Blue-green algae, or cyanobacteria, are a natural part of aquatic ecosystems and are often found in small or moderate amounts in Illinois lakes. They can grow rapidly when warm temperatures, sunlight, and excess nutrients — such as nitrogen and phosphorus, which are common in fertilizers — are present in the water. This rapid growth is called a "bloom," and some blooms can become harmful if the algae produce toxins such as microcystins. 

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

Because it’s not possible to tell by sight whether a bloom is producing toxins, all visible algae blooms should be treated as potentially harmful.

Harmful blooms can persist for extended periods depending on factors such as light penetration, water temperature and flow, pH levels, and the presence of nutrients and trace metals.

Pets, children, and people with weakened immune systems can become sick after ingesting, touching, or inhaling water affected by harmful blooms. Cyanobacteria and their toxins may cause symptoms such as skin irritation, headaches, vomiting, diarrhea, difficulty breathing, neurological effects, and other medical issues.