Electric current damages the body in three ways:
It's the electrical current that does the damage. Current equals voltage divided by resistance (I = V/R), but voltage is not a reliable indication of danger because the body's resistance varies so widely that it is impossible to predict how much current will flow through the body for a given voltage.
The actual resistance of the body varies depending upon the condition of the skin (moist or dry) at the points of contact . The skin resistance may vary from 1000 ohms for wet skin to over 500,000 ohms for dry skin. However, once the skin is broken through (for example, by the burning away of skin or by a wire piercing the skin) the body presents no more than 500 ohms resistance to the current.
The path through the body has much to do with the shock danger. A current passing from finger to elbow through the arm may produce only a painful shock, but that same current passing from hand to foot or through the chest from hand to hand may well be fatal. Therefore, the practice of using only one hand (keeping one hand behind your back) while working on high-voltage circuits is a good safety habit. Even better would be to disconnect all sources of power from the equipment you are about to repair. Do not rely on insulated tool handles, rubber-soled shoes, etc., to protect you.
A.C. is said to be four to five times more dangerous than D.C. For one thing, A.C. causes more severe muscular contractions. For another, it stimulates sweating, which lowers the skin resistance. Along those lines, it is important to note that resistance goes down rapidly with continued contact. The sweating and the burning away of the skin oils and even the skin itself account for this. That is why it's extremely important to free the victim from contact with the current as quickly as possible (but without endangering yourself) before the climbing current reaches the fibrillation-inducing level.
The frequency of the AC has a lot to do with the effect on the human body. Unfortunately, 60 cycles is in the most harmful range. At this frequency, as little as 25 volts can kill. On the other hand, people have withstood 40,000 volts at a frequency of a million cycles/sec or so without fatal effects.