Harmful Blue-Green Algae: Frequently Asked Questions

What are harmful blue-green algae?

Blue-green algae, technically known as cyanobacteria, are microscopic organisms that are naturally present in lakes and streams. Under certain conditions, blue-green algae can become abundant in warm, shallow, undisturbed, nutrient-rich surface waters that receive a lot of sunlight. When this occurs, blue-green algae can form blooms that discolor the water, or produce floating mats or scums on the water’s surface. Blooms can also form on rocks, along the shoreline, and at the bottom of a waterbody. These are called benthic blooms. It might be a harmful blue-green algae bloom if the water is blue-green, green, yellow, white, brown, purple, or red, has a paint-like appearance, or if there is scum on the water surface. Photo gallery of blue-green algae blooms.

What are the potential health effects of blue-green algae and blue-green algal toxins?

Some blue-green algae can produce toxins, some do not. However, exposure to any blue-green algae blooms can cause health effects in people and animals when water with blooms is touched, swallowed, or when airborne droplets are inhaled. Exposure to high levels of blue-green algae and their toxins can cause diarrhea, nausea or vomiting; skin, eye or throat irritation; and allergic reactions or breathing difficulties.

Pet and livestock illness/deaths can happen if animals consume large amounts of water containing blooms, scums or benthic mats. Immediately seek veterinarian care if noticing any symptoms in your animals. Learn more.

Recreational Exposures

Recreational exposures can occur while swimming, wading, fishing, or boating in areas with blue-green algae blooms. Exposures can occur by swallowing water, contact with the skin, eating fish caught from affected areas, and when airborne droplets are inhaled.

Household Exposures

Exposures can occur during household uses of surface water that is affected by a bloom, such as showering, bathing, or washing.  Exposures can occur by drinking or eating blooms or toxins, contact with the skin, and when airborne droplets are inhaled.

Health effects are not expected from recreational or household uses of surface water that is not noticeably affected by a blue-green algae bloom. However, it is possible that people who are more sensitive to low levels of blue-green algae might experience mild symptoms.

Drinking Water Exposures

If you are on a public water system, your drinking water is protected by your local water supplier to address possible contaminants, including blue-green algae and associated toxins.Water suppliers are required to treat, disinfect, and monitor drinking water for their customers. If contaminants are detected in your water supply above levels established to protect human health, you would be notified by your local water supplier and provided instructions.

People should never drink untreated surface water. If you are not on public water and use surface water for drinking, preparing food, cooking, and making ice, you are at risk of exposure to blue-green algae, algal toxins, and other common drinking water contaminants. Get connected to public water if you have the opportunity. Or consider installing a private well using a NYS Department of Environmental Conservation registered well driller. Even if you are treating surface water yourself with water filtration, chlorine, ultraviolet (UV) light, or other treatment, your water is still may not be protected from blue-green algae and their toxins, see Harmful Blue-Green Blooms: Understanding the Risks of Piping Surface Water into Your Home.

In addition to the health symptoms mentioned earlier, drinking water with high levels of blue-green algal toxins has been associated with effects on the liver and on the nervous system in laboratory animals, pets, livestock, and people. Livestock and pet deaths have occurred when animals consumed large amounts of accumulated algal scum from along shorelines and when animals groomed blue-green algal scums from their fur. Some limited evidence from human studies suggests that long-term consumption of untreated surface waters containing high levels of blue-green algal toxins could be associated with an increased risk of liver cancer. However, people in these studies also were exposed to other factors associated with liver cancer. As a result, it is unknown whether algal toxin exposure contributed to this risk. Because blooms typically don’t last long in waterbodies in the Northeast, long-term, continuous exposure to blue-green algae and associated toxins is unlikely. 

What is New York State doing about harmful blue-green algae blooms?

  • State and local health departments work with staff at regulated bathing beaches, state parks, and the Department of Environmental Conservation to monitor and respond to blooms, and help prevent people from being exposed.
  • Public water supplies in New York State that use surface waters are treated, monitored, managed, and have operational controls to prevent people from being exposed to blue-green algae, associated toxins, and other contaminants. 
  • The Department of Environmental Conservation maintains information about surface waters with blooms.

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

People should suspect that harmful blue-green algae blooms could be present in water that is noticeably discolored or that has surface scums. Colors might include shades of green, blue-green, yellow, brown, purple, white, or red. Water affected by harmful blue-green algal blooms often is so strongly colored that it can have a paint-like appearance. When it comes to drinking water, unpleasant tastes or odors are not reliable indicators of blue-green algal toxins, or other toxic substances.

What should I do if I suspect a bloom?

  • Know it. Be able to identify suspicious blooms so you can avoid them.
  • Avoid it. People, pets, and livestock should avoid areas with blooms or surface scums, or water that is noticeably discolored.
  • Never drink it. Never drink untreated surface water. Even if you treat it in your home with water filtration, chlorine, ultraviolet (UV) light, or other treatment; its still may not protected from blue-green algae and toxins.
  • Report it.
  • Stop using the water and consider medical attention for people and animals if symptoms such as diarrhea, nausea, or vomiting; skin, eye, or throat irritation; and allergic reactions or breathing difficulties occur after contact with surface waters.
  • Pet and livestock illness/deaths can happen if animals consume large amounts of water containing blooms, scums or benthic mats. Immediately seek veterinarian care if noticing any symptoms in your animals. Learn more.