How to calculate the safe AC frequency at high Voltages?
Humans in shoes have about 1,000 picoFarad capacintace to Ground, thru the shoes.

C = Eo * Er * AREA/dISTANCE 

C = 45PF/METER * Er * (10cm * 30cm) / 1cm * 1meter/100cm

C = 45e-12 * (Er = 5 for shoes) * 300cm * 1meter/100cm ~~~ 700 picoFarad

Now lets assume the frequency is 1MHz and the Voltage is 100,00 volts.

The displacement current, generated as electric fields change and charges rush around and back and forth to balance those fields, is

I = C * dV/dT

I = 700pF * [ 100,000 volts * d(1MHz)/dT  ]

I = 700pF * [100,000 * (2*pi * 1MHz)] = 700pF * 100,000 * 6.28e+6 radians/second

I = 700e-12 * 1e+5 * 6e+6 ~~~ 4,000 * 1e(-12 +5 +6) = 400 amps thru the point of entry of the arc, thru your body, and exiting your shoes.