Epsom salts - heavy metal contamination myth?
Here in the US, they sell different grades of Epsom salt, from USP down to plant food.  I'd presume that depending upon what the product will ultimately be used for, the manufacturer may use MgO with different purity levels - the lower the purity, the cheaper the product.  Even the "bath salts" grade contain trace amounts of arsenic and selenium (see the first link below).  Among the "medicinal" offerings, some are labeled "for external use" and others are labeled "for external and internal use."  I suspect that anything with the "for external and internal use" label would be more than safe enough to use in handwash.

Here are some links you might find helpful:

http://k.b5z.net/i/u/2182313/f/Tech%20Data%20Sheets/Tech_Data_Sheet-bath_salt-epsom_USP-3.pdf

http://www.drugs.com/otc/104235/pepsom-flying-p-epsom-salt.html