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  2. Harmful Algal Blooms (HABs) in Water Bodies

World Health Organization (WHO) 1999 Guideline Values for Cyanobacteria in Freshwater

The World Health Organization (WHO) developed guidance values for the relative probability of acute health effects to account for the possible exposures through recreational activities (contact, ingestion and inhalation) during recreational exposure to cyanobacteria based on cyanobacterial cells and chlorophyll-a levels.

WHO’s Guidelines for Safe Practice in Managing Recreational Waters. Summarized by the EPA from the World Health Organization. 2003. Guidelines for safe recreational water environments. Volume 1, Coastal and fresh waters. 
Guidance level or situationHow guidance level derivedHealth risksTypical actions

Relatively low probability of adverse health effects

20 000 cyanobacterial cells/ml
or 
10 ug chlorophyll-a/litre with dominance of cyanobacteria

From human bathing epidemiological studyShort-term adverse health outcomes, e.g., skin irritations, gastrointestinal illness

Post on-site risk advisory signs

Inform relevant authorities

Moderate probability of adverse health effects

100 000 cyanobacterial cells/ml
or 
50 ug chlorophyll-a/litre with dominance, of cyanobacteria

From provisional drinking water guideline value for microcystin-LR and data concerning other cyanotoxinsPotential for long-term illness with some cyanobacterial species health outcomes, e.g., skin irritations, gastrointestinal illness

Watch for scums or conditions conducive to scums  and further investigate hazard

Post on-site risk advisory signs

Inform relevant authorities

High probability of adverse health effects

Cyanobacterial scum formation in areas where whole-body contact and/or risk of ingestion/aspiration occur.

Inference from oral animal lethal poisoning.

Actual human illness case histories

Potential for acute poisoning

Potential for long-term illness with cyanobacterial species Short-term adverse activities health outcomes, e.g., skin irritations, gastrointestinal illness

Immediate action to control contact with scums; possible prohibition of swimming and other water contact activities

Public health follow-up investigation Inform public and relevant authorities

More information on WHO guidelines

  • WHO Toxic cyanobacteria in water: A guide to their public health consequences, monitoring and management
  • WHO Guidelines for Safe Recreational Waters Volume 1 - Coastal and Fresh Waters

Harmful Algal Blooms (HABs) in Water Bodies

  • Basic Information on HABs
    • Learn about HABs
    • What Causes HABs
    • Exposure to HABs
    • Effects of HABs
    • What You Can Do
  • Trends, Monitoring Results & Forecasts
    • HAB Monitoring and Trends
    • HAB Forecasts
    • HAB Advisories
  • Managing HABs
    • Managing HABs in Surface Water
    • Managing Algal Toxins in Drinking Water
    • Managing Algal Toxins in Recreational Waters
  • HAB Laws, Tools and Data
    • HABHRCA
    • HAB Tools
    • HAB Data
  • HAB Methods & Research
    • HAB Methods
    • HAB Research
  • HAB Resources
    • EPA Outreach on HABs
    • Labs that Analyze for Cyanobacteria and Cyanotoxins
    • State & Tribal HAB Programs and Resources
    • State & Tribal Toxin Thresholds and HAB Advisory Decision Methods
    • HAB Resources from Other Federal Agencies and Organizations
    • Recent Papers, Upcoming Meetings and Webinars
Contact Us about HABs
Contact Us about HABs to ask a question, provide feedback, or report a problem.
Last updated on July 2, 2025
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