Public Health & Safety: Harmful Algal Blooms


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Harmful Algal Blooms (HABs) Explained

A HAB can harm people, animals (including your pets), or the local ecosystem mainly via the production of toxins known here as cyanotoxins. Use caution during a HAB event (or a potential HAB event) as your health can be impacted when you are near the water, consume the water, or swim in the water.  

Where are toxic blue-green algae found?

Cyanobacteria are naturally-occurring photosynthetic bacteria found in many habitats (including recreational waters). Potentially toxic blue-green algae occur throughout Montana in any standing bodies of water exposed to the sun, e.g., lakes, reservoirs, stock ponds, eddies in slow moving water, and roadside ditches.

What does a bloom look like?

A bloom of potentially toxic blue-green algae appears as 'pea soup,' 'grass clippings,' or 'green latex paint.'  The algae usually are suspended in the water column or aggregated into floating mats; they do not grow from the bottom as do mosses or 'water weeds.'

The following photos show visual signs of cyanobacterial blooms, including surface water discolorations, scum or mat-like accumulations along the surface, and a fish kill.

How can you prevent Harmful Algal Bloom-associated illness?

It is difficult to determine if an algae bloom is harmful just by looking at it. If you do see or suspect a bloom, it is best to keep clear of the water and avoid any ingestion of the water. If you and/or your pet do go into the water, wash yourself off with clean water immediately after.

What are the symptoms of Harmful Algal Bloom-associated illnesses?

The potential health effects depend on many different factors (e.g., types of algae present, types of toxins present, etc.) and symptoms can start within a few hours of exposure and can last for 2-3 days. Skin exposure can result in skin irritation; and ingestion can result in abdominal pain, vomiting, and neurological effects. There is also potential for the inhalation of HAB containing water via the aerosolization (turned into tiny particle sized droplets) of the water. This may cause respiratory irritation or illness. If you think you have a HAB-associated illness, please contact your local health agency or call 1-888-849-2938. If you have any questions about symptoms you are experiencing, contact your healthcare provider or call the Montana Poison Control Center at 1-800-222-1222.

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