If you can't smell it, it can kill you

That's one of the strangest things about hydrogen sulfide (H2S), "rotten egg" gas, a familiar smell to visitors at hydrothermal basins like Yellowstone and Rotorua (New Zealand). In low atmospheric concentrations, it smells bad and alerts you that something might be rotten (Denmark and anywhere else, for that matter). But in higher concentrations, above 50 ppm if memory serves, it can't be smelt, and then it's toxic, deadly, suffocating.

And apparently, this is happening on French beaches where algae are decompositing!

Lethal algae take over beaches in northern France

"A man has also taken legal action after he was left seriously ill from breathing in fumes from the decomposing algae. Vincent Petit, a 27-year-old vet, had to be dragged unconscious from a patch of rotting algae a metre deep this month after the horse he was riding collapsed and died from fumes given off by the sludge on the beach. The horse died within minutes.

Last year, two dogs died while walking near piles of algae on a beach close by.

Local mayors said more than 70 beaches had been hit by the seaweed that has plagued the coastline for more than a decade, but which this year reached unprecedented levels."

I think it should be made clear that the algae themselves are not to blame here, except for doing what they do best, which is growing in eutrophic proliferation in the presence of sunlight and nutrients (supplied by agro-pollution), and then doing what comes naturally, which is dying, decompositing, and stinking. It's called anaerobic respiration, by the way, and that's what makes hydrogen sulfide. And just like anything taken to excess, too much H2S is a real bad thing.

Note: there are some blue-green algae that can be toxic, especially to pets, if water containing them is drunk, and of course, certain nasty dinoflagellates can get into the flesh of oysters and clams and other indiscriminate filter feeders, and concentrate enough neurotoxin in their flesh to kill anyone who eats the tainted shellfish.

Hey, if you want a neurotoxin buzz, there's always fugu.