This article is excellent, to the point, and absolutely supports the safety of zapping.
While they presented a basic presentation, they did not go into the important data.
The important factors in the equation are voltage, resistance, and current.
A long time back, when I first got into this, I contacted Underwriters Laboratory and passed them questions on electrical safety.
Since voltages below 12 volts DC as basically considered GRAS ( generally recognized as safe ) they do not normally do testing on these low voltage applications.
The reason behind this is as follows:
Suppose that a person was holding an electrode in their left hand which was wet with sea water ( most dangerous, closest to the heart ), placed the other electrode in a metal ( or plastic ) bucket of sea water ( highly conductive ), and stepped into the bucket, some might think that this was pure suicidal, but wait! What is the voltage and resistance.
The voltage being 10 volts DC, and the resistance from hand to foot in the salt water is above 1000 ohms. According to Ohm's Law, I=E/R or Current equals voltage divided by the resistance. In this case the current = 10/1000 or 0.01 amperes or 10 milliamperes. This is generally considered safe although it is a worst case scenario.
You see, a properly built zapper is designed to limit the current to below 10 milliamps even if the body resistance was zero.
This means that properly designed and built zappers are inherently safe. ( Some are not, caveat emptor ).
However, the article does not offer much insight into other important variables such as frequency.
One group of people who are highly familiar with the effects of frequency are the HAM radio operators. They know about frequencies and associated dangers. Cell Phone s and yes, even your wireless communication devices such as laptops and pads use frequencies in the GigaHertz range ( thousands of MegaHertz ) and these frequencies are thousands of times as dangerous as the audio frequencies ( AF ) and very low frequency ( VLF ) produced by zappers. Zappers are basically limited to about 1 MHz. So, if you put this stuff out to a HAM radio operator their response is likely to be 'So, what is the point?'
The fact is that 1 or 2 hours of Cell Phone exposure is more dangerous than a lifetime of zapping. Period! Flat out that much more dangerous.
So, if you want to find something technically condemning about zappers and zapping, best of luck to you.
Even a TENS unit is more than 10 times as dangerous as a zapper but they are FDA approved.