name since most blooms are blue-green or green in color.

What toxins are involved in these algal blooms?

The most common toxins in harmful algal blooms in Oregon are microcystins and cylindrospermopsin.

Why are these blooms a health concern?

Not all blooms are harmful, but some genera of cyanobacteria such as microcystins and dolichospermum can produce toxins that can cause serious illness or death in pets, livestock and wildlife. These toxins can also make people sick, and in sensitive individuals also cause a red, raised rash or skin, ear and eye irritation.

What are the health risks posed by exposure to these toxins?

Although these toxins are not absorbed through the skin, a red, raised rash or irritation of the skin and eyes can develop after contact with a bloom. If affected water is swallowed, you may experience one or more of these symptoms; headaches, cramps, diarrhea, nausea and vomiting, numbness, dizziness, fever. Children and pets are at increased risk of exposure because of their size and level of activity. The most severe reactions occur when large amounts of water are swallowed. Some symptoms of exposure can mimic food poisoning. Food poisoning symptoms usually go away fairly quickly once your body gets rid of the spoiled food. Symptoms of exposure to a cyanotoxin will go away, but not as fast. Again, if symptoms are severe or persist you should seek medical attention.

What causes cyanobacterial blooms?

It's difficult to know the specific cause of any one individual bloom. While we know of many factors that may contribute to cyanobacterial blooms, how these factors come together to create a bloom is not well understood. However, in general, sunlight, warm water temperatures, still water, and high nutrient (nitrogen and phosphorus) concentrations favor the development of these blooms. Nutrients can be naturally occurring (such as from wildlife and decaying vegetation) or arise from human inputs (such as fertilizer runoff, manure, failing septic systems, or forest management activities). While cyanobacterial blooms have been occurring since early in Earth’s history, global climate change is likely to increase factors that promote cyanobacterial growth. These factors are expected to increase the frequency and duration of cyanobacterial blooms.

Are these blooms a new problem?

No. The earliest reliable account of a cyanobacterial bloom dates to the 12th century and the toxic effects on livestock have been recognized for more than 100 years. Because bloom formation is linked to environmental conditions and nutrients that provide the elements necessary for cyanobacteria to multiply into a bloom, and conditions that foster blooms are increasing - higher air temperatures, more sunlight, lower snowpack and decreasing water levels creating warmer water temperatures, the problem with blooms is not likely to go away in the near future.

Why do blooms sometimes appear overnight?

Most cyanobacteria have evolved to be able to control their buoyancy. By being able to sink and rise they can move to where nutrient and light levels are at their highest. At night when there is no sunlight, cells lose their buoyancy causing them to float to the surface forming a surface scum. Scum can seem to appear overnight for this reason and linger until the wind and waves scatter the cells throughout the water and to other parts of a waterbody.

How will I know if a bloom is toxic?

Unfortunately, you can't tell if a bloom is producing toxins just by looking at it. Nor is the size of the bloom associated with the amount of toxins that can be produced. Because we don't know why or when cyanobacteria produce toxins it's impossible to predict when toxins are present unless sampling and testing is done.

When are recreational use health advisories issued for cyanobacterial blooms?

A health advisory is issued when bloom sampling data determines that cyanotoxin levels are above our Oregon Health Authority recreational use values (RUVs). We have established RUVs for people and for dogs, but advisories are only issued when levels are over the RUVs for people. You should know that even if an advisory is not in place on a water body (toxins are below RUVs for people, or sampling has not been performed), if you see what you believe to be a bloom, toxins may be high enough to be harmful to you and your pet. Dogs are very susceptible at extremely low levels and could result in your pet getting extremely sick, or in some cases cause death. If your pet exhibits any unusual symptoms after being in a water body you should seek advice from your veterinarian as soon as possible.

How can I protect myself when I am camping or recreating at a lake with a bloom?

Stay out of the affected water and keep children and pets away. Never drink or cook with affected water, and do not allow your pet to drink from the water body. Although toxins are not absorbed through the skin, if you or your pet come in contact with affected water, wash your skin or your pet thoroughly with another source of water and soap if available.

Can I treat toxins in the water to make it safe to drink?

