What are the drinking water impacts of harmful algal blooms?

Cyanobacterial and algal blooms are naturally occurring throughout the world.  At times they can be a nuisance and some of them can be harmful and produce toxins. The cyanotoxins can exist in different areas of a water body. Algal blooms can appear at the surface in the form of a mat or scum, be suspended at depth, or attached to rocks.
We are still learning how to manage the toxins and understand their health effects in water bodies as they relate to recreation, fishing, drinking water sources, and domestic animals. However, the United States Environmental Protection Agency (US EPA) has developed drinking water health advisories for two cyanotoxins, microcystin and cylindrospermopsin. The health advisories are discussed below.

It is important to keep in mind that health effects are dependent on how we interact with the affected water body. For example, a “Caution” sign posted at a recreational water body does not necessarily mean there are impacts to the local tap water. Or, a camper may be vulnerable using a portable water filter for drinking water on an impacted water body, while swimming could be safe. Concerns regarding specific water-related activities are covered through the pages of this portal. Water utilities in California are encouraged to monitor for cyanobacteria and their toxins. However, currently there are no California or federal regulations that require this testing.

Are there any guidelines in place for drinking water?

In 2015, the US Environmental Protection Agency published health advisories for two algal toxins, microcystin and cylindrospermopsin.

The health advisories describe concentrations of cyanotoxins in drinking water at or below which adverse health effects are not expected to occur over specific exposure durations. Health advisories are not legally enforceable standards and are subject to change as new information becomes available. Many states, including California, accept these values for public health guidance. See the table below for health advisory concentrations in drinking water. Concentrations are listed in micrograms per liter or µg/L (parts per billion).

10-Day Drinking Water Health Advisories
Cyanobacteria Toxin Children under Six Years Old  Adults
Microcystins 0.3 µg/L 1.6 µg/L
Cylindrospermopsin 0.7 µg/L 3.0 µg/L

If it is not safe to swim in the lake, is it safe to drink?

If a lake or stream is posted with “caution”, “warning” or “danger” signs, read the sign carefully for advice about using the water. You might be advised not to enter or drink the water or allow your pet to contact or drink it. Recreational lakes can also serve as sources of drinking water. However, tap water taken from a lake may be safe for drinking, even if it is not safe for swimming or other recreational uses. Lake water that is intended for drinking is often withdrawn at locations and depths that are not affected by cyanobacterial blooms and their toxins. Additionally, tap water is treated before being distributed to the public.  The treatment processes can remove and reduce cyanobacterial cells and toxins. If you have questions about the source(s) of your drinking water and the treatment processes, contact your local water supplier. For homes that are not served by a public water system, contact your city or county health office.

Is my tap water tested for cyanobacterial toxins?

Water utilities test their water regularly for chemical and microbiological contaminants to make certain that it meets state and federal drinking water standards. There are currently no regulatory standards for cyanobacterial toxins so testing is not required. However, your water system might be testing them voluntarily. Contact your local water supplier for more information on the specific testing they conduct on your water.

How can cyanotoxins impact my health?

There are gaps in our understanding of the health effects associated with cyanobacteria toxins.  Below is a table that summarizes potential health effects from exposure to drinking water that contains cyanotoxins. If you have any health concerns, contact your health care provider for more information. People with underlying medical conditions may be more susceptible to developing health effects. Tap water that is safe for people to drink is safe for pets to drink.

Cyanobacteria Toxin Possible Health Effects from Drinking Water
Microcystins
  • Nausea, Vomiting, Diarrhea
  • Liver damage
Cylindrospermopsin
  • Nausea, Vomiting, Bloody diarrhea, Abdominal pain
  • Kidney damage
  • Dehydration
  • Headache

Cyanobacterial neurotoxins, anatoxin-a and saxitoxin, are also found in California surface waters. There have been no documented reports of anatoxin-a or saxitoxin poisonings from treated drinking water, worldwide. Currently, US EPA does not have health advisories for anatoxin-a or saxitoxin in drinking water.

Acute symptoms of neurotoxin poisoning in non-drinking water cases:

  • Anatoxin-a
    In animals, high levels of anatoxin-a leads to loss of coordination, paralysis and death. There have been no documented cases of anatoxin-a poisonings in people.
  • Saxitoxin
    People poisoned by ingesting shellfish contaminated with saxitoxin experience mild symptoms such as tingling in the mouth, dizziness and nausea and moderately severe symptoms of incoherent speech, non-coordination of limbs, and difficulty breathing. Extremely severe shellfish poisoning cases have resulted in death. Saxitoxin concentrations are typically much higher in shellfish than in water.

Does my home tap water treatment device (e.g. filter) remove cyanotoxins?

We rely on public water systems to meet state and federal standards to ensure that tap water is safe to drink. We typically do not rely on home treatment devices for water treatment for a number of reasons, including maintenance, treatment applicability, certifications, and other factors.