Green film on the lake: dangerous or harmless? What cyanobacteria really do, and why the bathing threshold sits a thousand times higher than drinking water's.
The question that comes up every summer: is that greenish shimmer on the lake dangerous, or can I just jump in? The honest answer: it depends on the concentration — and the thresholds for that are more precisely defined than most people think.
That green film is usually cyanobacteria, commonly called blue-green algae. Some species produce toxins that can irritate skin and mucous membranes. Interestingly, the WHO guideline for bathing water sits at 1,000 micrograms of microcystin per liter, while drinking water is capped at just 1 microgram per liter — a thousandfold difference, because you swallow almost no water while swimming, whereas you drink liters of tap water.
So far, no confirmed poisoning cases from bathing have been documented in Germany or Europe — though cases in wild and domestic animals, which ingest far more water, have been recorded. Still, caution pays off: direct skin contact can trigger rashes, and swallowed water can cause gastrointestinal symptoms. Pregnant women, immunocompromised people and small children in particular should be careful around visible algae blooms.
Look for greenish-turquoise streaks, carpet-like buildup at the shore, or a water surface that looks like spilled paint. Clear water with just slight cloudiness is usually fine — a dense green film is the warning sign.
1,000 micrograms per liter for bathing versus 1 microgram for drinking seems contradictory at first. The difference comes down to intake: while swimming you occasionally swallow a mouthful, while drinking you take in liters daily.
Don't swim through dense green film, keep kids away from direct contact, and don't let anyone drink the water. Brief contact with clear water next to a bloom is far less critical than swimming straight through the film itself.
Even without visible algae: rinse skin and hair with clear water after swimming. That lowers the risk of skin irritation regardless of whether a bloom was visible that day or not.
Pregnant women, people with weakened immune systems and small children react more sensitively. With a visible bloom, choose a different body of water for these groups rather than hoping for the best.
No confirmed poisoning cases from bathing exist in Germany so far, but the symptoms from contact — skin irritation, gastrointestinal issues — can be real and unpleasant. They often appear only hours later, so stay alert if you suspect contact, especially with small children. Check your local health authority's current warnings before swimming.
Remember the "clarity to your knees" rule of thumb: if you can still clearly see your feet in knee-deep water, water quality is usually fine. If your feet blur even in shallow water, that's a sign of elevated turbidity or algae concentration — regardless of the official warning status.
By the visible concentration: dense, greenish-turquoise streaks or a paint-clump-like film are the warning sign. Light cloudiness without a recognizable film is usually unproblematic.
No confirmed poisoning cases from bathing have been documented in Germany or Europe so far. Still, skin and gastrointestinal irritation can occur from direct contact or swallowed water — especially in children.
Because the intake amount differs: while swimming you occasionally swallow a mouthful, while drinking tap water you take in liters daily. The WHO guideline accounts for exactly this difference in exposure.
With a dense green film: yes, especially at the shallow shore where kids swallow water more often. With just light, diffuse cloudiness and no visible film, caution makes sense but doesn't mean an automatic swimming ban.
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