[rec.autos] effect of microwaves on animals Long but has a good reference.

ornitz@kodak.UUCP (Barry Ornitz) (03/31/89)

In article <2841@spdcc.SPDCC.COM>, eli@spdcc.COM (Steve Elias) writes:
> can anyone provide me with any interesting references or opinions
> regarding the effects of microwave radiation, such as that used in
> a microwave building security system?  on humans, that is...
 
In article <538@penguin.UUCP> jac@penguin.UUCP (James Carter) replies:
>When I was in the USAF/ANG going to tech school at Keesler AFB, we used
>to have "mock-ups" of the equipment we would be working on once we got
>out on the flight line. Several such pieces of equipment were radar
>transmitters (sic) that were at varying levels of power. It was not
>uncommon to have a wrist watch (the wind up kind back then) magnetized
>into oblivion by accident.

The magnetization has nothing to do with the microwaves but rather with the
magnetron tube used to produce high power microwaves.  It is the magnet
associated with the tube that caused the problem with the wrist watches.

Steve continues:
> is continuous exposure at 5 to 30 feet from such a transmitter 
> something to be concerned about?  what power level should building
> alarm systems transmit at ?  are these enough questions?

As Frank Reid mentioned in an earlier posting, the power level from door
openers and microwave intrusion alarms is rather low.  In the U.S., these
devices are designed to meet FCC specifications as Field Disturbance Sensors.
I am not familiar with all brands available, but those of Alpha Industries
should be representative.  Typical power levels are 4 to 5 milliwatts output
for the Gunn oscillators, although 10 mw to 100 mw oscillators are available.
Alpha make a sensor module specifically designed for non-contact motion
sensing that is already FCC certified.  It is licensed as a Part 15 device and
has its own horn antenna.  They state that any change in the antenna or any
change in the window (or any change that will affect the radiation pattern)
will require recertification of the device by the FCC.  This device operates
with less than about 5 milliwatts output.  [Some police radars operate at
significantly higher power levels, especially the older ones.  You are not
likely to be exposed to these at extremely close range, however.]

To answer the question: is this safe?, the best source to consult is the
American National Standard ANSI C95.1-1982 - Safety Levels with Respect to
Human Exposure to Radio Frequency Electromagnetic Fields, 300 kHz to 100 GHz.
At the 10.525 GHz frequency used by these field disturbance sensors, the
maximum allowed power density is 5 mw/cm**2 as averaged over a 6 minute period.
To calculate the effective safe distance from the microwave source, you need
to know both its power output and its radiation pattern.  Since these are not
known to Steve, we can make some reasonable estimates.

In one of the Eastman Chemicals Division fiber producing areas, we use an
instrument based on microwave absorption to measure the moisture content of
the fibers.  A power level of around 100 milliwatts is used, and the exposure 
is essentially from an open-ended waveguide.  Using a 2x safety factor, i.e.
200 mw output, and the radiation pattern for an open-ended waveguide, I was
able to calculate that for distances more than several inches (3 or 4, if my
memory serves) from the end of the guide, the power density was below the 
accepted standard.

Using this information, we can estimate Steve's situation.  We can probably
assume a power level of 5 milliwatts.  Furthermore, a horn antenna is usually
used with these sensors.  A liberal guess as to the horn's gain over an open-
ended waveguide might be 13 decibels (probably 10).  In the most favored
direction (straight out from the horn), the effect is to amplify the output to
around 100 milliwatts.  This is the same situation that I calculated earlier.
We can therefore say that these units are safe for distances over 4 inches or
so from their antenna.  I suspect that this estimate is too high since it
includes several safety factors; my guess with real units is that the minimum
safe distance is more likely less than two inches.

This brings up another point, however.  The cornea of the eye is the most
sensitive portion of the human body to microwave radiation.  This is due to the
fact that the cornea, having no blood circulation, has no good way to remove 
any heat generated in the cornea by microwave absorption.  Therefore, the rule
of thumb when working around microwaves is **NEVER** under any circumstances,
look down the barrel of a waveguide no matter how low a power is being used.
With the field disturbance sensors Steve is talking about, his exposure
should be quite safe.  I would still avoid looking directly into the horn of a
field disturbance sensor closer than a foot or so to be safe.

And James continues: 
>I have accidently gotten in front of an open wave guide on a medium
>power transmitter. I lost a patch of hair on the back of my head about
>the size of a silver dollar. It eventually grew back, but I haven't been
>near any open tubes since!

James was lucky it was the back of his head.  The effect of the microwaves is
to heat the area and cause something like a deep burn for high exposures.  In
James' case, the hair follicles were probably heated enough to cause the hair
loss, but not enough to destroy them.  High power microwaves require great
care in their safe use.  Unfortunately, I have seen too many military radar
installations where this care was lacking.  

James concludes:
>I can remember a safety lecture we had about the radomes on some aircraft
>developing a static charge, but I don't remember if it was from air/dust
>particles passing it in flight, or was from the microwave transmitters for
>the active radar systems.

Once again, the microwaves have nothing to do with this.

To go back to Steve's original questions about references on microwave effects
on animals, I would suggest the ANSI standard mentioned above.  This standard
has over seventy references on the effects of electromagnetic radiation on
behavior, cataracts, immunology, teratology (birth defects), thermoregulation,
biorythms, endocrinology, hematology, and on measurements, standards, dosimetry
and other aspects of electromagnetic radiation.  Most libraries should be able
to obtain a copy of this ANSI standard.  The address of the American National
Standards Institute is: 1430 Broadway, New York, NY  10018.

I hope this has answered some of Steve's questions and cleared up some of the
misconceptions.
                                         Barry
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