HETA 95–0177–2617 Federal Occupational Health Seattle ...HETA 95–0177–2617 Federal...

HETA 95–0177–2617Federal Occupational Health

Seattle, Washington

C. Eugene MossEric J. Esswein

This Health Hazard Evaluation (HHE) report and any recommendations made herein are for the specific facility evaluated and may not be universally applicable. Any recommendations made are not to be considered as final statements of NIOSH policy or of any agency or individual involved. Additional HHE reports are available at http://www.cdc.gov/niosh/hhe/reports



This Health Hazard Evaluation (HHE) report and any recommendations made herein are for the specific facility evaluated and may not be universally applicable. Any recommendations made are not to be considered as final statements of NIOSH policy or of any agency or individual involved.


applicable. Any recommendations made are not to be considered as final statements of NIOSH policy or of any agency or individual involved. Additional HHE reports are available at http://www.cdc.gov/niosh/hhe/reports

http://www.cdc.gov/niosh/hhe/reports



ii

PREFACEThe Hazard Evaluations and Technical Assistance Branch of NIOSH conducts field investigations of possiblehealth hazards in the workplace. These investigations are conducted under the authority of Section 20(a)(6)of the Occupational Safety and Health Act of 1970, 29 U.S.C. 669(a)(6) which authorizes the Secretary ofHealth and Human Services, following a written request from any employer or authorized representative ofemployees, to determine whether any substance normally found in the place of employment has potentiallytoxic effects in such concentrations as used or found.

The Hazard Evaluations and Technical Assistance Branch also provide, upon request, technical andconsultative assistance to Federal, State, and local agencies; labor; industry; and other groups or individualsto control occupational health hazards and to prevent related trauma and disease. Mention of company namesor products does not constitute endorsement by the National Institute for Occupational Safety and Health.

ACKNOWLEDGMENTS AND AVAILABILITY OF REPORTThis report was prepared by C. Eugene Moss and Eric J. Esswein, of the Hazard Evaluations and TechnicalAssistance Branch, Division of Surveillance, Hazard Evaluations and Field Studies (DSHEFS). Desktoppublishing by Ellen E. Blythe.

Copies of this report have been sent to the Corps of Engineers and management representatives at FOH andthe OSHA Regional Office. This report is not copyrighted and may be freely reproduced. Single copies willbe available for a period of three years from the date of this report. To expedite your request, include aself–addressed mailing label along with your written request to:

NIOSH Publications Office4676 Columbia ParkwayCincinnati, Ohio 45226

800–356–4674

After this time, copies may be purchased from the National Technical Information Service (NTIS) at 5825Port Royal Road, Springfield, Virginia 22161. Information regarding the NTIS stock number may beobtained from the NIOSH Publications Office at the Cincinnati address.

For the purpose of informing affected employees, copies of this report shall beposted by the employer in a prominent place accessible to the employees for aperiod of 30 calendar days.

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Health Hazard Evaluation Report 95–0177–2617Federal Occupational Health

Seattle, WashingtonDecember 1996

C. Eugene MossEric J. Esswein

SUMMARYOn March 2, 1995, the National Institute for Occupational Safety and Health (NIOSH) received a requestfrom the Federal Occupational Health (FOH) office in Seattle, Washington, for technical assistance inassessing exposures to microwave (MW) and radiofrequency (RF) radiation at eight U. S. Army Corps ofEngineers (COE) field stations in Washington, Idaho, and Montana. Measurements were performed onMW/RF transmitting sources used in the radio communication network during June 12–20, 1995.

Analysis of the data logged measurements made on towers suggests that COE maintenance workers are notexposed to MW radiation in the 1.7 to 1.8 gigahertz (GHz) region. However, exposure above occupationalguidelines can occur from RF sources located either on the COE towers or on adjacent non-COE towers,operating mainly in the 100 to 300 megahertz (MHz) region. Occupational exposure to MW/RF sourceslocated in or around equipment buildings appear to be below guideline values. Workers who climb towerswill need to be trained on potential MW/RF exposures and techniques to reduce exposures.

NIOSH investigators determined that COE maintenance workers who climb towers are exposed toelectric and magnetic fields that can be in excess of applicable occupational exposure limits.Recommendations for lowering exposures are offered at the end of this report.

