Public Health Advisory: Spy Pond Not Safe for Swimming

Public health officials are advising people and their pets to stay out of Spy Pond due to high levels of microcystis algae.

In tests done Monday, the Massachusetts Department of Public Health found 85,000 cells per milliliter of the algae, about 21 percent higher than acceptable levels of 70,000 cells per milliliter, according to the Arlington Board of Health.

The algae can be toxic if ingested in high doses and can cause skin irritation and a rash after wading or swimming, according to the board. Ingestion can cause more serious health effects, such as muscle cramps, twitching and liver damage.

Microcystis is a cyanobacteria algae, or blue-green algae, that grows naturally beneath or on the surface of water. Under certain conditions, such as warm weather and an abundance of nutrients in the water, the algae may "bloom" or undergo an explosive type of growth that results in dense, floating mats of algae.

Although swimming is not permitted in the pond, people who fall in or come in contact with the water through other activities, such as boating, should be sure to shower after, according to the board.

Public health officials will continue to collect samples on a weekly basis. The advisory will remain in effect, however, until cell counts drop below acceptable levels for two consecutive weeks.