Persons are generally warned not to enter or drink water from algal blooms, or let their pets swim in the water since many pets have died from algal blooms. In at least one case, people began getting sick before warnings were issued. There is no treatment available for animals, including livestock cattle, if they drink from algal blooms where such toxins are present. Pets are advised to be kept away from algal blooms to avoid contact.
In some locations visitors have been warned not to even touch the water. Boaters have been told that toxins in the water can be inhaled from the spray from wind or waves. Ocean beaches, lakes and rivers have been closed due to algal blooms. After a dog died in 2015 from swimming in a bloom in California's Russian River, officials likewise posted warnings for parts of the river. Boiling the water at home before drinking does not remove the toxins.
In August 2014 the city of Toledo, Ohio advised its 500,000 residents to not drink tap water as the high toxin level from an algal bloom in western Lake Erie had affected their water treatment plant's ability to treat the water to a safe level. The emergency required using bottled water for all normal uses except showering, which seriously affected public services and commercial businesses. The bloom returned in 2015 and was forecast again for the summer of 2016.
In 2004, a bloom in Kisumu Bay, which is the drinking water source for 500,000 people in Kisumu, Kenya, suffered from similar water contamination. In China, water was cut off to residents in 2007 due to an algal bloom in its third largest lake, which forced 2 million people to use bottled water. A smaller water shut-down in China affected 15,000 residents two years later at a different location. Australia in 2016 also had to cut off water to farmers.
Alan Steinman of Grand Valley State University has explained that among the major causes for the algal blooms in general, and Lake Erie specifically, is because blue-green algae thrive with high nutrients, along with warm and calm water. Lake Erie is more prone to blooms because it has a high nutrient level and is shallow, which causes it to warm up more quickly during the summer.
Symptoms from drinking toxic water can show up within a few hours after exposure. They can include nausea, vomiting, and diarrhea, or trigger headaches and gastrointestinal problems. Although rare, liver toxicity can cause death. Those symptoms can then lead to dehydration, another major concern. In high concentrations, the toxins in the algal waters when simply touched can cause skin rashes, irritate the eyes, nose, mouth or throat. Those with suspected symptoms are told to call a doctor if symptoms persist or they can't hold down fluids after 24 hours.
In studies at the population level bloom coverage has been significantly related to the risk of non-alcoholic liver disease death.