Health Effects and Precautions

What is the Problem?

Cyanotoxins are produced by bacteria called cyanobacteria.   By producing oxygen, they are an essential part of the environment and have existed for millions of years. Under certain conditions, they multiply excessively and form visible clumps that can appear as cut grass in the water or blue-green, white or brown foam, scum or mats that can float on the water’s surface and accumulate along the shoreline and boat ramp area. 

On occasion, they produce toxins that can cause harmful effects in people and animals if exposed through ingestion, inhalation of aerosolized water or direct contact.  As environmental factors change, most harmful algal blooms resolve over time.   However, when cyanotoxins are known to be present, re-testing the water after it has cleared and allowing at least one month (two sampling events) to pass after no toxins are found is recommended.

Recommendations from the Statewide Guidance on Cyanobacteria and Harmful Algal Blooms:

Take care that pets and livestock do not drink the water, swim through algae, scums or mats, or lick their fur after going in the water.  Rinse pets in clean water to remove algae from fur.
Avoid wading, swimming, jet or water skiing in water containing algae blooms, scums or mats.
Do not drink, cook or wash dishes with untreated surface water from these areas under any circumstances; common water purification techniques such as camping filters, tablets and boiling do not remove toxins.
People should not eat mussels or other bivalves collected from these areas.  Limit or avoid eating fish from these areas; if fish are consumed, remove the guts and liver, and rinse filets in clean drinking water.
Get medical treatment immediately if you think that you, your pet, or livestock might have been poisoned by cyanobacteria toxins.  Be sure to alert the medical professional to the possible contact with cyanobacteria.  Also, make sure to contact the local county public health department at (707) 263-1090.

What are the health effects?

When the toxins are ingested in large amounts they can cause:
Sharp, severe stomach pain, accompanied by diarrhea and vomiting.
Liver damage that may take hours or days to show up in people or animals.
Numb limbs, tingling fingers and toes or dizziness.

Possible health effects of animal exposure to a toxic bloom:
•   Weakness, staggering, difficulty breathing
•   Convulsions
•   Vomiting or diarrhea

Signs of a toxic bloom may include: a large number of dead fish, waterfowl or other animals, or sudden, unexplained sickness or death of a cat or dog that has been exposed. It is not possible to tell if cyanobacteria are toxic by looking at them. There is ongoing research on potential health effects of exposure to the toxins produced by some species of cyanobacteria. The California Department of Public Health evaluates any new findings and makes updates to guidance materials as necessary.

For current cyanotoxin lab results, please visit the Big Valley Band of Pomo Indians cyanotoxin monitoring website: https://www.bvrancheria.com/clearlakecyanotoxins
For more information, please visit:

County of Lake Cyanobacteria brochure
County of Lake Cyanobacteria brochure (in Spanish)
California Harmful Algal Blooms Portal
Physician Reference Sheet
Domestic Animals and HABs
County of Lake Cyanobacteria Webpage
BloomWatch!
California Cyanobacteria and Harmful Algal Bloom (CCHAB) Network
California Surface Water Ambient Monitoring Program Freshwater HAB webpage
Centers for Disease Control and Prevention (CDC)
U.S. Environmental Protection Agency: CyanoHAB website