What Is a Harmful Algal Bloom?
The CDC says harmful algal blooms are the rapid growth of algae or cyanobacteria in water that can harm people, animals or the environment. It says algae and cyanobacteria, also called blue-green algae, are plant-like organisms that live in water and can quickly grow out of control, or bloom, and that some blooms produce toxins that make people and animals sick.
The EPA says health effects from cyanobacterial toxins in freshwater may range from a mild skin rash to serious illness or, in rare circumstances, death, and that short-term exposure during recreational activities can cause hay fever-like symptoms, skin rashes, and respiratory and gastrointestinal distress. It says microcystins are primarily liver toxins, with evidence indicating they can affect the kidney and reproductive systems as well.
The CDC says to stay out if water looks discolored, has scum or smells bad, and that if you do go in or touch water that may have a harmful algal bloom, to rinse off immediately with tap water from a sink, shower, hose or outdoor spigot. It warns that boiling water containing toxins will not make it safe to drink, because boiling does not remove toxins and can concentrate them.
In 2019 the EPA recommended recreational water concentrations of 8 micrograms per liter for microcystins and 15 micrograms per liter for cylindrospermopsin as protective of human health for swimming and other primary contact. In 2015 it developed drinking water health advisories for the same two toxins, and it says those advisories are not regulations or legally enforceable federal standards.
Sources checked September 15, 2026. Every point below links the page it came from.
The sources behind this answer
- Government sourceThe CDC defines harmful algal blooms as the rapid growth of algae or cyanobacteria that can harm people, animals or the environment, and says to stay out of discolored, scummy or smelly water. CDC ↗
- Government sourceThe CDC says algae and cyanobacteria, also called blue-green algae, can quickly bloom, and some blooms produce toxins that make people and animals sick. CDC ↗
- Government sourceThe CDC warns that boiling water containing algal toxins will not make it safe to drink, because boiling does not remove toxins and can concentrate them. CDC ↗
- Government sourceThe CDC advises rinsing off immediately with tap water after going in or touching water that may have a harmful algal bloom. CDC ↗
- Government sourceThe EPA says microcystins are primarily liver toxins, with evidence they can also affect the kidney and reproductive systems. US EPA ↗
- Government sourceThe EPA says cyanotoxin health effects range from a mild skin rash to serious illness or rarely death, and short-term recreational exposure can cause rashes and stomach and breathing problems. US EPA ↗
- Regulatory filingIn 2019 the EPA identified 8 micrograms per liter of microcystins and 15 of cylindrospermopsin as recreational water levels protective of human health. US EPA, Federal Register ↗
- Regulatory filingThe EPA says it developed drinking water health advisories for microcystins and cylindrospermopsin in 2015, and they are not legally enforceable federal standards. US EPA ↗
Sources checked September 15, 2026. A brand statement is the company’s own claim, which we report but have not verified. Our grades are model-based estimates, not lab tests.