Blue-green algae, or “cyanobacteria,” are a natural part of the aquatic environment. They’re often found in small or moderate amounts in Illinois lakes. But they can grow and reproduce quickly in warm, fresh water if nutrients such as nitrogen and phosphorous (both common in fertilizers) make their way into the water and if sunny, still conditions follow. This rapid growth is referred to as a bloom, and blooms can become harmful when the algae produce toxins, such as microcystins.

Harmful blue-green algal blooms are a common natural phenomenon that can form in any body of water. In Illinois, harmful blooms typically occur between June and September and can look like thick layers of blue, green, or brown scum or paint on the surface of the water. They may also smell, especially in warmer weather. 

Harmful blue-green algal blooms can last for extended periods of time depending on a variety of conditions, including light penetration, water temperature and flow, changes in pH, and the presence of nitrogen, phosphorus, and trace metals.

Ingesting, touching, and inhaling water with harmful blue-green algae can make pets, children, and people with weak immune systems sick. Cyanobacteria and their toxins can cause skin irritation, headache, neurological symptoms, vomiting, diarrhea, difficulty breathing, and other medical conditions.