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Algal Toxins/Cyanotoxins FAQ

In the summer of 2014 the City of Toledo, Ohio issued a “Do Not Drink” advisory for its 400,000 residents served by the Toledo Water System. The City was directed to do so by the Ohio Environmental Protection Agency (EPA) due to the presence of algal toxins in its drinking water which was the result of a toxic algal bloom on Lake Erie. This incident brought national attention to the potential risks posed by toxic algal blooms to drinking water.

In June 2015, the U.S. EPA issued 10-day drinking water health advisories, discussed in greater detail below, for two of the most common algal toxins, microcystin and cylindrospermopsin. The District responded by implementing a comprehensive algal toxin response plan that would prevent our drinking water from exceeding the health advisory levels. The District, along with two other water agencies, also funded a treatment study which concluded the treatment technologies currently used at our treatment plants are effective at removing algal toxins.

1. What is an algal bloom?
An algal bloom is an overgrowth of algae in lakes, ponds or ocean under certain conditions (e.g. warm water temperature and excessive nutrients in the water). Algal blooms are common from May to October, although blooms can occur as early as March and as late as December in warm climates such as the Bay Area.

2. When is an algal bloom harmful?
Algal blooms can be harmful when the algae are predominately blue-green algae, also known as cyanobacteria, because some species of cyanobacteria are toxin producers1.

Toxins produced by cyanobacteria are called cyanotoxins or algal toxins. In the U.S., four algal toxins (or cyanotoxins) have been found in freshwater: microcystin, anatoxin-a, cylindrospermopsin and saxitoxin2.

When people or animals are exposed to these toxins at the levels and for durations greater than the health advisory recommendations, they can become ill.

3. Are there regulations in place for cyanotoxins in drinking water?
Currently, there are no drinking water regulations for cyanotoxins. However, the U.S. EPA, in June of 2015, released 10-day health advisory levels for the two most common cyanotoxins: microcystin and cylindrospermopsin3. Health advisories are non-regulatory levels of potential concern at or below which adverse health effects are not expected. The health advisory levels for microcystins and cylindrospermopsin are in the table below; concentrations are in parts per billion (ppb). 

Drinking Water Health Advisory (based on 10 days of exposure) 

 Cyanotoxin  Bottle-fed infants and pre-school children  School-age children and adults
 Microcystins (ppb)



 Cylindrospermopsin (ppb)



Note: One part per billion is equivalent to one drop of water in an Olympic-size swimming pool.

The health advisory levels for bottle-fed infants and pre-school children are lower than that for school-age children and adults. Young children are more susceptible than older children and adults as they consume more water relative to their body weight.

According to the U.S. EPA, consuming drinking water with cyanotoxins at or below the advisory levels for 10 days or less is not expected to cause adverse health effects. Regular sampling of the District’s drinking water indicates no detectable levels of algal toxins since the sampling started in June 2015.

4. How can people or animals be exposed to cyanotoxins and what are the health risks?
People and animals can be exposed to cyanotoxins through recreational activities (e.g. swimming, wading) in waters when cyanotoxins are present, or by ingesting drinking water with cyanotoxins in it. Exposure may also occur by ingestion of food contaminated with the toxins (e.g. fish and shellfish).

Some symptoms related to exposure to cyanotoxins include fever, headaches, diarrhea, vomiting, allergic reactions, and muscle and joint pain. In severe cases, seizures, liver failure, respiratory arrest, and (rarely) death may occur4.

5. What will the District do if it finds cyanotoxins in its drinking water?
In response to the U.S. EPA health advisories, the District developed a cyanotoxin response plan in June 2015 based on the Ohio EPA model. The plan establishes a monitoring schedule and alert levels, identifies treatment strategies, and provides communication guidance in the event of a harmful algal bloom that results in cyanotoxins in excess of the U.S. EPA’s health advisories in our drinking water.

If cyanotoxins in our drinking water exceed the health advisory level, notifications will be made to the State Water Resources Control Board’s Division of Drinking Water, which is the state regulatory agency for drinking water, and our affected retailers immediately. Affected retailers will then distribute public notices to their customers. Examples of public notices have already been developed.

6. Have cyanotoxins ever been detected in the District’s water?
The District has been monitoring cyanotoxins since June 2015. On rare occasions, low levels of microcystin have been detected in our imported source water, however, the District’s water treatment processes have been effective at removing the microcystin. Consequently, no microcystin has been detected in the treated drinking water.

7. Are the District’s water treatment plants capable of removing cyanotoxins?
Yes. Based on a 2015 study commissioned by the District and two other water agencies, and various literature reviews, the District’s treatment processes are effective in removing cyanotoxins. The District’s treatment plants employ multiple treatment steps including coagulation, sedimentation, filtration, chlorination, ozonation, and advanced oxidation which are proven effective in removing cyanobacteria and cyanotoxins.

Additionally, the District’s water quality laboratory has developed analytical methods that can measure cyanotoxins accurately and quickly. The ability to test for cyanotoxins in-house has reduced the sample turnaround time to less than 1 day, compared to 4-7 days if analyzed at a commercial laboratory. It provides the District with valuable time to adjust sources or treatment when needed.

8. Are there cyanotoxin regulations in place for recreational waters?
On December 9, 2016, EPA recommended maximum concentrations of cyanotoxins in water for protection of human health while swimming or participating in other recreational activities in and on the water. The draft ambient water quality criteria for microcystin and cylindrospermopsin in recreational waters5 are guidelines, not a regulation.

 Cyanotoxin  Ambient Water Quality Criteria
Microcystins (ppb) 


 Cylindrospermopsin (ppb)


Recreational activities at District reservoirs are managed by the Santa Clara County Parks and Recreation Department; no body contact activities such as swimming are allowed.

9. Where can I find more information related to cyanotoxins?
Cyanobacteria and cyanotoxins:
Harmful Algal Blooms:;


[1] U.S. EPA (United States Environmental Protection Agency). Harmful algal blooms. (accessed May 2017)

[2] U.S. EPA (United States Environmental Protection Agency). Cyanobacteria/Cyanotoxins. (accessed May 2017)

[3] U.S. EPA (United States Environmental Protection Agency). 2015 Drinking Water Health Advisories for

Two Cyanobacterial Toxins. 2015. (accessed May 2017)

[4] U.S. EPA. (accessed May 2017).

[5] U.S. EPA (United States Environmental Protection Agency). Draft Human Health Recreational Ambient Water Quality Criteria/Swimming Advisories for Microcystins and Cylindrospermopsin. 2016. (accessed May 2017)