Toxic algae blooms prompt health warnings across multiple U.S. states and Northern Ireland

Rising summer temperatures and nutrient runoff have triggered toxic blue-green algae warnings across multiple U.S. states, including North Dakota, Massachusetts, and California, as well as Northern Ireland. These cyanobacteria blooms thrive in warm, stagnant waters and release fast-acting toxins that can cause severe illness in humans and have proved fatal to domestic pets. Public health officials urge swimmers to avoid discolored or cloudy water and caution that standard testing may not capture rapid, weather-driven pollution spikes.

Wild swimming health risks

• ▪Leptospirosis is a bacterial disease carried in the urine of rodents and livestock that can cause organ failure and death in swimmers.
• ▪Wild swimming in rivers and lakes can expose individuals to pathogens that cause gastrointestinal illnesses, with risks increasing for those who swallow water.
• ▪Children face a higher risk of falling sick from wild swimming than adults because they tend to swallow more water.

Combined sewer overflows

• ▪Heavy rain and storm events can overload urban sewer systems, causing water companies to release sewage-polluted water directly into rivers.
• ▪Urban rivers typically become microbiologically unsafe for swimming for at least 24 to 72 hours after heavy rain due to sewage overflows, runoff, and disturbed sediments.

Agricultural runoff pollution

• ▪Phosphorus stored in sediment and topped up by agriculture, wastewater, septic tanks, and household detergents stimulates blue-green algae growth in Lough Neagh.
• ▪Addressing nutrient pollution at its source, particularly from farming and water treatment, is the primary action required to tackle algal blooms.

Cyanobacteria bloom hazards

• ▪A dog died on July 23, 2026, shortly after swimming in a stagnant pool off the Eel River near Scotia, prompting California environmental health warnings.
• ▪The Massachusetts Department of Public Health issued an advisory on July 21, 2026, for a stretch of the Charles River following a dog's death from liver failure.
• ▪Cyanobacteria toxins can cause skin rashes, vomiting, diarrhea, neurological issues, and can be fatal to pets and livestock.
• ▪Cyanobacteria blooms thrive in warm, stagnant freshwater, and prolonged heat combined with lower water levels can accelerate their growth and concentrate nutrients.

Water quality testing limitations

• ▪Inland water sites like rivers and lakes generally lag behind coastal areas in water quality because they are more exposed to undiluted sewage runoff.
• ▪Official water quality tests at designated bathing sites may fail to capture real-time risks because pollution levels spike rapidly due to changing weather.

Safe swimming assessment guidance

• ▪Recreational users should avoid wild swimming in the days immediately following a storm or heavy rain event.
• ▪Swimmers should avoid water that is cloudy, opaque green, or has visible floating clumps, surface scums, or foam along the edges.