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Harmful Algal Blooms

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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.

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.

DOH is working closely with many other state entities, including the Department of Environmental Protection, the Department of Conservation and Natural Resources, the Fish and Boat Commission, and the Game Commission, to understand and prevent HABs from affecting Pennsylvania residents.

If you have any health-related questions about HABs, contact us at For other inquiries about HABs or to report a HAB, contact

Harmful Algal Blooms (HABS) in PA training This course describes harmful algal blooms (HABs) and explores the response strategies that have been employed by agencies in the Commonwealth of Pennsylvania. The learner will discover the coordination and implementation of response activities among Commonwealth agencies to minimize the public's exposure to HABs and reduce negative impact of HABs.