The link between BMAA (from toxic algae blooms) and neurodegenerative diseases in predisposed people is basically causal and proven at this point, but it's allegedly suppressed by Big Fishing and Aquaculture.
You may be right. There are many other components contributing to the problem in Finland, too.
So maybe Sauna is good, but where Sauna is popular is usually a place that is very cold and humid, which is not good.