Harmful Algae Bloom FAQs

Harmful Algae Bloom FAQs

Blue-green algae blooms can make pets, children and people with weak immune systems sick if they ingest or inhale it.

What is blue-green algae?
Blue-green algae, or “cyanobacteria,” are a natural part of the aquatic environment. They’re often in Illinois lakes in small or moderate amounts but can grow and reproduce quickly in warm, fresh water that’s rich with nutrients such as nitrogen and phosphorous. This rapid growth is referred to as a bloom.

What are harmful algae blooms?
Harmful blue-green algae blooms occur when the algae produce toxins, such as microcystins. In Illinois, harmful blooms typically occur June through September and can look like thick layers of blue, green, or brown scum or paint on the surface. They may also smell, especially in warmer weather.

Ingesting, touching and (rarely) inhaling water with harmful blue-green algae can make people and animals sick. The most common effects are skin irritations resulting from contact with the water. Less common symptoms include nausea, vomiting, throat irritation, allergic reactions and difficulty breathing.

What causes harmful algae blooms?
If large amounts of nutrients like nitrogen and phosphorus (common in fertilizers from lawns and farms) wash into warm waters during heavy storms and then sunny, still conditions follow, harmful blue-green algae blooms can form.

How long do harmful blooms last?
Harmful blue-green algae blooms do not last for specific periods, but factors contributing to their duration include light, water temperature and flow, changes in pH, the presence of nitrogen, phosphorus, and trace metals.

Are harmful blooms common in DuPage?
Harmful blue-green algae blooms are a common natural phenomenon that can form in any body of water.

Do they harm wildlife?
It depends on the concentration of toxins and the duration of the bloom. Canines (pet dogs included) are particularly susceptible. Drinking the water can cause seizures, organ failure and death. Other wildlife can be affected, but it’s less common. Wildlife has evolved with and been exposed to harmful algae blooms for centuries, so many have developed ways to avoid the potentially dangerous effects.

Does the District monitor lakes?
When harmful blue-green algae blooms are suspected, the Forest Preserve District tests for microcystins in the water. If levels exceed 5 parts per billion, it informs the public and the Illinois Environmental Protection Agency. It then regularly tests to determine when the bloom dissipates.

What does the District do to control blooms?
Some measures can help lower the risk of harmful blooms, but once a bloom forms there is little to do but let it run its course. Microcystins are released as algae die, so treating with herbicides can actually increase the level of toxins.

Can I eat fish from lakes with harmful blooms?
Research suggests it should be safe. Microcystins primarily affect the digestive tracts and livers of animals, so a fish’s muscle tissue (the part people eat) should have low levels. However, handling fish during an active bloom increases the risk of skin exposure and should be limited or avoided.

Is it safe to boat with harmful blooms?
Boating is typically safe, but always use your best judgment if active blooms are present. Avoid contact with discolored, scummy water, and wash with soap and water or rinse thoroughly with clean water if you do.

What can I do to avoid harmful blooms?
The safest thing is to treat every bloom as dangerous. Keep pets out of the water, and do not come in contact with it yourself. If you do, wash thoroughly as soon as possible.

If I think I see a harmful bloom in a DuPage forest preserve what should I do?
Contact the Forest Preserve District with the location, or fill out a HAB report form on the Illinois EPA website

For more information visit dph.illinois.gov.


Report a Concern

The Forest Preserve District of DuPage County offers scores of nature-related programs and activities for grownups, kids, families and groups year-round. Click below to register for programs, reserve shelters and campsites or purchase permits, and we'll see you soon!
Reserve Facility
Purchase Permit