Actually J... the term Mad As A Hatter was used to describe the advanced stages of Mercury poisoning brought on by the most popular fur treatment in the mid 1800's "carroting". Mercury Nitrate was brushed on to cheaper furs and smoothed out by hand, as cheaper furs didn't mat easily and were usually not uniformly dense. Mercury Nitrate poisoning causes a variety of fun symptoms like twitching uncontrollably, tooth and memory loss, slurred speech, depression and extreme anxiety. Finer furs, like beaver, have serrated edges, easily mat, and are generally more uniform in density, thus they don't require carroting. Only the finest hat makers could afford beaver fur however, and the rest were unfortunately driven mad, and eventually killed by their craft.
Oh yeah. I had probably heard that. Well, in any case, formaldehyde is used in fabric manufacturing, and you can feel its effects (I can, anyway) by wandering around in a fabric store for a while. I get a headache and start feeling dizzy within 15 minutes or so.