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Home > Health Hazards & Infectious Diseases > Food and Water > Blue-green Algae (cyanobacteria) > Health risks - Blue-green Algae (cyanobacteria)

Health risks - Blue-green Algae (cyanobacteria)

Health risks - Blue-green Algae (cyanobacteria)

What are blue-green algae?

Blue-green algae, technically known as cyanobacteria, are microscopic organisms that are naturally present in lakes and streams. They are usually present in low numbers. Blue-green algae can rapidly increase in warm, shallow, undisturbed surface water that receives a lot of sunlight. When this occurs, they can form blooms that discolour the water or produce floating rafts or scum on the surface of the water.

What are the potential health effects from drinking or coming in contact with water containing blue-green algae?

Some blue-green algae produce toxins that can pose a health risk to people and animals when they are exposed to them in large enough quantities. Health effects can occur when surface scum or water containing high levels of blue-green algal toxins are swallowed, through contact with the skin, or when airborne droplets containing toxins are inhaled while swimming, bathing or showering.

Consuming water containing high levels of blue-green algal toxins has been associated with effects on the liver and on the nervous system in laboratory animals, pets, livestock and people. Livestock and pet deaths have occurred when animals consumed very large amounts of accumulated algal scum from along shorelines.

Direct contact, or breathing airborne droplets containing high levels of blue-green algal toxins during swimming or showering, can cause irritation of the skin, eyes, nose and throat and inflammation of the respiratory tract.

Recreational contact, such as swimming, and household contact, such as bathing or showering, with water not visibly affected by a blue-green algae bloom is not expected to cause health effects. However, some individuals can be especially sensitive to even low levels of algal toxins and might experience mild symptoms such as skin, eye or throat irritation or allergic reactions.

Avoiding exposure to blue-green algae

Never drink untreated surface water, whether or not algae blooms are present. Untreated surface water may contain other bacteria, parasites or viruses, as well as algal toxins that could cause illness if consumed.

People not on public water supplies should not drink surface water during an algal bloom, even if it is treated, because in-home treatments such as boiling and disinfecting water with chlorine or UV and water filtration units do not protect people from blue-green algal toxins.

People, pets and livestock should avoid contact with water that is discoloured or has scum on the surface. Colours can include shades of green, blue-green, yellow, brown or red. If contact does occur, wash with soap and water or rinse thoroughly with clean water to remove algae.

Stop using the water and seek medical attention if symptoms such as skin, eye or throat irritation, allergic reactions or breathing difficulties occur while in contact with untreated surface waters. However, swimming, bathing or showering with water not visibly affected by a blue-green algae bloom is not expected to cause health effects.

Do not eat the liver, kidneys and other organs of fish caught in the water body. Be cautious about eating fish caught in water where blue-green algae blooms occur.

Where to get more information:

Blue-green Algae (Cyanobacteria) and their Toxins (Health Canada)

Information about blue-green algae: Background, potential impacts to human health and safety (Ontario Ministry of the Environment)

Blue-green algae: Information for cottagers and home owners (Ontario Ministry of the Environment)

Blue-green algae: Information for drinking water system owners and operators (Ontario Ministry of the Environment)

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