Measurements and research from various sources tell us that
-1mA is the sensibility threshold.
-5mA to 10mA hurts.
-40mA causes heart disturbance.
-60mA to 100mA is lethal.
These are reactions of a healthy human body. Much lower currents could be lethal for persons with heart malfunctions, pacemakers or other implanted electronic devices.

The resistance of human skin is highly variable depending on several different variables, but the two main variables are whether the skin is clean or dirty. Clean skin often has a resistance of about 500 ohms. Dirty skin can have electrical resistances of up to several million ohms. 

resistance of dry skin is usually between 1,000-100,000 Ohms >> 10K ohm average

10 mega ohms is the average resistance used by Dr. Becker for a silver/platinum battery

zinc versus silver is four times greater voltage difference than is silver versus platinum

40 mega ohms >> zinc vs silver

300 to 1k ohm internal flesh >> 650 ohm average

20 K ohms / 650 ohms = 30.76923076923077 factor

40 mega ohms zinc/silver / 30.77 factor = 1.3 mega ohms for zinc/silver battery across two layers of skin plus internal flesh

33 mega ohms carbon/zinc / 30.77 = 1.07 mega ohms for zinc/carbon battery across two layers of skin plus internal flesh

Carbon = +0.52

Zinc = -0.7618

Platinum = +1.188

Silver = +0.7996

A - B = |C|

Platinum - Silver = 0.3884

Silver - Zinc = 1.5614

Carbon - Zinc = 1.2818