Toxic algae blooms are growing on lakes and beaches across the US, and health departments warn they can poison kids and kill pets

A girl uses a stick to try and scoop algae from an algal bloom off the beach at Maumee Bay State Park in Oregon, OH on August 3, 2014. Ty Wright for The Washington Post via Getty Harmful algae blooms can endanger aquatic wildlife and human swimmers. Blooms may appear as greenish scum or brightly colored patches like spilled paint.

Algae grows out of control when the water is warm, slow-moving, and rich with nutrients. As lakes across the country sprout foul-smelling, greenish mats of aquatic bacteria, health officials are warning residents about the risks of toxic algae poisoning. Most algae and similar single-celled organisms are harmless or even beneficial to marine ecosystems.

However, algae can “bloom” rapidly under certain conditions, endangering wildlife and making it unsafe for people to swim. Blooms of cyanobacteria, also known as blue-green algae, have been reported at Lady Bird Lake in Austin, Texas; Lake Pillsbury in Northern California; and Doctors Lake in Clay County, Florida. Freshwater algae blooms are typically greenish in color, while saltwater algae species may dye the coastlines red, earning the name “red tides.

“Swimming in or near contaminated water can put humans at risk of toxic algae poisoning, although animals are more likely to be affected. Symptoms of algae poisoning may include rashes, diarrhea, vomiting, coughing, or wheezing. More than 300 emergency room visits were related to exposure to harmful algae blooms between 2017 and 2019, according to a report from the Centers for Disease Control and Prevention.

Environmental researchers have expressed concern that harmful algal blooms are already becoming more common due to the climate crisis. As blooms continue to thrive, it’s only a matter of time before reports of algae-associated illness spike as well. Algal blooms may produce toxins and harmful gassesFor an algae population to grow out of control, it needs warm, slow-moving water that is rich with nutrients.

Climate change has created the “perfect” conditions for algae to bloom, according to the Environmental Working Group. Warming temperatures and rising carbon dioxide levels both make waters more hospitable to all species of algae and cyanobacteria (also known as blue-green algae). According to the Environmental Protection Agency, cycles of drought and extreme rainfall also increase the risk of runoff, filling waters with extra nutrients that cause algae to bloom.

Not all algal blooms are harmful, but public health officials generally recommend staying out of funky-looking water. Certain types of algae are toxic to humans and animals, while others can deplete the water’s oxygen supply or release harmful gas into the environment, according to the CDC. Humans can get sick from toxic algae if they swim in or around contaminated water, or if they consume food or water that’s been contaminated.

Small fish and shellfish in affected areas ingest toxins from algae, and those toxins may travel up the food chain. Toxic algae poisoning may cause stomach pain and breathing problemsSymptoms of harmful algae-related illness vary depending on whether contaminated water was ingested, accidentally inhaled, or contacted the skin. CDC surveillance from 2017 to 2019 found that respiratory illness was more common than upset stomachs, rashes, and other symptoms of toxic algae poisoning.

However, researchers noted that cases where people reported coughing, congestion, and trouble breathing may have been connected to a large-scale “red tide” along the Gulf of Mexico in 2018. Algae-associated illness has been more pronounced in small children who play in shallow waters and need less exposure to get sick. According to the CDC, nearly half of people ill from algae exposures in 2019 were younger than eight years old.

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