Boston: Health Advisory: Charles River Lower Basin Affected By Cyanobacteria Algae Bloom

Thursday, August 20, 2026 ~ Boston residents are being warned to avoid contact with the Charles River Lower Basin due to a cyanobacteria algae bloom, according to a press release from the Boston Public Health Commission (BPHC) on Thursday, August 20, 2026.

The BPHC has issued an advisory after an assessment of the pond by the Massachusetts Department of Public Health (DPH) staff revealed high levels of cyanobacteria, which exceed DPH guidelines for recreational waterbodies in Massachusetts. Cyanobacteria are known to produce toxins that can be harmful to both humans and pets.

As a precautionary measure, residents are urged to avoid direct contact with the water and any areas where algae concentration is present, including on shore. This includes swimming and other water activities. Additionally, dogs should not be allowed to swim in or drink from the affected waterway.

In case of accidental contact with the water or algae on the shoreline, individuals are advised to rinse off immediately. The BPHC has also provided a map with the precise location of the affected area at the bottom of their press release.

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Blue-green algae blooms can make water appear murky and sometimes give it a greenish tint, resembling pea soup or paint. These blooms can produce toxins that may cause illness if ingested by humans or pets. The primary concern for humans is ingestion while swimming, but direct skin contact and inhalation of water droplets containing blue-green algae or toxins can also pose a risk. For dogs, ingestion of contaminated water or scum that has washed ashore or gotten onto their skin or fur is the main concern.

Exposure to blue-green algae can cause skin and eye irritation as well as respiratory issues. Ingestion may result in acute gastrointestinal symptoms such as vomiting and diarrhea. In more severe cases where toxins are present in high levels, there may be more serious health effects such as liver and kidney damage or neurological issues. It is especially important to keep children and small pets away from the affected area due to their smaller body weights.

If a dog has been exposed to an algae bloom and shows symptoms such as vomiting, staggering, drooling, or convulsions, it is crucial to seek immediate veterinary care. Dogs have been known to ingest the scum that washes ashore or lick it off their fur, leading to serious health complications. In Massachusetts and other states, there have been documented cases of canine deaths due to harmful algae ingestion.

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The BPHC advises residents to check the website mass.gov/algae-blooms for an updated list of all bodies of water with algae bloom advisories. These blooms are most common in the summer months when warm and sunny conditions promote algae growth. The duration of a bloom can vary from weeks to just a few days. The DPH will continue to monitor cyanobacteria levels in the affected area and conduct sampling for laboratory analysis once the bloom has dissipated. The advisory will be lifted only after two rounds of samples collected a week apart show levels below 70,000 cells/milliliter of water.

For any questions, concerns, or further information, residents can contact the Boston Public Health Commission's Environmental and Occupational Health Division at (617) 534-5965.

In conclusion, residents are advised to take necessary precautions and avoid contact with the Charles River Lower Basin until further notice from the BPHC. By following these guidelines and staying informed about the situation, we can ensure the safety and well-being of ourselves and our pets.

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