10 Deadly Things People Used To Think Were Harmless

By Peter Austin / February 5, 2016

3. Hatting

€œMad as a hatter€ is a phrase that was first used in the mid-1800s, denoting the fact that someone is a little bit loopy. But what does it actually mean?

Well, it's actually all got to do with mercury, a chemical famous for two things; being in thermometers and being really bad for you if it ever gets out of the thermometer.

Sadly, that second little detail wasn€™t discovered until well after 17th century hat makers began using it to make felt from animal fur. The vapours they were periodically exposed to led to muscle tremors, delirium, shyness and eventual dementia, hence the phrase and the fact that we hardly use mercury for anything anymore.