The damage electricity does to the body is determined by the following:
1. The type and magnitude of the current.
2. The resistance of the body at the point of contact.
3. The pathway of the current.
4. The duration of the current flow.
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The type and magnitude of the current has a profound influence on the injury that is caused by an electrical shock. Direct current (DC) doesn't cycle (i.e. it doesn't change between positive and negative) and isn't as dangerous as alternating current (AC).
The effects of AC depend largely on the frequency and low frequency AC (50-60 Hz) is more dangerous than high frequency current. One has to wonder which bright spark decided that we need AC at 50 - 60 Hz in our houses!
DC tends to cause the muscles to contract and this often makes one pull away from the source. AC often causes muscle tetany and "freezes" the hand to the source, thereby increasing the time exposed to the electricity. The "let-go" current is the highest amperage at which you will still be able to pull your hand away when you receive a shock.
For DC this is about 75 milliamperes (mA) and for AC it's about 15 mA depending on the person's muscle mass (the higher the muscle mass, the higher the cut-off amperage).
Altered function Generally speaking, the higher the voltage and the amperage, the more dangerous the current. This is true for both DC and AC and they both alter physiological function and cause physical damage.
Altered physiological function includes involuntary muscle contractions, seizures, ventricular fibrillation (the heart beats so fast that it doesn't pump any blood), denaturation of proteins, blood clotting problems, dehydration, and cardiac and respiratory arrest.
Physical damage includes burn wounds, tissue necrosis, fractures, and torn muscles and tendons. The body resistance is measured in ohms/cm2 and is basically determined by the thickness of the skin. Thick calloused skin like the soles may have a resistance as high as 2-3 million ohms/cm2, normal skin's resistance is typically 20-30 thousand ohms/cm2 and wet thin skin 500 ohms/cm2.
If the skin is punctured (e.g. cuts, needles) or the current is applied to a mucous membrane (e.g. mouth, rectum, vagina), the resistance may be as low as 200-300 ohms/cm2. If the resistance is high, large surface damage may occur with little deeper damage. It the resistance is low, the person may suffer little if any skin damage, but may still die from cardiac arrest if the current reaches the heart.
When a radio falls into a bathtub, a person will die of cardiac arrest without suffering any burn wounds. Simplified, the higher the resistance, the more heat is generated in the skin and the lower the chance of deep damage and vice versa.
Pathway through the body
The pathway of the current through the body is crucial in determining the injury. If the current passes through the heart (head/hand/foot to hand/foot) it may be fatal. 220v at 60Hz AC travelling through the chest for a fraction of a second may induce ventricular fibrillation at currents as low as 60-100mA (300-500mA if DC).
If the current has a direct pathway to the heart like in the case of a pacemaker, less than 1mA is needed (AC or DC). The most common entry point for electricity is the hand, followed by the head. The most common exit is the foot. The duration of the current flow, just like the duration of the exposure, is directly proportional to the amount of injury to the body (the heart being the exception as small currents at low voltages may be fatal).
The longer the exposure, the more heat is generated and the deeper the electricity penetrates the body. Lighting is a good example, because in spite of the high voltage, very little burn wounds are caused because of the very short duration of flow.
It short-circuits all electrical systems and that's why people look normal on the outside but are confused/unconscious and then drop down (cardiac arrest). Well there you have it - the ABC of the physics of electricity.
Now go and impress your friends, but please don't let anyone tell you that an electrical shock is good for a healthy person. - Dr Bram van Niekerk, Cyberdoc
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