On June 29, suspicions of blue-green algae or cyanobacteria at Pirita beach arose after lifeguards reported their possible presence in the swimming water and raised a purple warning flag. The Health Board collected a water sample on Monday and the results came back positive on Tuesday.

"The sample contained a large concentration of blue-green algae. The dominant species is not known to be toxic in the Baltic Sea, but it can cause skin irritation when present in large quantities. We also found very high levels of a potentially toxic species of blue-green algae that poses a health risk to swimmers if the water is swallowed or through direct skin contact. Spending time in such water is not recommended and swimming may be particularly dangerous for children and pets," explained Kätlin Lahi, senior inspector at the Health Board's Northern Regional Department.

Also on June 29, there were suspicions of blue-green algae at Haabneeme beach in Viimsi Municipality. The Health Board has not confirmed those suspicions.

Earlier this June, the Health Board also detected blue-green algae at the Harku Lake swimming area. Testing identified potentially toxic species at levels that could pose a risk to both people and animals if the water is swallowed.

According to Lahi, an intense blue-green algae bloom has persisted throughout the Gulf of Finland since mid-June and the algal mass has now begun to break down. However, it is during this process that algal toxins are released into the water and people should avoid swimming in bodies of water where visible accumulations of algae are present. As the algae decompose, they may turn bluish-green or yellow.

According to Andres Jaanus, a researcher at the University of Tartu's Estonian Marine Institute, the most favorable conditions for blue-green algae blooms occur when the water temperature reaches 20 degrees Celsius (68 degrees Fahrenheit) or higher and the water contains sufficient light and nutrients, especially nitrogen and phosphorus. "Rainfall carries additional nutrients into bodies of water from both land and the atmosphere, which can promote and prolong algae blooms," Jaanus said.

Blue-green algae also require stable conditions in the upper layer of the water column to develop. Windy weather mixes the algal mass and can disperse it into deeper layers of water. Along the coast and at beaches, the accumulation of blue-green algae also depends on wind direction. "For example, in the Gulf of Finland, winds from the west and north are favorable for algae accumulating," Jaanus said.

According to the researcher, the blue-green algae season in Estonian waters typically lasts from June through September. In recent years, strong blooms have already been observed in June in the central and western parts of the Gulf of Finland. Depending on the region, peak blue-green algae season usually occurs in July or August.

According to the Health Board, symptoms of blue-green algae poisoning may resemble those of the flu and include redness of the skin and eyes, diarrhea, fever and muscle pain. The agency recommends washing with clean water and soap after swimming whenever possible. Anyone who suspects poisoning should contact their family physician or call the Poison Information Center's hotline at 16662.

According to the Health Board, signs of blue-green algae may include yellowish-green water, green accumulations along the shoreline, a musty odor and flakes floating on the water's surface.

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