HABs: Harmful Algal Blooms

Algae are plant-like organisms that sustain marine life. They contribute to the food chain and to the oxygen that keeps waterbodies 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 algal blooms, known as HABs. In Florida, HABs can be found among our saltwater, freshwater, and brackish waterbodies.

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 low, non-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.

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, brown, or green 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.

Health Impacts

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.

Avoid Exposure

Do not swim in or around red tide because the toxin can cause skin irritation, rashes, 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. 

Fish Kills

Red tide can kill fish and other marine life. Avoid areas where dead fish can be seen - don’t swim or walk in these areas. Additionally, keep your pets away from these areas.

Consuming 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.

Consuming 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. Marine gastropods, commonly called sea snails (conch, whelk, etc), feed on the bivalves, and 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
• Slurred speech
• Dizziness

Neurological symptoms can progress to partial paralysis and respiratory problems.

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. Blue-green algae is not always at the water's surface; it can appear at all levels of the water column.

Health Impacts

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.

Pets can become sick from blue-green algae so keep them out of those areas and away from contaminated marine animals and fish. Animals can exhibit effects of toxin exposure before humans, especially if they eat the algae from the water or clean their fur after swimming through a bloom with toxins. See the Education and Resources page for more information on protecting your pet.  

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

Avoid Exposure

Water from areas with blue-green algae can make people and animals sick, so it is advised to stay away from these areas.

Do not cook or clean dishes with water contaminated by algae blooms. Boiling the water will not eliminate toxins.

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. 

Consuming Fish

Fish tested from water with blue-green algae have shown that cyanotoxins do not significantly accumulate in the edible parts (muscle or fillet) 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.