Personal water filtration devices for camping or hiking have not been proven to be effective, and boing water will not remove the toxins. In fact, boiling can increase the toxins in the water. Not all home filtration devices used to purify water drawn from an intake in a water body affected by a bloom are adequate at reducing cyanotoxins. However, for microcystin the global public health organization NSF International has developed a test method (NSF protocol 477) Drinking Water Treatment Units - Microcystin that verifies a water filter's ability to reduce microcystin to below the health advisory levels for drinking water set by the EPA. Certified systems for the reduction of microcystin can be found here. If you do not own one of the certified systems, or there are other cyanotoxins present, the only way to know if an in-home treatment system is effective at removing or reducing these toxins is to have your drinking water tested when a bloom is identified near your intake.

Can any chemicals be used to get rid of an cyanobacterial bloom?

Unfortunately, sprays that would be effective against cyanobacteria would harm the river or lake’s beneficial aquatic plant and animal life. These blooms naturally dissipate when conditions no longer favor their growth, such as cooler water temperatures, higher water flow, low sunlight and lower nutrient (nitrogen and phosphorus) concentrations.

Is OHA investigating specific sources of cyanobacterial blooms?

OHA’s role is to advise the public on how to reduce exposures once a bloom has occurred. Natural resource agencies are better suited to investigate the causes of blooms, and to work on actions that may reduce the frequency of conditions that favor cyanobacterial blooms.

How big can cyanobacterial blooms get, and where are they most prevalent?

Cyanobacterial blooms change rapidly over the course of days and weeks. It is therefore difficult to characterize the size of the bloom in terms of acreage covered or volume of water affected. Not all areas of a lake or other water body may be equally affected. Blooms are often most concentrated around the water’s edge, and in coves, backwater areas, and arms of lakes and reservoirs.

What does a cyanobacterial bloom look like?

A bloom’s appearance can change over the course of its life, and different types of cyanobacteria can begin to grow once an initial bloom begins to die. Therefore, it is very important to continue visually monitoring a bloom. There are two major groups of cyanobacterial blooms. One can look foamy, scummy or thick like paint and is often blue-green, brownish red, pea green or white in color. The other looks like a dark green or black slimy mat that can have a smelly, offensive odor. Sometimes cells can be suspended in the water column, making it look bright green without a layer of visible scum. As blooms begin to die, they can disperse in the water. OHA’s website has photographs of different types of blooms based on the type of cyanobacteria involved.

What precautions should people take when using power boats on water bodies where there is a recreational use advisory for a cyanobacterial bloom?

Although inhalation from boat spray is an exposure route, it is a low-risk exposure compared to ingestion (drinking). That said, high-speed water activities such as water skiing, wake-boarding or power-boating should be avoided in areas where recreational advisories are in place, as these activities can increase incidental ingestion through falling and can aerosolize the water, making it easier to be inhaled. Although toxins are not absorbed through the skin, people who have skin sensitivities may experience a puffy red rash in the exposed area.

Are there treatments available if someone gets sick from a cyanotoxin exposure?

There are no antidotes for cyanotoxins. Health care providers may be able to treat symptoms such as vomiting or diarrhea. If you experience prolonged symptoms, you should drink plenty of fluids. This will help you avoid dehydration or electrolyte problems. If symptoms persist beyond 72 hours, you should seek medical attention and let your health care provider know that you may have been exposed to cyanotoxins from a bloom. Although it may be difficult to know if symptoms are related to cyanotoxin exposure, persistent symptoms should not be ignored. People with underlying liver or kidney disease may want to visit a health care provider for these or other symptoms and should avoid any further exposure.

Dogs will exhibit symptoms after the first hour of exposure. Because dogs are susceptible to these toxins at extremely low levels it is very important to get your pet to the veterinarian as soon as possible if they show signs of diarrhea, vomiting, breathing problems, difficulty walking or standing or loss of appetite.

When should people see a health care provider?

If you have severe diarrhea, vomiting, skin irritation or other related symptoms, or you experience these symptoms for more than 72 hours, you should seek medical attention to prevent severe dehydration or other problems. People with ongoing liver or kidney conditions should seek medical evaluation if they think their condition is worsening. Although it may be difficult to know if symptoms are related to cyanotoxin exposure, persistent symptoms should not be ignored.

What about exposure at recreational facilities, like campgrounds and resident camps?

