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Re: RF biological hazards? (fwd)



---------- Forwarded message ----------
Date: Mon, 4 May 1998 07:35:31 -0400 (EDT)
From: "Mark S. Rzeszotarski, Ph.D." <msr7-at-po.cwru.edu>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Re: RF biological hazards? (fwd)

Hello All:
R M Craven said in part:
<craven-at-globalnet.co.uk>
>I was reading a  IEEE conference proceeding recently, which showed
>that "skin depth" in people was several tens of cm at a few MHz,
>decreasing with frequency. I haven't got the reference to hand but >if
anybody wants me to find it, I'll do so.
>Basically, RF currents do not flow over the outer mm of human skin:
>they reach significant penetration depths <snip>
        Skin Depth specifically applies to conductors with uniform
conductivity across their cross section.  The human arm, however, is quite
anisotropic (nonuniform) when it comes to electrical conductivity.  Nerves
and blood vessels conduct electricity much better than muscles, fat and the
outer skin layer.  As a result, the heat produced by having electricity pass
through the body can cause substantial damage.  This may not be sensed by
our nervous system, which primarily detects that which is close to the skin
surface.  It's sort of like burning your retina with an arc welder - you
don't sense the damage caused by the ultraviolet light until after you have
received excessive amounts because your retina has no pain sensors built in.
        As the RF frequency increases, there is a tendancy for the current
to flow on the outside of the conductor via skin effect, and this is seen in
humans.  At very high voltages, the current also tends to distribute itself
at the surface of the conductor electrostatically, and these factors help
reduce the risks associated with drawing a spark from a small tesla coil.  
        My biggest concern is not the RF so much as having an arc pass
through my body and make its way to the primary, which has deadly 60 cycle
high voltage on it.  If you choose to draw arcs from your coils, be
especially careful to avoid the possibility of having an arc jump back to
the primary.  You family members will appreciate this.
Regards,
Mark S. Rzeszotarski, Ph.D.