Is Red Tide Harmful to Humans? What to Know About Florida's Recent Algae Bloom

Residents along Florida’s southwest coast are experiencing a naturally occurring phenomenon known as a “red tide,” algae blooms that can kill marine life and affect human health.

According to the Florida Fish and Wildlife Conservation Commission (FWC), red tides occur when algae multiply to levels higher than usual. These algae blooms generate what are known as brevetoxins, which can kill marine animals including fish and birds. The presence of Karenia brevis, the microscopic red tide organism, discolors the water around it—hence the name.

Red tides aren’t unexpected—they occur almost every year in Florida’s waters—Farnaz Tabatabaian, MD, associate professor and allergy and immunology physician at the USF Health College of Medicine in Tampa, FL, told Health. She warned of red tide’s disruptive nature, noting the legitimate health concern for those who live near a red tide.

Dr. Tabatabaian explained that Florida red tide occurs on the coast and near the shore waters of the West Florida Shelf, and less frequently in other parts of the Gulf of Mexico. Depending on the concentration and range of the blooms, it can last just a few weeks up to several months.

“Red tide has been reported back as early as the 16^th century, but it has undergone several morphological changes with the newest genus, Karenia described in the early 2000s,” Dr. Tabatabaian said.

“These toxins impact various organisms including fish, turtles, marine mammals, birds, and humans in various ways,” she continued. “So yes, these blooms are an ongoing and serious threat to local economies and public health.”

A Common Occurrence

The current 2022-2023 red tide event along the southwest coast of Florida began in October 2022 and is currently in its fifth month, Kate Hubbard, PhD, director of the Center for Red Tide Research at the FWC, told Health.

Dr. Hubbard explained that these algae blooms “typically quiet down” in the winter heading into spring, with May through July being “the quietest months for red tide.”

“This bloom is dynamic over the course of a day, and from day to day and week to week. We have seen conditions worsen and improve several times since it started and even over the course of the past few weeks,” she said. “We have several tools that are helpful for providing information about the current status given that conditions can shift rapidly.”

Dr. Hubbard pointed to the FWC’s Red Tide Current Status map and report, the University of South Florida (USF) Bloom Tracking Forecast, and the National Oceanic and Atmospheric Administration (NOAA) Respiratory Forecast as some tools those affected can turn to for current information and updates.

How a Red Tide Affects Air Quality

Dr. Tabatabaian emphasized how important it is to note that the primary avenues for human exposure are through either ingestion or inhalation.

“The aerosols contain a high level of brevetoxin 20 to 50 times greater than in the seawater. The wind delivers the contaminated aerosol onto the beaches, which can cause symptoms immediately upon exposure,” she explained. “Individuals might experience symptoms even after leaving the beach because these toxins have been measured up 2.6 miles away from the shore.”

Inhaling or Ingesting Red Tide Toxins

According to Dr. Tabatabaian, some symptoms that can result from ingesting red tide-released toxins include abdominal pain, diarrhea, hot and cold sensations, paresthesias (numbness or tingling), and ataxia (gait problems).

Dr. Hubbard echoed these thoughts and said that gastrointestinal symptoms like nausea and vomiting, slurred speech, and dizziness, could result from ingesting red tide.

Additionally, symptoms from inhaling these toxins can include wheezing, non-productive cough, sneezing, a runny nose, and conjunctivitis. Dr. Tabatabaian stressed that people who already have respiratory illness might have "worsened or exacerbated symptoms" by coming in contact with a red tide.

“Some people experience respiratory irritation—coughing, sneezing, tearing, and an itchy throat—when the red tide organism is present and winds blow aerosolized brevetoxins onshore,” Dr. Hubbard added. “Offshore winds usually keep respiratory effects experienced by those on the shore to a minimum. The [Florida] Department of Health advises people with severe or chronic respiratory conditions, such as emphysema or asthma, to avoid red tide areas.”

Dr. Hubbard recommended individuals be on the lookout for neurological symptoms that could progress into “partial paralysis and respiratory problems.”

“Swimming in red tide may cause rashes, skin irritation, or burning eyes,” she added.

Coming in Contact With Red Tide

Dr. Tabatabaian urged anyone concerned about red tide to stay up to date with their given county’s red tide reporting. Individuals should refrain from swimming in areas that are possibly affected.

“If you have symptoms that are fairly rapid onset after being in close proximity to the beach or shoreline, it is likely from the red tide,” she said. “If symptoms persist, you should reach out to a medical professional.”

The Florida Department of Health reports that you should wash off with soap and water immediately if you come in contact with red tide while at the beach. You might also get relief from persistent respiratory symptoms by going to an air-conditioned space. Additionally, those who don’t have chronic respiratory problems or asthma, for instance, can turn to over-the-counter antihistamines.

If you notice dead fish washing up ashore en masse, avoid that area. The department reiterates that you should absolutely refrain from swimming in these areas. Similarly, don’t eat or harvest “distressed or dead fish” or other marine life that comes from or even nearby an area afflicted by red tide. They do say that fish caught live and healthy can be eaten if they are filleted and rinsed thoroughly with fresh water.

These rules apply to shellfish as well. Shellfish like lobsters and shrimp, for just two examples, may be contaminated by the red tide toxins, There is the potential for you to suffer from neurotoxic shellfish poisoning (NSP) if you consume contaminated shellfish. Again, if you experience serious symptoms like slurred speech, nausea, and vomiting–and they persist—contact your medical provider.

Dr. Tabatabaian reasserted that you check local red tide reports and avoid going for a swim or fishing in afflicted areas. “If you have underlying pulmonary conditions,” she continued, “it’s recommended that you avoid going to the beach and decrease exposure if possible.”