Although blue-green algae is present in small quantities in most bodies of water, they become harmful for health when they multiply.

Blue-green algae

Blue-green algae are small micro-organisms that have been present on earth for the past 3.5 billion years and that are naturally present in many bodies of water. Although we call them "cyanobacteria", these micro-organisms have common characteristics with algae. Among them, is the blue and green pigment, which is responsible for their typical blue-green colour.

When they multiply excessively, blue-green algae form “algal blooms”. An algal bloom appears when micro-organisms develop in very abundant quantities and become visible to the naked eye. An algal bloom can affect an entire body of water or a portion of it. Typical characteristics include:

• green or turquoise-coloured water
• water texture resembling that of paint or broccoli soup, and
• scum (foam) on the water surface

Multiplication of algae

Blue-green algae multiply especially in summer, during the months of July, August, and September. Certain factors promote growth:

• climate changes that increase water temperature
• increased UV rays caused by the deterioration of the ozone layer (contrary to other algae, blue-green algae use UV rays to proliferate)
• the use of certain herbicides in agriculture
• slow current or stagnant water, and
• phosphorous water

Phosphorus is a natural nutrient. Human activity increases their concentration in runoff waters, which end up in our bodies of water. This is why it is important to monitor or to minimize:

• the use of compost, manure or fertilizer, especially on riparian lands
• the use of non-compliant or old septic installations
• untreated sewage disposal in watercourses
• excessive deforestation near watercourses, and
• the destruction of buffer strips and of wetlands near watercourses

Therefore, all citizens have a role to play in the fight against blue-green algae by adopting healthy practices for the environment.

The consequences on health

Toxins produced by blue-green algae can lead to various consequences on health, for instance:

• gastroenteritis (stomach aches, nausea, and vomiting)
• headaches
• skin irritations due to physical contact with contaminated water, and
• throat irritation due to ingestion of contaminated water

The presence of scum on the water surface is generally indicative that these toxins are present in sufficient quantity to cause health problems.

Your pharmacist can recommend solutions to relieve your discomfort if you are bothered by these symptoms. Be sure to mention that you have been in contact with blue-green algae contaminated water to all healthcare professionals you deal with.

Prevention methods

The best way to prevent the harmful consequences of blue-green algae is to avoid coming into contact with it.

• Do not use contaminated water to brush your teeth or to cook.
• Do not try to make it safe for drinking by boiling it. Boiling water does not destroy the toxins produced by blue-green algae.
• Avoid water sports or swimming in blue-green algae contaminated water.
• If you absolutely must go into the water, keep a three-meter distance between you and the algal bloom or scum.
• Immediately wash all the areas of the body that have come into contact with the contaminated water.
• Supervise children and pets so they do not play with algal blooms or scum.
• Do not use algicides to destroy blue-green algae. These products cause the algae to burst, which releases significant quantities of harmful toxins into the water.

It is important to report the presence of blue-green algae to the Ministère de l’Environnement et de la Lutte contre les changements climatiques. The government monitors contaminated bodies of water and can recommend actions to be taken by the municipalities involved. Water is a valuable resource that must be protected. As a society, the fight against blue-green algae concerns us all!

For additional information about other swimming-related health issues, read the following text: Infections related to swimming.

Remember that your pharmacist is available to answer any health-related questions you may have.