Algal blooms in bodies of water across the nation are increasing as a result of climate change, farming practices, storm and wastewater runoff and other environmental issues.
They're naturally occurring, but produce toxins that get into the air, water or food, and can cause illness in humans and pets.
They also deplete the oxygen in water, and that kills fish, mammals and birds.
The Centers for Disease Control and Prevention (CDC) has launched a new website with public information about harmful algae, and is asking state and local health departments to keep track and report the blooms.
CDC epidemiologist Virginia Roberts says they're easy to spot - most of the time.
"The slimy green stuff, sometimes it will look like thick paint in the water," she explains. "There are multiple colors that it can be. You can find algae and algal blooms in fresh water, in salt water. They're often very visible, but sometimes you don't even see them there."
When the blooms get into drinking water, it causes it to have weird odor, often musty or earthy smelling.
The Arkansas Department of Health requires samples of public drinking water be taken and storage tanks to be well maintained.
Roberts says breathing in these toxins can cause coughing or respiratory problems, and swimming in water with a harmful algal bloom can cause skin rashes.
People also can get sick from eating fish or shellfish or drinking tap water contaminated with the toxins.
"We don't have a lot of data to really pinpoint where across the country that this is the biggest issue," Roberts says. "So this system will be able to collect information that will inform our understanding of where blooms occur, how frequently they occur and whether they're more or less of a problem over time."
Roberts adds the algal blooms cause a huge financial hit to the country in health care costs for people and animals that have been exposed.
The fishing industry loses $38 million a year, and the recreation and tourism industries also lose millions every time a body of water is closed because of too much algae.