WARSAW, Poland (AP) — Authorities in Poland have banned swimming at over 50 Baltic Sea beaches after hot weather led to the toxic growth of bacteria in the unusually warm sea.

Emergency water rescuers are telling vacationers on the hot sandy beaches from Swinoujsce in the west to Gdynia in the east not to enter the sea, where thick green-brown cyanobacteria colonies have grown and pose a health threat.

Regional sanitary authorities have issued warnings, saying that contact with the bacteria may cause allergies and rashes. Drinking contaminated water can also lead to serious digestive problems.

The Baltic Sea has not seen such intense growth of cyanobacteria for 12 years. It results from exceptionally high air temperatures of 34 degrees Celsius (93.2 degrees Fahrenheit) that have raised the temperatures of the usually cold Baltic.