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It's the same way in chemical plants that handle H2S or other dangerous chemicals. I've always had a beard, but I'd have to shave it when installing or servicing our equipment in those plants.

Firefighters also aren't allowed to grow beards for the same reason.

WWI, not II

Beards went in and out of fashion a lot between the Renaissance and WWII.

Yeah, IIRC the Greeks were big into beards, while the Romans shave them most of the time.

I mean in between the renaissance and WWII the fashion has changed a lot.

They declined during the early modern period, took off after 1850, went out of fashion in the early 20th century, were common place in the 60's and 70's, and declined again until the recent revival this century.