Due to intense heat reaching around 35 degrees Celsius in parts of the country after the rainy season, blue-green algae levels are rapidly increasing in the four major river systems nationwide. 

Blue-green algae, which are considered harmful, have been detected at levels of 10-thousand cells per milliliter in parts of the Geumgang River, while also rising rapidly at water sports facilities in the Nakdong River.

According to K-Water and the National Institute of Environmental Research on Friday, algae levels are currently at 'caution' at eight locations, including five in the Geumgang River system and three in the Nakdong River system.

A 'caution' alert is issued when the number of harmful blue-green algae cells exceeds one-thousand per milliliter for two consecutive weeks. A 'warning' alert is issued when cell counts exceed 10,000 per milliliter, and an 'outbreak' alert is issued when cell numbers surpass one million.

While no alerts have been issued for the Han River, Yeongsan, and Seomjin River basins, the number of blue-green algae in these areas has also increased.