Millennia, actually. There is clear evidence that people understood the medical importance of copper in ancient Rome, and even some evidence of Bronze-age Egypt

Millennia, actually. There is clear evidence that people understood the medical importance of copper in ancient Rome, and even some evidence of Bronze-age Egypt and ancient India (which is to say it's clear copper was used in medicine, but it's unclear people had evidence it helped). That's 2000 years definitely, and 5000 years possibly. Prior to disinfectants, hospitals used a lot of copper. It only became a relic with industrial-grade disinfectants. That said, the timelines aren't such that it seems useful for use in masks. Viruses and bacteria survive on copper for hours, instead of days. There are a lot of uses of copper which aren't grounded in science and make no sense (superstitious nonsense). But copper for things people touch a lot? Doorknobs? Definitely makes sense.