Harmful Algal Blooms, commonly known as HABs, are excessive growths of certain types of algae in water bodies, such as lakes, rivers, and oceans. The overgrowth of these blooms can result in harmful effects on the environment, aquatic life, and public health. The most problematic HABs also produce toxins. 

HABs can be green, blue-green, red and sometimes brown or black. They may look like pea soup or spilled green paint, as well as surface scum, mats or films on the water. They also could appear as green dots or globs floating below the surface and discolor or leave streaks in the water.

If you suspect a HAB is occurring, ADEQ recommends:

• Notifying the local land owner or manager of the waterbody
• Contacting the local public health department 
• Filing a report through the ADEQ Arizona Water Watch mobile app | Learn More >
• Emailing ADEQ | Email >

ADEQ also recommends always practicing healthy recreational water habits. Enjoying the outdoors is an important part of an active, healthy lifestyle. ADEQ recommends following these simple, healthy habits in waters with visible algae:

• STAY AWAY from algae and scum in the water – AND – keep children and pets away from algae in the water or on the shore
• DO NOT DRINK or use this water for cooking
• AVOID EATING FISH from the water until the bloom is dissipated as toxins can build up in fish tissue 
• WHEN IN DOUBT, STAY OUT

Algae and bacteria are natural parts of all water bodies. HABs occur when certain types of algae grow rapidly into large populations in the water. This excessive growth can be caused by an overabundance of nutrients, such as nitrogen and phosphorus. These nutrients may be from natural sources, such as decaying organic matter, or from activities, such as agricultural runoff or untreated sewage discharge. Environmental conditions such as increased temperature, nutrients, sunlight and shallow or stagnant water increase the chance of HABs.

When people come into contact with water contaminated by HABs that are toxic, they may experience health problems such as skin rashes, respiratory irritation, nausea, vomiting, diarrhea, and allergic reactions. Ingesting water or seafood contaminated with toxins from HABs can lead to more severe health issues over time, including liver damage or neurological problems. Children are at a higher risk of experiencing symptoms, which could be a result of having a lower body weight than adults, as well as lacking knowledge of the risks of ingesting natural and potentially contaminated water.

HABs can create hypoxic (oxygen depleted) conditions in the water, harming fish and other aquatic organisms. Domestic animals and wildlife can also get sick if they drink toxins in the water, potentially leading to death. Toxic HABs can be especially harmful to pets, such as dogs. 

Only some algae bloom and only some produce toxins. In Arizona, algal blooms of cyanobacteria (“blue-green algae”) that may produce cyanotoxins are generally of most concern.  

Cyanotoxins are made by cyanobacteria. Cyanotoxins are generally toxic to the liver, but some may also affect the nervous system. Scientists are still studying how and at what levels cyanotoxins affect people’s health. It is not yet known how many different cyanotoxins exist and there is not a test for all the different types, nor is there a way to know when an algae is going to produce the toxin, to what extent, and for how long. 

Currently, there are no medical tests that can directly identify cyanotoxins in people or easily prove that an illness is related to cyanotoxin exposure. Blood samples may be helpful in evaluating patients for physical responses to toxin exposure. Usually when people are concerned about illness from cyanotoxin exposure, their doctor will collect information about the symptoms, timing and nature of exposure (e.g., if a person was swimming in the water or drank the water). The physician will look for other possible causes of the illness and will determine the most probable cause based on many factors.

Exposure to cyanotoxins in people is rarely fatal alone, but may worsen other potentially fatal conditions (e.g., alcohol poisoning, sunstroke, asthma). There are many more cases of dogs and livestock dying from cyanobacteria exposure, likely because dogs and livestock are more likely to directly ingest the algae when drinking or cleaning their fur after swimming. 

There are numerous types of cyanotoxins, and not all have been identified. Cyanotoxin research is an emerging field of science, and researchers continue to identify new types. Some cyanobacteria are able to produce multiple kinds of cyanotoxins at certain times.

Illness caused by cyanotoxins may occur through drinking contaminated water, eating algae, ingesting contaminated fish or shellfish, or coming into physical contact with the toxin through your skin while swimming or recreating. High winds can also spray contaminated water near the shore. Simply being present in the general area of HABs does not necessarily mean that exposure has occurred; however, when you recreate in any natural waters, it is wise to be aware of the potential exposure to any contaminants that might be present.

It’s important to be cautious during a HAB; avoid swimming or participating in other recreational activities in waters with visible algal blooms or if you notice any unusual odor, color, or foam on the water surface. Pay attention to local advisories and warnings issued by health authorities or environmental agencies to ensure your safety. Do not ingest water that contains a potential HAB. It is difficult to treat water containing algal toxins, so boiling or filtering water for drinking is not advised.

You should promptly use a towel to dry off and shower in clean water as soon as possible. If shower facilities are not readily available, rinsing off with any available clean water may be helpful as an interim measure. Make sure to also rinse your pets so they do not lick algae off their fur. 

Most of the reported illnesses and deaths in the U.S. relating to cyanobacteria exposures have occurred in pets and livestock. While there are still many unknowns about cyanotoxins effects on public health, children are considered to be at greater risk of harmful effects than adults. The elderly and people with underlying diseases (such as liver disease) may be at higher risk than average due to weaker liver function. Some sensitive individuals with asthma and related conditions could experience worsening of their symptoms due to vapors released from decaying algal mats or high winds spraying contaminated water into the air.

Research indicates that toxins from harmful algae can accumulate in fish tissue over time. When a potential HAB is present, be cautious, and do not consume the fish. 

In general, ADEQ recommends that you check the local fishing advisories for a specific water body, especially if you plan to consume the fish. For more information on Arizona fish consumption advisories (unrelated to HABs) | Learn More >

While we cannot eliminate harmful algae or bacteria from our natural environment, individuals can help reduce the frequency and impacts of HABs by using fertilizers responsibly and properly managing waste (e.g., septic systems, stormwater, camping waste, pet waste). Communities and industries can implement practices that reduce nutrient runoff, improve wastewater treatment, control stormwater run-off, and anything else necessary to protect water quality as a whole.

For ponds and small lakes, there are treatment options and many lake or pond owners actively manage to prevent HABs. However, there are currently no effective treatments for large lakes. The best course of action is to implement programs to prevent HABs by reducing the nutrients entering the water and preventing stagnation in the lake.

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EPA – Harmful Algal Blooms >
EPA – Harmful Algal Blooms FAQ >
EPA – Cyanobacterial Mobile App >
CDC – Harmful Algal Bloom-Associated Illness >