Keywords: SIC 4899 (Communication Services, Not Elsewhere Classified) Electromagnetic fields,Radiofrequency, Microwave, safety, MW, RF

TABLE OF CONTENTS

Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii

Acknowledgments and Availability of Report . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Evaluation Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4Microwave/Radiofrequency Radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4Body Currents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5Adams Ridge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5Creston Butte . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5Mt. Spokane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6Albani Falls Dam . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6Black Mountain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7Libby Dam . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7King Mountain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8Tony Mountain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8Tower Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8Site Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Health Hazard Evaluation Report No. 95–0177

INTRODUCTIONOn March 2, 1995, the National Institute forOccupational Safety and Health (NIOSH)received a request from the Federal OccupationalHealth (FOH) office in Seattle, Washington, fortechnical assistance in assessing exposures tomicrowave (MW) and radiofrequency (RF)radiation at eight U. S. Army Corps of Engineers(COE) field stations in Washington, Idaho, andMontana during June 12-20, 1995. Theseradiation measurements were performed toevaluate potential occupational exposure to COEpersonnel from MW/RF transmitting sources usedin the radio communication network. The FOHoffice in Seattle provides occupational healthconsulting services to the COE Seattle district andhad asked NIOSH to assist in evaluatingoccupational MW/RF radiation exposures amongworkers servicing the network.

BACKGROUNDThe Seattle district radio communication networkis made up of 16 stations located in Washington,Idaho, and Montana to maintain communicationamong the COE–operated dams that control waterand generate electricity in the three state areas.The network operates in the frequency regionfrom 1.715 to 1.828 Gigahertz (GHz) at a powerlevel of 1 Watt (W). The transmitter operates ina continuous mode and is always on, but the useof the network is quite intermittent. The COEantennas on top of the towers are directional, andworkers do not normally operate in the beam path.

Each network station consists of a tower and asmall, adjacent equipment building housingelectronic components and accessories to operatethe transmitter and antenna. There are at least twoCOE antennas located on each metal tower,typically 50 to 60 feet above the ground; one isused to transmit, the other to receive. At someCOE stations, there are other transmitting sourcesmounted on the COE tower operated either by

city, state, COE, or other federal agencies (a“shared” tower configuration). Transmittingsources which are in a shared configurationoperate at frequencies and power levels differentfrom those used by the COE network. There arealso COE stations which have equipmentbuildings and towers owned by non–COEagencies (a “non–shared” tower configuration).These non–shared towers have transmittingsources which operate at other frequencies andpower levels than the COE sources. In manycases these non–shared towers are located next tothe COE towers, creating a potential for radiationemissions to be incident on workers climbingCOE towers.

The COE towers are typically serviced at leastonce per year. During a typical service period,two workers determine communication statusbetween two adjacent stations. After establishingcontact and verifying communication andelectronic compatibility, one of the two workersmoves to another site, and the process is repeateduntil all of the stations are serviced. While mostof the maintenance/service work occurs in theequipment building, it may be necessary to realignthe antenna or to check and confirm electrical andmechanical connections on the tower. If it isnecessary to climb the tower, the worker dons asafety climbing belt and an ascending device,notifies the other worker on the radio, and ascendsthe tower.

METHODSAt each of the eight stations, radiationmeasurements were performed in the equipmentbuilding, at the base of the tower, and during a sixto nine–minute tower climb. The climber wore aback pack containing battery–powered MW/RFmeters, and a datalogger which stored radiationexposure data as a function of time. The climberwould come to the tower base where the NIOSHinvestigator would activate the meters anddatalogger in the back pack. Upon completion ofthe climb, the system would be deactivated, the

Health Hazard Evaluation Report No. 95–0177 Page 3

climber’s back pack removed, and the stored datadown–loaded to a portable laptop computer. Eachclimb was videotaped, and the timer on thevideotape was compared with the data loggedfrom the MW/RF meters to determine locationswhere highest levels of radiation were measured.

MW/RF exposures can also occur to maintenanceworkers from “leaks” in transmitter cabinets,conduits, and transmission cables. Theselocalized exposures exist on the tower from cablesleading to the transmitting sources. Such hot–spotreadings may be shown on time–intensity figuresas peaks lasting less than five seconds and canoccur whenever the probes come into contact withthese spots.

MW/RF radiation levels were measured using thefollowing equipment:

< A Holaday Model 3002 survey meter usingtwo probes: a model STE for the electric (E) fieldand a model STH for the magnetic (H) field. TheE–field probe is designed to cover the frequencyrange from 0.5 to 6000 megahertz (MHz) andmeasures the E–field strength in units of voltssquared per meter squared (V/m)2. The lowestmeter indicating level (LMIL) for thisprobe–meter combination system is 500 (V/m)2.The H–field probe is designed to cover thefrequency range from 5 to 300 MHz and measuresthe H–field strength in units of amperes squaredper meter squared (A/m)2. The LMIL for thisprobe–meter combination system is 0.005 (A/m)2.

