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Related:
Veritasium made a video about it.
TLDR: negative ions may have slight positive effects on general mental health but we can't produce negative ions without harmful waste gas (ozon). The best way to get your dose of negative ions is go outside, preferably close to waterfalls
It's interesting, some customs around the world talk about waterfalls and specifically spending time next to them or under them. I wonder if this could be an empirical basis for a local custom?