Cyanobacteria FAQs

What causes organisms to form "blooms"?

When blue green algae multiply rapidly over a short time it is called a “bloom”.  It is known that light, temperature, and the water’s nutrient content play roles in bloom formation.  Under favorable conditions a large bloom can form overnight, and rise to the surface as a huge unsightly mat of pond scum.  A blue green algae bloom can also lie below the surface of the water.  Slow-moving water or water rich in run-off from farms or sewage treatment plants seem to be common places for the development of blue green algae blooms.  Blooms can also spontaneously disappear or move to different parts of a pond or lake.

What causes some blooms to be toxic?

There are thousands of blue green algae species and many more are still being discovered.  Most actually do not produce toxins that are harmful to humans or animals.  However, some types of blue green algae can produce substances that are known to be toxic.  These blue green algae include Microcystis, Cylindrospermopsis, Anabaena, Nodularia, Oscillatoria and Aphanizomenon.  These blue green algae produce toxins within their cells that are then released when the die or are ruptured.  It is not possible to tell just by looking at a bloom whether it is toxic.  Over time, these toxins are diluted and eventually break down and disappear.  However, some of these toxins may remain in fish and shellfish that have consumed blue green algae.  It is not yet clear why these blue green algae make toxins.

How do people and animals get exposed to these toxins?

People and animals (including pets) can be exposed when a person or animal swims through a bloom or drinks untreated water containing high levels of blue green algae and/or their toxins.  Water skiers, users of jet-skis, and other using water for recreational purposes may be exposed through inhalation of water droplets with high levels of bloom related toxins.  Farm workers can also be exposed if untreated water is used for irrigation.

There was a report in 1996 of human deaths in Brazil due to high levels of cyanobacterial toxins in water used for kidney dialysis.  The dialysis patients were especially vulnerable to toxins because they were already sick.  the water responsible for these deaths, which had not been treated, came from a reservoir with a major blue green algae bloom.  It is important to not that this was a very unusual event in that already ill people were exposed to untreated water directly into their bloodstreams.  This is not a typical route of exposure to blue green algae toxins.

Chronic exposure can occur when people consume the blue green algae toxins at low levels over many years in drinking water, or possibly food such as shellfish.  The effects of chronic exposure to low levels of cyanotoxins are not well understood, and need further research.

What are the health effects of the blue green algae toxins?

There have been case reports through the years of people and animals becoming acutely ill after drinking untreated water found to be contaminated with blue green algae toxins.  Their health complaints have ranged from skin irritation or rash, to flu-like stomach ailments, particularly diarrhea.  There have been many reports of animals dying after drinking untreated scummy pond or lake water with blue green algae overgrowth.  Liver abnormalities have been found in laboratory animals exposed to blue green algae toxins, and also in humans who became ill after drinking water later found to be contaminated with blue green algae toxins.  Persons having recreation and/or occupational exposure to blue green algae blooms have reported hay fever-like symptoms including itchy, watery eyes, and/or asthma.  These symptoms usually clear up in several days to weeks, with treatment only for symptoms.

The question of the long term, chronic effects of exposure to small quantities of blue green algae toxins is still under study.  In the mid-1980's, studies were done in china, where people were drinking untreated water contaminated with blue green algae toxins.  It was found that drinking contaminated pond and ditch water was associated with high rates of liver cancer.  When the quality of drinking water sources was improved in these areas, the rate of liver cancer decreased.  Subsequent studies in mice have shown that some blue green algae toxins cause pre-cancerous damage to both the liver and the bowel.  These toxins may be a special risk to people with liver disease, such as hepatitis.  How many cases of cancer can be attributed to blue green algae toxins in the U.S. (where drinking water is of high quality) remains unknown.

How can I prevent exposure to these toxins?

Most people will avoid a blue green algae bloom automatically because the blooms that rise to the surface tend to be icky-looking and smelly.  Sometimes however, pets and children will not have the good sense to stay away.  It is critical that pets and children are kept from getting into blue green algae blooms and the surrounding water.  There are numerous cases dating back to the 1800's of pets and livestock dying after drinking blue green algae contaminated water.  Children also are more vulnerable generally to environmental toxins than adults.

Swimming in water that is visibly rich in blue green algae is a bad idea.  Swimmers have reported rashes, hay fever-like symptoms and even pneumonia associated with blue green algae blooms and their toxins.  Abdominal cramps, nausea, diarrhea, and vomiting may occur if the swimmer swallows the untreated water.  Even water-skiing, showering or cleaning with this water may make people sick because the toxins may be absorbed from water breathed through the nose.  Unfortunately, boiling water does not remove or destroy these toxins.

What can be done to control blue green algae growth?

Blue green algae grown can be controlled by minimizing sources of nutrients (such as phosphates and nitrogen) into water bodies.  These nutrients can be from fertilizer fun-off, agricultural run-off (such as manure) or sewage. 

  • Plant shrubs along canal borders
  • Only fertilize to within ten feet of a water body
  • Do not over fertilize

What can be done to prevent the toxins from causing illness?

While blue green algae growth can be controlled with the preventive use of copper sulfate, do not use this or any common algaecide when a bloom has already formed because they can cause cells to dump toxins into the water.  There have been reports of human illness in Australia linked to the post-bloom use of copper sulfate in drinking water sources.

Research is ongoing about how to remove the small traces of blue green algae toxins from drinking water taken from surface water sources (i.e. rivers, lakes).  Normally, standard drinking water treatment processes are enough to break down whatever toxins enter the treatment systems.  However, when massive blooms occur in source waters, small amounts of toxins can remain even in the treated drinking water.

This is a newly discovered problem that is being taken seriously by drinking water quality officials.  When drinking water treatment facilities realize that there is a blue green algae bloom in progress, they can take steps to remove the bad taste, odor and most toxins by adding activated charcoal and other steps to ensure the safety of the drinking water.

Of note, the World Health Organization (WHO) has established guidelines for the safe level of toxins in drinking water as well as possible treatment processes to remove these toxins.  Unfortunately, these are expensive measure that cannot be maintained over long periods.  Researchers in Australia, US and US are all looking for more pracital means of safeguarding the water supply from blue green algae toxins.

Watching our water for blue green algae blooms is going to become important in preventing toxins from causing illness.  Because there is limited monitoring for blue green algae blooms, it is important to report unusual bloom activity to your local water quality officials.


Last updated: July 31, 2012