< A Narda electromagnetic radiation monitormodel 8616 connected to either a Narda H–fieldisotropic probe model 8633 (10 to 300 MHz) or anE–field isotropic probe model 8621B (0.3 to40 gigahertz [GHz]). Both field probes, whenconnected to the monitor, measure field intensitiesin milliwatts per square centimeter (mW/cm2)over their respective frequency region. The LMILis 0.05 mW/cm2 for the 8616/8633 system and0.01 mW/cm2 for the 8616/8621B systems.

< A Metrosonics Industrial HygieneDatalogging System was used to collect the datagenerated by the MW/RF measurement systems.The Metrosonics system includes the modeldl–3200 datalogger and the ms–3200 MetrosoftSoftware. The menu–driven software controlsand sets up the datalogger. The battery–operateddatalogger uses a serial RS–232 communicationformat that collects data at one sample per second.The system can provide statistics for minimum,average, and maximum values. Data from thesystem was exported to a softwear package toobtain selected time–intensity figures for thisevaluation.

< Body currents resulting from exposure toE–fields in the RF frequency region (i.e.,frequencies below 300 MHz) were evaluatedusing a body current detector system. This systemis based on the principle that when RF energy isabsorbed, electric currents are induced within thebody. These body currents can be measured byusing a foot current sensor designed to respondonly to currents induced by external E–fields.The body currents were measured by having theworker stand on a 6–millimeter thick, 32 by32–centimeter polyethylene sheet clad on bothsides with copper. The current from the upperplate, where the worker stands, passes to the lowercopper plate, which is in contact with the groundthrough a non–inductive carbon resistor located inthe center of the bi–layered sensor. The RFcurrent across the resistor is measured with acalibrated RF milliammeter. All currentmeasurements were made with the workerstanding on the sensor either in front, or under, thetower structure. The sensor could also be placedin direct contact with the tower structure as ameans to determine the current induced onto thetower structure (i.e., contact current) fromexposure to either shared or non–shared sources.When used in this contact current mode, bodycurrents could not be recorded. The presence ofcontact currents merely confirms that currents arebeing induced on the given tower from some fieldin the immediate vicinity.


< The source frequencies were measured usingan Optoequipment Handi–Counter Model 3000battery–powered frequency counter. Knowledgeof the frequency is critical in evaluating MW/RFexposure relative to occupational standards sincethe guidelines are frequency dependent.

Workers who climb the tower are not normallyexposed to the radiation produced by the COEnetwork, since the transmitted signal is aimedaway from the tower. Radiation exposure couldoccur, however, from other COE or non–COEsources mounted on the COE tower, or fromadjacent “non–shared” towers. Furthermore, itwas not known for how long any source on anytower (“shared” or “non–shared”) would transmit.Unfortunately, the NIOSH investigators did notutilize a frequency analyzer that would haveprovided information about what signals(frequencies) are produced at a given time.Therefore, precise knowledge of the frequenciesfrom all available sources present in and aroundthe tower and during the actual climb is notavailable for this evaluation. What is known,however, is the dominant frequency observed bythe frequency counter at a given time of the climb.It is those frequencies which are used to compareexposures with occupational criteria.

EVALUATION CRITERIA

Microwave/RadiofrequencyRadiationAbsorption of MW/RF energy can adverselyaffect a worker’s health. Human and animalstudies indicate that this type of radiation cancause harmful biological effects due to excessiveheating of body tissues. MW/RF radiation canpenetrate the body and cause heating of internaltissues. The body’s heat sensors are located in theskin and do not readily sense heating deep withinthe body. Therefore, workers may absorb largeamounts of radiation without being immediatelyaware of the presence of such energy. There have

been reports that workers exposed to MW/RFfields from radar equipment, MW/RF heaters andsealers, and radio/TV towers have experienced awarming sensation some time after being exposed.

Many of the observed biological effects ofexposure to MW/RF radiation can be attributed toa rise in body temperature. The heating effect ofMW/RF within the body depends on the amountof energy absorbed by the skin. The rate ofabsorption, denoted as the specific absorption rate(SAR), is measured in watts per kilogram (W/kg)for the whole body or parts of the body. The SARdepends on many factors such as the frequencyand intensity of the radiation, size and shape ofthe exposed worker, and the worker's orientationin the radiation field. The human body absorbsmaximally in the frequency range of 30 to 300MHz. Outside this range, much less energy isabsorbed by the body from the radiation field.

