Boston: Health Advisory: Officials Investigating Charles River for Cyanobacteria Algae Bloom

Tuesday, July 21, 2026 ~ Boston residents are being warned about a potential cyanobacteria algae bloom in the Charles River, according to a press release from the Boston Public Health Commission (BPHC) on Tuesday, July 21, 2026. The affected area stretches from Kendrick Street in Needham to Spring Street in West Roxbury and includes popular recreational spots such as Millennium Park and Cutler State Park.

The Massachusetts Department of Public Health (DPH) is currently awaiting test results to confirm the presence of cyanobacteria that exceeds DPH guidelines for recreational waterbodies in the state. Cyanobacteria, also known as blue-green algae, can produce toxins that can be harmful to both humans and pets.

As a precautionary measure, residents are advised 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, pets should not be allowed to swim in or drink from the affected waterway.

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If anyone comes into contact with the water or algae on the shoreline, they should rinse off immediately. Blue-green algae blooms can make the water appear murky or like pea soup and may produce toxins within their cells or in the surrounding water.

The primary concern for humans is ingestion of water containing blue-green algae while swimming. However, direct skin contact and inhalation of water droplets containing blue-green algae or toxins can also cause irritation and respiratory issues. Ingestion of these toxins can lead to acute gastrointestinal symptoms such as vomiting and diarrhea. In severe cases, these toxins can affect liver, kidney, and neurological function and even result in death.

For pets, ingestion of contaminated water or scum that has washed ashore or gotten onto their skin or fur is a major concern. If a pet shows symptoms such as vomiting, staggering, drooling, or convulsions after being around an algae bloom, owners should seek immediate veterinary care. Ingestion of harmful algae has been known to cause canine deaths in Massachusetts and other states.

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The advisory will remain in effect until the bloom has dissipated and two rounds of water samples, collected a week apart, show levels below 70,000 cells/milliliter. The DPH will continue to monitor the cyanobacteria levels in the affected area and conduct sampling for laboratory analysis.

Residents with questions or concerns can contact the Boston Public Health Commission's Environmental and Occupational Health Division at (617) 534-5965. The BPHC also advises checking the website mass.gov/algae-blooms for an updated list of all bodies of water with algae bloom advisories.

Harmful algae blooms are most common during warm, sunny weather in the summer months. While they can last for weeks, they may also dissipate quickly. The BPHC urges residents to take caution and avoid contact with affected waterways until the advisory is lifted.

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