If drinking water comes from a public water system, campgrounds and residential camps should follow the drinking water advisory issued by the local community or public water system. Check the system’s website for information. If facilities or camps pull drinking water directly from an affected lake and use in-home-style treatment, an alternative source of water should be used unless the treatment system has been tested to ensure that it can remove the specific cyanotoxin present. Boiling and camp-style filters are not effective at removing cyanotoxins. In fact, boiling will split cells open, releasing any toxins present. Tap water may be safely used for cleaning surfaces in facilities. Also, follow any public health advisory for recreational waters if issued by the Oregon Health Authority. Information on Oregon Health Authority recreational water advisories can be found online on our Recreational Advisories page.

Can I get the recreational water quality results from OHA’s website?

The public can access recreational data that triggers a recreational use advisory on the cynaobacterial (harmful algae bloom) website here: OHA does not post data below the recreational health advisory levels on the website.

Why are the levels that trigger an advisory for recreational and drinking water different from each other?

Recreational use advisories are based on concentrations of cyanotoxins measured in a water body—a reservoir, lake, or river. Drinking water advisories are based on concentrations of cyanotoxins measured in the finished water after treatment. Cyanotoxin concentrations are typically much higher on the water body than in finished water. Further, the amount of water people incidentally swallow while swimming, water skiing, etc. is much lower than when people use it as a primary drinking water source.

Can I eat fish or shellfish I catch in a water body under a recreational use advisory due to cyanotoxins?

Fish caught in affected waters pose unknown health risks. If you choose to eat them, remove all fat, skin and organs before cooking, because toxins are more likely to collect in these tissues. Crayfish (crawdad) muscle can be eaten but discard all organs and liquids before preparing. It is illegal to harvest clams or mussels from freshwater lakes and you can be fined by the Oregon State Police if caught.

Who does the sampling, testing and reporting of results for recreational water bodies?

There is no statewide monitoring system. In Oregon, monitoring for cyanobacterial blooms on recreational waters is done by whichever agency or organization manages the water body on a voluntary basis. Monitoring for recreational purposes is not required by law or rule, and water body management agencies and organizations decide whether to test, how often to test, and where to test. They also choose the laboratory that performs sample analysis on the recreational water samples collected. OHA’s recreational sampling and advisory guidance documents provide information on best practices for water body managers.

How often is sampling performed to determine whether a recreational advisory is lifted or stays in place?

This is usually decided by water body managers for individual water bodies. OHA guidance recommends that sampling and analysis for cyanotoxins be conducted every other week after the first incidence of a visible bloom. However, OHA’s role is not regulatory, which means we cannot require water body managers to sample and analyze waters used for recreation. That said, if an advisory is issued based on data submitted to OHA, the water body manager must provide data to lift the advisory once the bloom is gone.

How many water bodies does OHA monitor as part of the Harmful Algae Bloom Surveillance Program?

OHA does not monitor any water bodies for cyanobacteria or cyanotoxins. Monitoring and sampling is at the discretion of the designated management agency. Such agencies include, but are not limited to, U.S. Army Corps of Engineers, U.S. Forest Service, Oregon Parks and Recreation Department, cities and counties. In general, these agencies have little to no dedicated financial and staff resources to carry out sampling and analysis throughout the season. Since OHA has no regulatory authority over monitoring, we can’t require agencies to monitor or sample cyanobacteria or cyanotoxins, even when it is highly recommended.

Recreational Advisory Levels

Health advisory values are different for recreational and drinking water. 

The difference between the two reflects the amount of water children and adults incidentally ingest while swimming, water skiing, etc., and the amount of water ingested when drinking from the tap. 

Since much more water is ingested through drinking water, the health advisory values for drinking water are much lower than for recreational use advisories.

Recreational Use Values (RUVs) for Cyanotoxins in Recreational Water (in μg/L)

  • Microcystin: 8
  • Cylindrospermopsin: 15
  • Anatoxin-a: 15
  • Saxitoxin: 8

Recreational Water

The recreational use advisory values are based on incidental ingestion of water while people are recreating in water bodies affected by a cyanobacterial bloom and the cyanotoxins that may be produced. 

Since people are not intentionally drinking water while swimming, water skiing, wake-boarding or during other recreational activities, the amount of water incidentally ingested over time is much smaller than the amount ingested by children and adults when drinking tap water. 

For this reason, OHA uses the recreational values when determining exposures for children and adults while showering, washing hands, cleaning countertops, washing/rinsing teething rings, and all other activities that could result in incidental ingestion.

English HAB Brochure Spanish HAB Brochure

Download the brochure:

English  |  Spanish

Your browser is out-of-date! It has known security flaws and may not display all features of this and other websites. Learn how