With many U.S. waterways reaching their highest temperatures at this time of year, colonies of algae in lakes, ponds, and even the ocean can “bloom”—grow far more rapidly than normal. While most won’t hurt you, algae overgrowths that may be toxic, called harmful algal blooms, or HABs, are on the rise. And that has public health officials concerned.  

HABs—“mats” of algae that typically turn water the color and consistency of pea soup—have become more common and larger in size over the last few decades, according to the National Oceanic and Atmospheric Administration. More than half of all states report having harmful algal blooms every year in at least one freshwater body, and all coastal states in the U.S. have reported toxic algae in marine waters in recent years.  

HABs contain tiny toxin-producing organisms, and exposure to them may cause health problems such as skin rashes, gastrointestinal issues, and breathing difficulties. These can range from mild to severe and can be deadly to animals.  

Like benign algae blooms (and it can be tough to tell the difference between the safe and the toxic), HABs are most common in late summer and early fall, which can interfere with safe swimming, boating, and fishing. “Unfortunately, this peak season coincides with the time of year that many people are out recreating in surface waters,” says Bryan Brooks, Ph.D., an environmental scientist at Baylor University in Waco, Texas.

Harmful algal blooms can form in any body of water, but they’re more common in those that receive a lot of runoff from agricultural and residential fertilizers or sewage overflows. This runoff, combined with sunlight and warm temperatures, can encourage an HAB to blossom.  

Here’s what you need to know now about harmful algal blooms and how to protect your family—and pets:

Risks to People and Pets

Exposure to a toxic algal bloom can cause one or more of the following: diarrhea, vomiting, throat irritation, allergic reactions, and breathing difficulties. It can also cause rashes, hives, or blisters, especially on the lips and in areas covered by swimsuits.

People (and pets) can be exposed by swimming in or swallowing any water with a toxic algal bloom, eating seafood contaminated by an HAB, or simply breathing in the toxins, according to the Centers for Disease Control and Prevention.

“The greatest danger in freshwater is through ingestion,” says Todd Miller, Ph.D., an environmental health specialist at the University of Wisconsin-Milwaukee.

Although anyone can be sickened by exposure, children are at higher risk because they tend to spend more time splashing around in lakes and ponds, and swallow more water than adults, he says. Symptoms can appear within a few hours to a few days.

In pets, symptoms tend to be more severe and usually arise within 30 minutes of exposure. A dog that swam in water with a toxic algal bloom, for example, might seem disoriented and stumble around or experience difficulty breathing, or even experience seizures or paralysis. Some pets have died after contact with an HAB.   

What We Don't Yet Know

Researchers aren’t sure why the blooms are becoming more common, but climate change and an increasing volume of pollution from surface runoff are possible factors.

Health experts also don’t yet have a good handle on how often people are sickened by HABs or what the long-term health impacts might be, says Lorraine Backer, Ph.D., M.P.H., an environmental epidemiologist at the CDC’s National Center for Environmental Health in Atlanta. “It’s an emerging public health issue that we are just now beginning to explore and quantify.”  

One program that may help this effort: Last year, the CDC launched a national reporting system for public health, animal health, and environmental health professionals—designed to track human and animal illnesses from toxic algae exposure.

Protect Yourself From Toxic Algae

Be informed. Go to your local or state health department’s website to determine whether there have been any warnings about HABs or contaminated fish or shellfish in the area. You can also check for fish and shellfish advisories nationwide.

Avoid water you’re unsure of. Don’t enter any water where a harmful algal bloom advisory is posted. And if unposted water is green, brown, golden, or red (HABs can occur in a range of colors); has a mat of algae, foam, or scum on top; you spot dead fish or other dead animals in or near it; “or just doesn’t look or smell good, stay out,” Backer says. Keep pets out, too.

Take action after exposure. If you or your pet comes into contact with water that you think might contain a harmful algal bloom, rinse off right away with clean water. That’s especially important for dogs—they may continue to lick the toxins off their fur after they exit the water. “Watch for any unusual symptoms and call your primary care physician if you have any questions or concerns,” Backer says. If you think your pet has been exposed, contact your veterinarian immediately—the onset of severe symptoms may constitute a medical emergency for your pet.

Report it. If you see what you think is an HAB, let your local or state health department know. Some toxic algal blooms—like the massive 2014 Lake Erie HAB, which shut down the city of Toledo’s water supply and sent some 60 people to the hospital—get noticed quickly. But equally hazardous blooms on smaller lakes or reservoirs often go unreported, according to Brooks.

Pay attention to messages from your water utility. Though they’re rare, there are occasional reports of HABs in drinking water. Follow advisories your local water utility may have in place, and if you’re still concerned or want to take an extra step, a simple charcoal filter on your faucet can be a good backup measure, says Miller. (See more on water filters here.) 

Take steps to help prevent or limit HABs. “If you live near a water body, be careful about how much fertilizer you use on your property,” Backer says. And, she says, consider leaving an unmowed or naturally growing strip of land between your lawn and the water as a buffer to help reduce runoff and filter out substances that can increase the likelihood of an HAB.