water coming out of a hose


Microcystins (which includes total microcystin, microcystin-LA, microcystin-LF, microcystin-LR, microcystin-LY, microcystin-RR, microcystin-YR) and nodularin are toxins produced by certain species of cyanobacteria. Cyanobacteria, formerly known as blue-green algae, are sometimes found in surface water when conditions favor growth and formation of algal blooms.

Cyanobacteria are capable of releasing toxins that can persist for weeks to months, and microcystins are the most commonly observed cyanotoxins in the United States. Toxicity to the liver is the greatest concern, but microcystins can also irritate the skin, eyes and throat. After exposure, symptoms may take hours or days to appear.

Nodularin is similar to microcystins in chemical structure. Therefore, while there isn’t as much information on nodularin as there is on microcystins, based on what information is available we assume that nodularin and microcystins have similar properties, including health effects.

People can be exposed to microcystins by:

  • ingesting fish or shellfish from water containing microcystins;
  • having direct skin contact with water containing microcystins through bathing, showering, swimming or wading;
  • breathing airborne microcystins while boating, waterskiing or recreating in waters with microcystins present; or
  • consuming drinking water containing microcystins.
Drinking water treatment generally removes intact cyanobacterial cells and low levels of cyanotoxins from source waters, but during a severe algal bloom, some microcystins may escape treatment.


There is limited data on the effect on humans of drinking water containing microcystins; there is data from animal studies that indicate the potential for harm to the liver. Recreational exposure to cyanobacterial blooms has been reported to lead to allergic reactions, including hay fever-like symptoms, skin rashes and gastrointestinal distress. The U.S. Environmental Protection Agency recommendations for controlling both drinking water and recreational water exposure are intended to protect people against damage to the liver.
The best way to know if microcystins are in your source water or treated drinking water is to contact your water utility or state public health agency. Depending on the source of your water, climate and potential for nutrient pollution, your utility may or may not monitor for microcystins and other cyanotoxins. For example, if your water comes from groundwater sources, it’s unlikely to contain microcystins.

Typically, groundwater wells are not expected to contain microcystins. However, if your well is affected by surface water it could contain microcystins.

If your well is influenced by surface water then you should take additional steps to address several potential contaminants. EPA has information available on their private drinking water well page. Your local health department is another reliable source of information regarding steps you should consider.

Currently, microcystins in drinking water are not regulated by EPA. However, several states have guidelines for monitoring and addressing microcystins and other cyanotoxins in drinking water. Contact your local water utility to determine how cyanotoxins are addressed in your state.

In 2015, EPA issued a drinking water health advisory for microcystins. Health advisories are on-regulatory levels to assist public health professionals. No negative health impacts are anticipated at these concentrations. EPA has provided two different ten-day health advisory levels – 0.3 microgram/liter for bottle-fed infants and young children and 1.6 microgram/liter for school-aged children and adults.

Regulators, utility managers and public health officials use health advisories as they consider what management practices are necessary and when additional actions are necessary if elevated microcystin levels are observed.

The Safe Drinking Water Act (SDWA) requires EPA to publish a list of substances that could potentially be of concern and warrant further study. This list is known as the Contaminant Candidate List (CCL), and microcystins are on the list.

Under the SDWA, EPA is also required once every five years to issue a new list of up to 30 unregulated contaminants to be monitored by public water systems, known as the Unregulated Contaminant Monitoring Rule. Microcystins and nodularin are included in the fourth round of UCMR (UCMR4). Testing for UCMR4 starts in January 2018 and will continue through December 2020. The data from this nationwide monitoring will inform and support EPA’s decisions on regulatory actions to protect public health.

If you are concerned about microcystins in your drinking water you may consider purchasing a home treatment device. NSF International certifies some filters as effective in reducing microcystins (NSF Protocol 477). To make a well informed and cost-effective decision, consider:

If you decide to use a home treatment device, be sure to follow manufacturer instructions for proper care and maintenance of the device.

Bottled water in the United States is regulated by the U.S. Food and Drug Administration and is required to meet standards equal to EPA’s tap water standards. There are also individual state standards to consider. In most cases, you must contact the bottled water manufacturer for information about microcystin levels.