Blue-Green Algae and Harmful Algal Blooms (HABs)


Blue-green algae are microscopic organisms that occur naturally in Illinois lakes and streams. When certain conditions are present algae can reproduce very quickly. These conditions include; warm, shallow, undisturbed surface water that receives a lot of sunlight. The rapid growth of algae is called a “bloom.” Algal blooms can discolor the water or produce floating scums on the surface of the water, especially along shorelines. These blooms are primarily a concern during the summer months. Certain types of blue-green algae can produce algal toxins that could pose a health risk to people and animals when they are exposed to them in large enough quantities. These are collectively called Harmful Algal Blooms (HABs).

Surface water affected by blue-green algae often is strongly colored such that it can develop a paint-like appearance.

How can blue-green algae or HABs be harmful to my health?

While most blue-green algal blooms are not harmful, some produce algal toxins. Health effects can occur when surface scums or waters containing high levels of algal toxins are swallowed, come in contact with skin, or when airborne droplets containing toxins are inhaled. Symptoms can include nausea, vomiting, skin or throat irritation, allergic reactions or difficulty breathing. The toxins produced by blue-green algae can affect the liver and nervous system when water is consumed in sufficient quantities. The safest thing to do is to treat every algal bloom as if it could be dangerous. Recreational contact such as swimming, bathing, or showering with water not visibly affect by a blue-green algae bloom is not expected to cause health effects.

How do I know if I am being exposed to blue-green algae?

The blooms look like blue or green paint spilled into the water, thick puffy blue or green foams on the surface of the water, or swirling colors beneath the surface of the water. Blue-green algae blooms can also have distinct smells. They can smell grassy or septic, and in some cases the smell can cause nausea.

Are children more vulnerable than adults to blue-green algal toxins?

Yes. This is mainly due to children’s low body weight; it takes fewer toxins to make children sick from exposure to blue-green algae. Also, children tend to have more sensitive skin than adults, so a skin rash or reaction is more likely. Children should always be supervised when swimming in any body of water.

How can I reduce exposure to blue-green algae or algal toxins?

People should avoid contact with water that is discolored or has scum on the surface. This includes but is not limited to: swimming, water skiing, tubing, boating, etc. If contact does occur, immediately wash with soap and water or rinse thoroughly with clean water to remove algae. Restrict the access of pets and livestock to this water. This is especially important for pets because they may lick the algae off their fur to clean themselves. Never drink untreated surface water, whether or not algae blooms are present. Water from lakes, rivers, or streams may contain other bacteria, parasites or viruses, as well as toxins, that all could cause illness if consumed.

Can I eat fish caught in water with high amounts of blue-green algae or algal toxins?

Toxins from algae can accumulate in the entrails (guts) of fish and occasionally in the muscle (filet) of fish. Levels in fish depend upon the severity of the bloom in the area where the fish are caught. In general, fish that are caught in areas of a waterbody where major blue-green algae blooms occur may be safe to eat, as long as the entrails of the fish are discarded. However, there is some uncertainty about the levels of algal toxins that can accumulate in filets, so anglers may want to wait a week or two after algal blooms are over before eating fish from waters where a bloom is occurring. Care should be taken that animals are not fed or allowed to eat the entrails of these fish.

Do not swim or wade through algal scums.Do not let dogs drink lake water during an algal bloom.
Do not boat, water ski or jet ski through algal blooms.Do not let dogs eat algal scum, or lick it off their fur.
Do not fish from lakes where algal scum is present.Wash your dog off with clean water immediately if your dog swims or wades in water during an algal bloom.
Always shower off with soap and water after swimming in a lake. 

For more information, you may contact:

Dr. Thomas Hornshaw
Illinois EPA

Gregg Good or Teri Holland
Surface Water Section
Illinois EPA

Barb Lieberoff
Community Relations
Illinois EPA

Additional Information