(Lansing State Journal, January 8, 1992)
Question submitted by: M. Faraday of Okemos
Electricity is dangerous because of its disruptive effect on the life support organs of the human body. We characterize electricity by two quantities: voltage and current. Voltage is measured in units called volts and current is measured in units of amps. The primary factors determining the effect of electricity on the body are the amount and path of the current passing through the body.
Currents of less than 0.02 amp may produce sensations ranging from tingling to sharp pain. A more serious effect occurs if the current causes muscles to contract. A person touching a live wire with their outstretched hand may literally not be able to let go of the wire due to the current's effect on the muscles. Experienced electricians who must sometimes deal with wires which may be live always use the backs of their hands to move the wires; if a current were present, the contacting muscles would cause the hand to pull away from the wire. Currents from 0.03 to 0.07 amp will begin to impair the ability of the person to breathe.
The most dangerous range of currents is from 0.1 to about 0.2 amp. Currents in this range can cause death by initiating fibrillation (uncontrollable twitching) of the heart, which stops the regular flow of blood to the rest of the body. Currents much larger than 0.1 amp do NOT result in fibrillation and instead stop the heart completely. If the duration of the current is short, the heart will usually start to beat by itself after the current is removed.
The amount of current which passes through a body depends on the ratio of the voltage of the electricity to the resistance of your body. The smaller the resistance (or the larger the voltage), the more current passes through the body. As examples, an AA battery provides voltage of 1.5 volts, a car battery 12 volts and an electrical outlet 110 volts. The resistance (which is measured in ohms) of the human body can range from one hundred to one million ohms. Wet skin has a much lower resistance to current than dry skin. This is why electrical appliances warn against use while in the shower or bath; although the voltage of the appliance may not be sufficient to send large currents through a dry body, the same voltage may result in a very dangerous current in a wet body. If the resistance of a body is as low as 100 ohms, a voltage as small as 20 volts can lead to fatal currents.
The path of current through a body also determines the magnitude of the effect. Current entering a body wants to travel a path of least resistance and exit through the part of the body touching the ground. Because the heart is on the left side of the body, touching a live wire with your left hand means that the shortest path to the ground involves passing through your heart. If current travels through the right side of the body, the current is less likely to affect the heart.
Although electricity has simplified many aspects of our lives, care must be taken when using electrical appliances. According to the Lansing Safety Council, 714 people in the US - 24 of those in Michigan- were killed by accidents in the home involving electricity. Because the potential for damage depends on the size, path and duration of the current and the resistance of the body, all electricity should be treated with respect and care.