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The Florida Department of Health works to protect, promote, and improve the health of all people in Florida through integrated state, county, and community efforts.

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HABs: Harmful Algae Blooms

Aquatic Toxins Program


image of ocean with algae bloom

Algae are plant-like organisms that sustain marine life. They contribute to the food chain and to the oxygen that keeps water bodies healthy, but sometimes, when conditions are right—often warm water and increased nutrients—certain algae can quickly grow and overpopulate. These foam- or scum-like masses are called blooms and can be pushed to the shore by winds, waves, tides and currents. Some blooms release toxins that make ecosystems, animals and people sick; scientists call these harmful algae blooms (HABs). In Florida, HABs can be found among our saltwater, freshwater and brackish water bodies.

HABs usually happen in summer or early fall and can affect small and large areas of a waterbody. Blooms can move around and change in a waterbody due to tides, winds and naturally fluctuating amounts of the algae. While a specific location might not constantly have a bloom, the bloom itself can continue for weeks, months or over a year.

As some HABs decay, they release a bad odor due to hydrogen sulfide, a foul smelling gas that is a naturally occurring and colorless product of decomposition. It can smell like rotten eggs. The human nose is very sensitive and can smell the unpleasant odor of hydrogen sulfide at very low levels. The Florida Department of Health completed testing in 2016 and 2018 for the presence of hydrogen sulfide in the air during HAB events. The results from both studies indicated very lownon-hazardous levels. The smell can cause respiratory irritation, but is not harmful to your health. If you are irritated by the strong odor or have breathing difficulties, move away from the source of the smell. Symptoms should end a short time after the odor disappears or the exposure to the odor ends. Animals can also be impacted by harmful algal blooms. For more information available to veterinarians and pet owners, see Education and Resources.

image with text red tide

There are many HAB species in the Gulf of Mexico. Karenia brevis (K. brevis), a single-celled, naturally occurring organism belonging to a group of algae called dinoflagellates, is the common cause of red tide in Florida. Red tide can discolor water to appear red or brown, and it produces potent neurotoxins, called brevetoxins, that can be harmful to the health of both people and animals. Brevetoxins can be released into the air or water when wind and waves break open the algae cells. Respiratory irritation can occur when exposed to brevetoxins.

SYMPTOMS? Even if you are not prone to respiratory issues, you should be careful. Red tide can cause coughing and sneezing and can irritate your eyes, nose and throat. Reactions to red tide can be worse for people with asthma, emphysema, bronchitis or any chronic lung disease. You can get relief from respiratory symptoms by being in an air-conditioned space. For people without asthma or chronic respiratory problems, over-the-counter antihistamines can help. If your symptoms do not improve, contact or visit a doctor or call the Florida Poison Information Center at 1-800-222-1222.

SWIMMING  Do not swim in or around red tide because the toxin can cause skin irritation, rashes and burning and sore eyes.  If you come into contact with red tide, wash off with soap and water, especially if your skin is easily irritated. 

DEAD FISH  Red tides can kill fish and other marine life—avoid contact with and don’t swim or walk in these areas. Keep your pets away from these areas.

RED TIDE AND FISH  Do not harvest or eat distressed or dead fish (or any animals) from or near a red tide. Fish caught live and healthy can be eaten if filleted and rinsed thoroughly with fresh water.

RED TIDE AND SHELLFISH   Check local harvesting status at Florida Department of Agriculture and Consumer Services. Bivalve mollusks, including clams, oysters, mussels and scallops are filter feeders that can concentrate toxins—these and other shellfish, if harvested from red tide areas, can be contaminated with brevetoxins. The muscle of the scallop is generally free of toxin but the rest of the scallop is not (recipes using scallop muscle are safe to eat). In addition, marine gastropods, commonly called sea snails (conch, whelk, etc), feed on the bivalves, therefore, they can also be contaminated with brevetoxins during a red tide. Your safest choice is to not harvest or eat molluscan shellfish or marine gastropods from red tide affected areas. 

You could suffer from Neurotoxic Shellfish Poisoning (NSP) if you eat contaminated shellfish. NSP symptoms include: nausea and vomiting; tingling of the mouth, lips and tongue; and slurred speech and dizziness. Neurological symptoms can progress to partial paralysis and respiratory problems.

image with text blue green algae

There are several species of blue-green algae (also known as cyanobacteria) that can occur in Florida’s fresh or brackish waterbodies, many of which have the potential to produce toxins. If people or animals splash in the water or if boats create wakes, the algae cells can be broken apart, and the cyanotoxins can release into the air. The toxins mix with water droplets and spray, causing people and animals to potentially inhale the toxin. Though contact with the algae may cause skin irritation, these toxins cannot pass through your skin easily, so swallowing large amounts of contaminated water is what can typically cause illness. Blue green algae blooms can be described as discolored water in a variety of colors such as blue, bright green, brown or red and appear as paint-like streaks on the surface of the water, dense scum, or foam that can emit unpleasant odors. Pets can become sick from blue-green algae so keep them out of those areas and away from contaminated marine animals and fish.

SYMPTOMS? For some people, exposure to blue-green algae can cause rashes, stomach cramps, nausea, diarrhea and vomiting. People who are very sensitive to smells can have respiratory irritation. Sometimes, high exposures of toxin can affect the liver and nervous system.

If you come into contact with blue-green algae or think that blue-green algae has made you sick, contact or visit a doctor or call the Florida Poison Information Center at 1-800-222-1222.

CONTAMINATED WATER  Water from areas with blue-green algae can make people and animals sick—stay away from these areas. Do not cook or clean dishes with water contaminated by algae blooms. Boiling the water will not eliminate toxins.

SWIMMING  Do not swim in or around blue-green algae. If you come into contact with blue-green algae, wash off with soap and water, especially if your skin is easily irritated. 

BLUE-GREEN ALGAE AND FISH  Fish tested from water with blue-green algae show that cyanotoxins do not significantly accumulate in the edible parts—muscle or filet—of fish, but can in other organs. Prior to consuming, rinse fish fillets with tap or bottled water, throw out the guts, and cook the fish well. Do not eat shellfish in waters with blue-green algae blooms.