parazapper
Which zapper, there are differences between the various models. And which mode, because the frequency makes a difference.
The important thing to know is that the lower the frequency, the more your nerves and muscles notice it. This is especially true at frequencies below 1000 Hz. That is one reason that Dr. Clark chose 30,000 Hz, because it could not be felt by the body although the voltage was just as strong.
So, if you are using 15 Hz, 20 Hz, 72, Hz, then you are going to notice the signal.
I have never had anyone report any nerve damage but I have heard of a very few cases from one zapper that is often plugged into the wall.
The tingle is normal at lower frequencies but the tingling and slight burning can be reduced by using tap water on the paper towels rather than strong salt water solution.
The zapper is safe and the output is weaker than a TENS unit.