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Recreational Health Advisory Guidance for Harmful Algal Blooms

Local health departments (LHDs) in Utah have the authority to post health advisories and close water bodies. LHDs use recreational health thresholds established by the Utah Department of Health and Human Services (DHHS) to determine if and when a bloom presents a human health risk. The Division of Water Quality (DWQ) supports LHDs through a monitoring and sampling program that prioritizes waters that are at risk for cyanobacteria blooms, experience high recreational use, or serve as drinking water sources. DWQ interprets and reports sampling results, and along with DHHS, makes recommendations to LHDs on the issuance of recreational health advisories based on sampling data and the best available science.

Utah’s recreational health advisory guidance for HABs was last updated in 2023. It is based largely on guidance from the Environmental Protection Agency (EPA). In 2019 the EPA provided guidance on adverse human health effects from recreational exposure to two cyanotoxins: microcystin and cylindrospermopsin. Utah’s guidance also integrates the latest scientific information on the health impacts of toxigenic cyanobacteria cell count densities and a third cyanotoxin, anatoxin-a.

What can trigger a Harmful Algal Bloom Advisory?

Utah’s HAB Guidance uses four independent indicators to protect recreational users. If any of these indicators is exceeded, a recreational health advisory is recommended:

  • Toxigenic cyanobacteria cell density
  • Microcystins
  • Cylindrospermopsin
  • Anatoxin-a
Warning Advisory
Avoid primary contact recreation (e.g., swimming, waterskiing, wading)
Danger Advisory
Stay away from the waterbody
Over 100,000 cells/mL of HABs
Over 8 μg/L of microcystinsOver 2000 μg/L of microcystins
Over 15 μg/L of cylindrospermopsin or anatoxin-aOver 90 μg/L of anatoxin-a
Over 8 μg/L of saxitoxinOver 75 μg/L of saxitoxin
Under a Warning Advisory there is potential for short-term effects (e.g., rash, nausea, vomiting, diarrhea) or long-term illnessUnder a Danger Advisory there is potential for short-term effects, long-term illness, or death

Latest Guidance Documents

When should an advisory be issued and/or a water body closed?

Flowchart Description: Utah DEQ Harmful Algal Bloom Response
This flowchart outlines the step-by-step process used by the Utah Department of Environmental Quality (DEQ) and its partners to respond to a suspected harmful algal bloom (HAB).

1. Initial Report
The process begins when a potential HAB or a related illness in a person or animal is reported. The first step is to contact the Utah 24-Hour Spill Line at (801) 536-4123.

2. Site Investigation
After a report is made, the DEQ, a local health department, or another trained partner visits the site. They work to confirm the bloom's presence and investigate any reported illnesses.

3. Is a HAB Present?
A decision is made based on the site investigation:

NO: If no HAB is present and there are no related illnesses, the process ends with "No Action," and the event is documented.

YES: If a HAB is visually confirmed, the response proceeds to the next stage.

4. Initial Response and Sampling
For a confirmed HAB, the following actions occur:

The local health department and the waterbody manager are notified.

Warning signs may be posted to alert the public.

Water samples are collected for detailed analysis.

5. Is There an Associated Human or Animal Illness?
The presence of a related illness is a critical factor:

YES: If a human or animal illness is linked to the bloom, a Danger Advisory is immediately issued. This includes posting danger signs and sending a press release. The advisory remains in effect until two consecutive water samples, taken at least one week apart, show that toxin levels are below the health-based threshold.

NO: If no illnesses are reported, the collected water samples are sent for laboratory analysis to determine the specific risk level.

6. Laboratory Analysis and Health Advisory Thresholds
The lab results are compared against specific health-based thresholds to determine the appropriate advisory.

A Warning Advisory is issued if sample results exceed the following values:

Blue-green algae cell counts: Greater than 20,000 cells per milliliter (cells/mL)

Microcystin toxin: Greater than 4 micrograms per liter (µg/L)

Cylindrospermopsin toxin: Greater than 4 micrograms per liter (µg/L)

The presence of Anatoxin-a is also assessed. If Anatoxin-a is detected at any level, a Warning Advisory is considered.

No Advisory is issued if lab results are below all of these thresholds or if Anatoxin-a is non-detect.

If a Warning Advisory is issued, it includes posting warning signs and a press release. The advisory stays in place until two consecutive samples, taken at least one week apart, are below the health-based thresholds. The local health department can then lift the advisory.

DHHS and DEQ recommend that LHDs use the table and accompanying decision algorithm when determining the appropriate level of health risk and public health action for a given waterbody. If a LHD receives reports of human or animal illness or death that is plausibly linked to cyanobacteria, an immediate public health advisory is recommended.

  • Once an advisory is issued, the Guidance recommends at least two weeks of measurements indicating the hazard has passed before removing the advisory.

Other Harmful Algal Bloom Guidance

The recommended standard operating procedure (SOP) for collecting samples for HABs and HAB cyanotoxins are described in DWQ’s Standard Operating Procedure for the collection of Phytoplankton Samples during Harmful Algal Blooms. View the video overview, How to Sample for Harmful Algal Blooms.

The recommended SOP for collecting samples for carcasses of fish and wildlife associated with a HAB event is described in the U.S. Fish and Wildlife Service (FWS) and DWQ Guidance for Fish and Wildlife Mortality Sampling.

The recommended SOP for preparation and deployment of SPATT (Solid Phase Adsorption Toxin Testing) bags is described in the Standard Operating Procedure for Solid Phase Adsorption Toxin Testing (SPATT) Assemblage and Extraction of HAB Toxins publication.

Guidance for harmful algae mats (benthic cyanobacteria)

Beginning in 2025, the State of Utah has also released specific advisory guidance for harmful algae mats, i.e., benthic cyanobacteria. Existing harmful algal bloom (HAB) guidance cannot be applied to evaluating risk at waterbodies where harmful algae mats are present. Rather, mat coverage percent is used as the primary metric in evaluating human health risk. This new mat-specific guidance is recommended for use in both standing and flowing waters.


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