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Blue-green algae (cyanobacteria): season and precautions
July to August.
These are general seasonal patterns for the Nordic and Baltic region, not a live forecast. Always check the current local advisory — the weather service, health authority or water-quality notice — before you head out.
The risk
In warm, still water, cyanobacteria can multiply into a bloom that looks like spilled paint, greenish streaks or a scummy film on the surface. Some blooms release toxins that irritate the skin and eyes and, if swallowed, cause stomach upset, and they can be dangerous to small children and to dogs that drink or lick the water.
How to stay safe
Do not swim where the water is discoloured, streaky or scummy, and keep children and pets well away. If in doubt, treat the water as unsafe: a simple check is that a bloom clouds a glass of the water rather than settling out. Rinse off with clean water after swimming, and never let a dog drink from an affected shoreline. Follow local water-quality notices, which are updated through the bathing season.
Good to know
Dogs are the most common serious casualties, as they drink the water and lick their fur afterwards.
Other seasonal risks
Ticks (Lyme disease and TBE)
Ticks wait on grass and low vegetation in forests, meadows and gardens and attach to skin as you brush past.
Mosquitoes
Mosquitoes breed in still water and are at their fiercest around bogs, lakes and wet forest, particularly in the long northern twilight.
Forest and wildfire risk
After a run of dry days the forest floor, heath and peat dry out and ignite easily.
Pollen and hay fever
The northern pollen season is compressed and intense.
Thin ice
Ice on lakes, rivers and the sea forms and melts unevenly.
Source: Seasonal patterns follow standard public-health and safety guidance for the region; confirm with the current local advisory. · checked 2026-09-04
Frequently asked questions
When is blue-green algae (cyanobacteria) worst?
Warm, calm, sunny spells on lakes and the sea.
How do I reduce the risk from blue-green algae (cyanobacteria)?
Do not swim where the water is discoloured, streaky or scummy, and keep children and pets well away.
Enjoy the outdoors year round
TempoLife helps you keep an outdoor habit going safely through every season.