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Hot weather increases the risk of blue-green algae waterblooms. The risk of blue-green algae should be evaluated before swimming and any other use of water.
Ranta vihreänä sinilevästä

The department of Jyväskylä Environmental Health reminds the general public that you must not swim in blue-green algae infested water, nor use it for bathing, in sauna for steam or give it to your pets to drink. Do not use such water for watering or washing vegetables.

The poison of blue-green algae cannot be destroyed by boiling; in other words, you must not use the water even after boiling. 

Not all blue-green algae is toxic, but some of the algae produce liver and neurotoxins or compounds that irritate your skin. Blue-green algae may cause red or itchy skin, oral or nasal symptoms, vomiting and diarrhoea.

How to identify blue-green algae 

In calm and warm weather, blue-green algae can form a greenish film floating on water. A small amount of blue-green algae may look like greenish or yellowish specks on water. You can check, whether there is blue-green algae in water with either a stick or glass test.   

The stick test: Poke the algae with a stick. Lift some of the mass from water with the stick. If algae hangs from the stick, it is not blue-green algae. Blue-green algae breaks to particles at the touch.   

The glass test: Scoop some water with algae in a glass. Leave it for an hour. If you see greenish or bluish particles floating on water, it is blue-green algae.