Glossary
Harmful algal bloom
A fast overgrowth of algae or cyanobacteria that can make people and pets sick. It can look like scum, foam or paint.
A harmful algal bloom, often shortened to HAB, is the rapid growth of algae or cyanobacteria in water that can harm people and animals, according to the CDC (opens in a new tab). In fresh water, the most common cause is cyanobacteria, also called blue-green algae.
How to recognize one, per CDC:
- foam, scum, mats or what looks like paint on the surface;
- water that has turned green, blue, brown or red;
- water that smells bad.
Blooms can cause mild to serious illness, and animals can get very sick or even die within hours of contact. CDC's advice is to stay out, and keep pets out, if the water looks discolored, has scum or smells bad. CDC also suggests checking online whether a swim area is monitored, under advisory or closed before you go, especially after heavy rain (CDC (opens in a new tab)), and not swallowing lake or river water.
See is the water safe to swim in? and choosing a safe place to swim.
What you can do today: find your state or local beach water quality page and bookmark it.