It's important to change the paper toweling after each zapping otherwise you will drive copper into your system via your skin as it comes through and concentrates in the toweling.
The discoloration of the toweling is copper ions being driven into the paper towel due to current. This has also fueled my concern about zapping with a mouth full of metal/mercury. The same process can happen there but to a less extent because of several factors, one being proximity. I zap on my feet and abdomen. I contacted Hulda's people about this concern and they said they haven't done any tests one way or another regarding this but sugested to zap on my feet to be sure.