Health Advisory Alert Lifted on Cyanobacteria Algae in the Charles River

The Cambridge Public Health Department is alerting residents that the health advisory alert on the cyanobacteria algae in the Charles River lower basin has been lifted. This alert, which was first issued in July and then expanded in early October, was partially lifted last week. 

On Tuesday, October 21, 2025 the Massachusetts Department of Public Health (MDPH) collected follow-up samples at three locations in the Charles River. Laboratory analysis showed cyanobacteria concentrations were below MDPH’s guidelines at all three locations. Therefore the alert has been lifted.

Algae and cyanobacteria (also called blue-green algae) are plant-like organisms that live in water. If the blue-green algae are producing toxins, the health effects can be serious for people and for pets. Animals can get very sick or even die within hours of coming into contact with harmful algae or cyanobacteria. More information about harmful algal blooms can be found on the MDPH’s website.