Why do coins smell so bad?

By Alan Chenkin, Novice coin collector (numismatist) and hobbyist
The bane of any coin collector is the smell that can haunt some of your coins.  No one wants smelly coins, but there must be a way to manage it.  Not wanting to suggest you Fabreeze your coins (Don’t do it), I did some digging on the web:
Coins Don’t Smell, You Do – Live Science
There also may be a reaction between the coins and your hands that creates an odor, related to the conductivity of the coins and their PH[1] .
Best suggestions here are to wear gloves, keep the room well ventilated, and wash your hands before and after. In the “Old days” of coin operated slot machines, your hands would be black from the dirt on the coins, and every casino would have a cup of moist hand towelettes for cleaning it off your hands.
Don’t use this with rare of collectible coins! –  EZEST Coin Cleaner Although it is ok to clean dirty pocket change or novelty (not highly collectible) coins.  Cleaning collectible or rare coins will actually diminish their value.  If you want to clean coins without harsh chemicals, consider an ultrasonic coin cleaning kit, under $75 at Amazon, Or from eBay.
I use these when handling coins in my collection:
If you can’t wait for the gloves to arrive, use a special coin cleaning tool, like this:
old sock for coin cleaning
Also known as an old sweatsock.  usually there are a few near the dryer, that you can recommission as coin wipers.
And is the smell is persistent, try a little aromatic spray:
This old method may work for you too….
(Image from Shutterstock)
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Footnotes
Edited from my Quora answer, June 19, 2017