Have any outside sources (not the people who created or manufacture zappers) tested whether or not these things actually do kill bacteria, viruses, parasites, etc?
Yes. Dr. Thiel's zapper study showed that 95 percent of zapper users reported positive results.
>- Also, has an outside source tested whether or not it is truly safe to use? The report above addresses this to a small extent.
However, consider the following. There are over a million zapper users and there has only been one major problem reported to the FDA. Someone did not follow instructions and used the zapper with a pacemaker.
>- I'm looking for concrete proof of the zapper's safety and effectiveness
You mean like the FDA's approval for Bextra, Vioxx, etc :^)