A harmful algal bloom (HAB) occurs when certain kinds of microscopic organisms multiply and produce toxins in a waterbody or waterway. The microscopic organisms that most commonly cause HABs in Pennsylvania's fresh and brackish waters are cyanobacteria, or blue-green algae. While cyanobacteria are a natural part of many aquatic ecosystems, under certain conditions, like high nutrients and warm temperatures, some kinds of cyanobacteria can produce cyanotoxins. HABs can form at any time but most often in late summer or early fall. HABs may move and their toxicity may change rapidly, requiring ongoing surveillance and response throughout the season.

People and animals can encounter HABs that are in the environment by physically touching, ingesting, and inhaling cyanobacteria and/or cyanobacteria toxins while swimming and boating; eating fish caught in contaminated water; using contaminated water to prepare food; or drinking contaminated water. For dogs and livestock, eating scum or algae and licking fur after swimming in contaminated water could be HABs exposures.

In high enough concentrations, cyanotoxins from HABs can be harmful to people, pets, livestock, and wildlife that come in contact with or ingest them. Those who encounter these toxins can experience severe illness including vomiting; diarrhea; neurological symptoms; skin, eye, nose, or throat irritation; or even death.

To coordinate monitoring, response, and communications about HABs in Pennsylvania, several Commonwealth agencies and commissions formed the Pennsylvania HABs Task Force, which includes representatives from:

• the Pennsylvania Department of Health;
• the Pennsylvania Department of Environmental Protection;
• the Pennsylvania Department of Conservation and Natural Resources;
• the Pennsylvania Department of Agriculture;
• the Pennsylvania Fish and Boat Commission;
• the Pennsylvania Game Commission; and
• the Pennsylvania Emergency Management Agency.