Human Exposure to RadioFrequency Radiation
I am not going to cover the pseudoscientific arguments of human
exposure to radio frequency radiation; I address
only the current ANSI limits as related to human exposure to radio
frequency fields. However, keep in mind that cellular telephone
companies have run into groups that are using this
pseudoscience to
delay or stop deployment of cell phone installations via city and
county governments.
Once 802.11 deployment gets more popular, these groups may have an
impact on your deployment. After all, they know what
"microwave ovens can do" and
802.11b runs at the same frequency.
The FCC's concern is:
At the present time there is no federally-mandated radio frequency
(RF) exposure standard. However, several non-government
organizations, such as the American National Standards Institute
(ANSI), the Institute of Electrical and Electronics Engineers Inc.
(IEEE), and the National Council on Radiation Protection and
Measurements (NCRP) have issued recommendations for human exposure to
RF electromagnetic fields.
[...]
On August 1, 1996, the Commission adopted the NCRP's
recommended Maximum Permissible Exposure limits for field strength
and power density for the transmitters operating at frequencies of
300 KHz to 100 GHz. In addition, the Commission adopted the specific
absorption rate (SAR) limits for devices operating within close
proximity to the body as specified within the ANSI/IEEE C95.1-1992
guidelines.(See Report and Order, FCC 96-326.) The
Commission's requirements are detailed in Parts 1
and 2 of the FCC's Rules and Regulations [47 C.F.R.
1.1307(b), 1.1310, 2.1091, 2.1093].
—From http://www.fcc.gov/oet/rfsafety
This breaks down to exposure limits for workers exposed around the
equipment and for the general public. At 2.45GHz, it is
4.08mW/cm2 for unlimited time exposures
for workers and 1.63mW/cm2 for 30 minutes
for the general public. As this energy is absorbed over time, you can
raise or lower the mW/cm2 for a controlled
situation by decreasing or increasing the time exposed. It would be
hard to regulate this in a wireless setup, so you
shouldn't apply any "time versus
exposure" calculation for the public.
The Office of Engineering and
Technology's (OET) Bulletin 65 (August 1997),
"Evaluating Compliance With FCC Guidelines for Human
Exposure to Radiofrequency Electromagnetic
Fields," (http://www.fcc.gov/oet/info/documents/bulletins/#65)
shows how to calculate these fields.
For example, a near-field calculation of a two-foot aperture dish
(24dBi) with 1/4 watt of power applied (maximum EIRP for
point-to-point) results in a one-foot area in front of the dish that
is considered "controlled," and a
two-foot area (also in front of the dish) where exposure to the
general public should be limited. You can comply with these
regulations by placing your dishes out of the way; say, above
"head height."
The FCC has a page that covers many of these issues at http://www.fcc.gov/oet/rfsafety/.
The FCC also has a page on "Cell Phone
Facts." This page is designed for end users of
RF-emitting equipment and it tries to demystify some of the concerns
about RF exposure. You can find the site at http://www.fda.gov/cellphones/.
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