Q & A: The Human Body’s Resistance

please tell value of human body resistance
- thiru (age 21)
india

Thiru -

There are a lot of factors involved and not every person has the same electrical resistance.  For instance, men tend to have lower resistance than women.  Just like for the resistors used in electronics, the resistance of a person's arm depends on the arm's length and diameter.  Resistance goes up with length and down with diameter.  Since men tend to have thicker arms and legs (more muscle), they usually have lower resistance.  (An implication of this is that the lethal current for men is higher than that for women.)  A rough value for the internal resistance of the human body is 300-1,000 Ohms.  Naturally, the resistance also depends on the path that electricity takes through the body - if the electricity goes in the left hand and out the right foot, then the resistance will be much higher than if it goes in and out of adjacent fingers. 

Within the body, the tissues with the greatest resistance are bone and fat - nerves and muscle have the least resistance.  That said, the majority of the body's resistance is in the skin - the dead, dry cells of the epidermis (the skin's outer layer) are very poor conductors.  Depending on the person, the resistance of dry skin is usually between 1,000-100,000 Ohms.  The skin's resistance is much lower if it is wet or burnt/blistered.  This means that when a person is electrocuted in real life, the body's resistance drops as the skin is burned.  To determine a person's total resistance, just add together the resistance of each part of the body - remember that the electricity must pass through the skin twice (on the way in and on the way out), so the total resistance is:

        R_total = R_skin(in) + R_internal + R_skin(out)

Another interesting point to consider is that in addition to acting like a resistor, the epidermis acts like a capacitor if placed in contact with a piece of metal (the underlying tissue is like one plate of a capacitor and the metal surface is like the other plate - the dry epidermis is the less-conductive material or "dielectric" in between) .  In cases of electrocution by a DC voltage source, this capacitive property has little importance.  But if the electrocution is by an AC source, the epidermis's natural resistance is "shorted out", allowing the current to bypass that part of the body's resistance and making the body's total resistance much lower.

-Tamara


Reference: R. Fish & L. Geddes, Medical and Bioengineering Aspects of Electrical Injuries, c2003 Lawyers & Judges Publishing Company, Inc.

(published on 10/22/2007)

Not a question, just a sincere THANK YOU for providing a clear and precise answer. I have come across several webpages trying to address this question with vague and sometimes ridiculuous answers. One other page in fact pointed out the AC/DC difference but did not explain the reason (epidermis as dieletric)- now it makes sense.
- Marco (age 32)
San Diego, USA

Thanks!
Mike W.

By the way, you can see the original post, it's # 6793.
LeeH

(published on 08/05/09)

do we get electricity shocks in periods when we touch high voltage AC source? also when dc current passes throgh us do we vibrate or experience shocks in period? in one of ur privious answers uve given that Rtotal = Rskin(in) + Rinternal + Rskin(out),i dont agree as u cannot consider as a series connection,it is a parallel one.
- raghavendra
bangalore,karnataka ,india

I think the previous answer is correct.  In an electrical shock the current first has to pass through the skin, then it has to pass through the body,  finally it has to pass through the skin again. That sounds like series to me. 

LeeH

(published on 08/25/10)

I know this will sound stupid but here it goes... Can the human body stop watches and laptops?
- Rosemary (age 37)
Dickinson, TX USA

I find it hard to believe a human body stopping a watch, unless the human hand dropped the watch from a high distance.  Stopping a laptop though is a different matter.  I once destroyed my laptop's wireless card on a cold winter's day by scuffling across a rug and touching it before discharging my self.  A big spark did it.    Other than static electricity I can't see other human body phenomena that would do it, except clumsiness.

LeeH

(published on 08/22/11)

How does the human body act as capacitor?
- aru (age 25)
chennai,tamil nadu,india

Electrical capacitance is defined by the amount of static charge on a body divided by the voltage of the body with respect to some reference ground.   For an isolated  spherical body of radius R with an accumulated charge Q the voltage with respect to a ground at infinity (or very far away compared to R), is V = Q/(4*pi*epsilon_o)/R   which results  in C = 4*pi*epsilon_o*R_.
For R =  1 meter  C  =  111 picoFarads.    Actual  measured values of human body capacitance (to distant ground) vary from 100 to 200 picoFarads.   By the way, epsilon_o is the experimentally measured permitivity of free space.

LeeH

(published on 12/13/11)

which is the most dangerous low voltage or high voltage?
- Ariel (age 17)
gensan.phil.

The danger of electrical shock depends mainly on the total current flowing through the body.  The current, according to Ohm's Law, is proportional to the voltage divided by the total resistance of the circuit.  This includes the skin resistance plus the body's internal resistance.  The skin resistance can vary by a factor of 100 or more depending whether the skin is dry or moist with salty sweat.   For the same value of skin resistance the higher the voltage, the more danger of severe shock.  For the same value of voltage, the wetter the skin contact, the more danger.   Take your pick.

I have marked this answer as a follow-up to question number 6793 that discusses some other aspects of electrical shocks.

LeeH

(published on 06/21/12)

Hey, thanks for your time I was in the lab, and was testing a 100k resistor. With the resistor in my hands, the multimeter read 47k ohlms. Didn't matter how hard or soft i held it. 3 others of my piers all read 87k olhms or greater. Why would there be such a difference?
- matt (age 29)
Canada

This sounds quite peculiar. What's particularly odd is that 47kΩ is a standard resistor value. Ignoring that, could your hands have been sweaty?

Mike W.

(published on 04/28/13)