"There is some algae in the water, so we have warned people to be careful when swimming, not to swallow water and to shower afterwards," Antti Poenkae, chief of environmental health at the Helsinki Environment Institute, told AFP.

Young children and dogs are particularly sensitive to the algae, called Nodularia Spumigena but more commonly referred to as blue-green algae. It can cause flu-like symptoms, vomiting, fever, diarrhea, nausea, headaches, as well as itchy and burning skin, but it is not life threatening.

It has erupted in the Baltic Sea off Sweden's southeastern coast and in the Gulf of Finland on Finland's southern coast.

So far this month, it has poisoned some 30 people in Finland, mostly children who have swallowed algae-infected water while swimming, according to local health authorities.

The algae typically blooms during warm weather, and has recently become a seasonal phenomenon during hot summers as it thrives in the brackish and increasingly polluted waters of the Baltic Sea.

"It seems that it's becoming more common because of the higher amounts of phosphor, nitrogen and organic material from sewage discharged into the Baltic Sea from the surrounding countries," Poenkae noted.

The algae appears in the sea as strings of blue, green or yellow scum or occasionally just murky, cloudy waters.

Earlier this week the Finnish coast guard reported a 300-kilometer (185-mile) long belt of algae some 20 to 50 kilometers (12 to 30 miles) wide, stretching from the southwestern tip of Finland across the Baltic to the Aaland islands.

And in Sweden, the algae has been observed around the Baltic holiday islands of Oeland and Gotland and in the Stockholm archipelago.

If the warm weather continues, officials said the outbreak was likely to worsen.

TERRA.WIRE