13 - Laserfiche WebLink

Home Browse Search

III How do exposures by 14 different biological-validation workingQ D. DO BASE STATIONS COMPLY WITH to radio waves groups,each made up of experts in a specific field THE 1992 ANSI AND OTHER SAFETY from base stations of biology.The groups looked for study results that STANDARDS? compare to those indicated a measurable effect,regardless of whether permitted by applicable the power flux density required for that effect In short,yes.Gauging actual exposures from safety standards? caused measurable tissue warming(the so-called cellular base stations requires specialized equipment thermal effect).The most sensitive indicator and measurement techniques.'5 Measure-ments turned out to be behavioral.Laboratory animals can be time-consuming,but the techniques are ABase stations would modify their performance of trained tasks now well established.Participants at the Federal result in public when the absorbed dose of power was about four Focus Symposium shared their experiences measur- exposures well below warts per kilogram of body mass.A safety factor of ing actual signals in the vicinity of base stations. the limits permitted 10 for occupational exposures was then imposed. The consensus of the participants was that expo- not only by the 1992 The committee increased the safety margin sure levels from cellular base stations do not even ANSI standard, but another five-fold to arrive at the stricter limit for begin to approach ANSI limits except in the also by all other stan- uncontrolled exposures.14 immediate vicinity of energized transmitting dards under considera- antennas. tion by the FCC. As one government official stated at the Federal Focus National -- Symposium on Wireless FCC STATEMENTS ABOUT RF EXPOSURE FROM "worst-case"ground-level exposures near the-: Transmission Base BASE STATIONS typical cellular tower are on the order of 1 pW/cm1 Station Facilities the or less. Calculations corresponding estion of adverse The FCC's December 1994 "Information On Human to a "worst Exposure to Radiofrequency Fields From Cellular case"situation(all transmitters operating simu/ta alth effects fromRadio Transmitters"states the following about neously and continuously at the maximum licensed base stations is "almost base stations: power)show that in order to be exposed to levels a nonissue in terms of near the 1982 ANSI limits for cellular frequencies; actual exposure levels." The signal from a cellular base station antenna is an individual would essentially have to be in.the- essential/y directed toward the horizon in a tela- main transmitting beam(at the height of the tively narrow beam in the vertical plane.For exam antenna)and within a few feet of the antenna. This p/e,the radiation pattern for an omnidirectional makes it extremely unlikely that a member of the antenna might be compared to a thin doughnut or general public could be exposed to RF levels in.-: pancake centered around the antenna,while the excess of those guidelines. pattern for a sector[directional]antenna is fan- shaped,like a wedge cut from a pie.As with all Potential exposure can also be compared to the forms of electromagnetic energy,the power density more restrictive limits recommended by the from a cellular transmitter decreases rapidly National Council on Radiation Protection and (according to an inverse square law)as one moves Measurements(NCRP),the International Radiation away from the antenna. Consequently,normal Protection Association(IRPA),or the Institute of ground-level exposure is much less than exposure Electrical and Electronics Engineers(IEEE). The very close to the actual antenna.Measurements IEEE guidelines(IEEE/ANSI C95.1-1992)have been made near typical cellular towers have shown that recently adopted by ANSI to replace the 1982 ground-level power densities are well below limits guidelines mentioned above. The NCRP and ANSI/ recommended by RF/microwave safety experts. IEEE guidelines recommend a limit for exposure of At a frequency of 869 MHz(the lowest base station the general public(or exposure in "uncontrolled" frequency used),the RF protection guidelines of environments)of about 580 pW/cm2 at 869 MHz the American National Standards Institute(ANSI The corresponding IRPA recommendation is about C95.1-1982), which is used by the FCC and others, 435 pW/cm2 The exposure levels measured at recommend that human exposure should be limited ground level around typical cellular towers are to a power density of about 2900 micro watts per hundreds or thousands of times lower than the square centimeter(pW/cm2),as averaged over any above limits. The FCC has recently proposed • six-minute period. This limit is many times greater than RF levels found near the base of a typical adopting the new IEEE/ANSI guidelines for thepurposes of evaluating environmental RF fields cellular towers.Measurement data obtained from from transmitters such a cellular radio antennas. various sources have consistently indicated that 20