dangers of elctric frequency?

It would have to be much higher than 100 Hz. You'd have to be at MHz and up and then it concentrates near the skin and causes burns there instead of instant electrocution. It known as the skin effect. Higher frequency signals concentrate at the surface of a conductor and do not penetrate.

On airplanes (military ones at least) power is provided at 400 Hz in order to save weight (transformers and motors are smaller for a given power rating). Believe me, there is no safety in this higher frequency. It can, and has, electrocuted workers just as well as 50 Hz or 60 Hz power.

Nikola Tesla generated high frequency, high voltage, currents that did no apparent harm to the human body, but they weren't practical as a way to deliver energy. He wanted to "beam" power through the air to the world without considering how to meter it or who would pay for its generation and transmission. Could only have worked if everything were free and involved no effort requiring compensation. As he found out by going bankrupt promoting his world power vision.

As for "skin effect," it is true that higher frequencies penetrate less into the surface of conductors, but that doesn't make them any safer if there is enough power (voltage and current) available to do damage.

It's the voltage (read "pressure") not frequency which causes skin penetration.

Additional info, it's current (read "amount") that kills.

It's true that higher frequencies demonstrate what is known as 'skin effect', which causes the current to flow along the outside of a conductor, but all electric current flows along the outside of a conductor. The higher freqs are just more susceptible to problems issuing from that.

That's not true at all!

In 1891, Tesla made the discovery of how high frequency electrical current can penetrate the body deeply, causing many beneficial effects that restored and revitalized him. (he tested the prototype machines on himself)

The machine that he called the "electrical massage" produced high frequency AC current in the 1- 1.5 MHz range that, when passed through the body, produced a warming effect that improved circulation and speeded healing of wounds.

Diathermy in modern times

http://en.wikipedia.org/wiki/Diathermy

Tesla: High Frequency

http://www.pbs.org/tesla/ll/ll_hifreq.html