A.9 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/.