Is electric field of capacitive sensing circuit harmful and is it radiation?
This type of capacitive sensing circuit works by changing the voltage applied to the sense pad, and calculating the time required for the pad to reach that voltage.  When you are near the pad, the capacitance is changed, thus changing the charge/discharge time.  

The Arduino Capsense library uses this technique.  

The voltage output is only as high as the MCU operating voltage (3.3V, 5V etc) and will vary in frequency depending on whether a touch is detected or not.  The capsense library has about 2s timeout period waiting for the pad to change states, then the process is repeated.  Even so, the maximum frequency couldn't be more than a couple MHz.  With oscillating input to a metal pad, there would be some radiated EMF.  However, this would be quite weak and harmless, but explains why you see an "alternating current" field.

Connect your oscilloscope and watch the pads being charged/discharged, or set up a counter in the capacitive sensing algorithm to output the approximate period, then you can calculate the frequency.

I see no way for this to produce high voltages or harmful levels.