The most influential standard for occupationalexposure to MW/RF radiation is the Institute ofElectrical and Equipment Engineers (IEEE)standard published by the American NationalStandards Institute (ANSI) and known as ANSIC95–1991. The IEEE committee concluded thata SAR of 4 W/kg represents the thresholdabsorption level above which adverse healtheffects may arise as body temperature increases.A safety factor of 10 was then applied to reducethis SAR to 0.4 W/kg as the maximum permissibleexposure limit, averaged over the entire body.The standard uses dosimetry measurements ofMW radiation to calculate the power density limitnecessary to achieve an SAR of 0.4 W/kg whenaveraged over a six–minute period. Table 1shows the maximum permissible exposures(MPEs) presently considered safe by the IEEE asa function of frequency.

The Occupational Safety and HealthAdministration (OSHA) has a radiation protectionguide (defined as the radiation level which shouldnot be exceeded without careful considerations ofthe reasons for doing so) of 10 mW/cm2 averagedover any possible six–minute period (29 CFR


1910.97 [1991]). This standard is applicable forfar field measurements and is not useful inevaluating near field exposure scenarios, whichare of concern in this evaluation.

Body CurrentsIn addition to E– and H– field exposure limits, theIEEE C95–1991 committee has adopted a bodycurrent limit of 200 milliamperes (mA) asmeasured through both feet. This value of200 mA limits the partial body SAR to levels lessthan 20 W/kg in the extremities and protectsagainst electrical shocks and burns.

RESULTSData was collected in two formats. The firstformat was a walk–around data collection modeand was obtained by measuring incident E– andH–field intensities at various locations in andaround the towers, including the equipmentbuildings. Approximately 200 measurements ofthis kind were made at the eight stations.

The second collection format was denoted“data–logged results,” and consisted of 26 eventsat the eight stations. The MPEs for the IEEEC95–1991 standard refers to values averaged overany six–minute period for frequencies less than15 GHz. The data–logged results obtained at thestations were used to determine the maximumsix–minute average for both electric and magneticfields, and compared to the MPE levels.

Adams RidgeThis station is located about 120 miles west ofSpokane and sits on approximately 1 acre that issurrounded by an 8' high locked fence areaadjacent to a dirt country road. Measurementswere performed on June 12–13, 1995. The stationis located next to a farmer’s equipment barn thatcontains tractors and typical farming equipment.

There are two transmitting sources on thisproperty, the COE microwave network and aDepartment of Defense (DOD) 1000 W HighFrequency (HF) single sideband system thatoperates within the frequency region from 2 to30 MHz. Although the COE personnel do notservice or come in contact with the DOD system,measurements were performed on the systembecause of the possibility of it contributing tooccupational exposure to COE workers whenclimbing the tower. All measurements made inthe equipment building were below LMIL levelsfor frequencies at the station. The farmer did notreport any electromagnetic interference (EMI)problems and EMF levels in the adjacent farmbuilding were all below LMIL.

Data recorded from tower climbE–field ranges: 21.2 to 3250 (V/m)2 during a6:30 minute (min) climbH–field ranges: 0.002 to 0.05 (A/m)2 duringan 8:00 min climbMaximum body current: 1 mA at tower baseduring both climbsMeasurements made on DOD CommunicationSystem: 2 to 30 MHz, 1000 WAt 10 feet (on ground) from the center ofoverhead line: 1 x 103 (V/m)2

At feed point 6 feet above ground: 2 x 105

(A/m)2

Tower contact current 12 feet above ground:19 mA

Figure 1 is a time-intensity plot of the E–fieldmade at Adams Ridge showing the variation offield strength with time during the climb on theCOE tower. It is possible that the DOD systemhad some influence on the E–field exposureswhile climbing the COE tower. Figure 1 indicatesmore activity at the beginning and end of theclimb than in the middle. The video tape indicatesthat the climber was on top of the tower around9:34 and stayed there until 9:36 when the descentbegan. Since the height of the DODcommunication system was lower than the COEtower, this increased activity may be due to theDOD source, or may be due to hot–spots, as


explained earlier. The H–field time-intensity plot(Figure 2) was not as active as the E–field.

Creston ButteThis station is located about 40 miles fromSpokane and contains a multitude of transmittingsources and towers. The station is located on asmall rise and is accessible by a small dirt road.Measurements were made at this station on June13, 1995, around noon. The entire area issurrounded by a fence and the gates to the sitewere locked. The equipment building used by theCOE for their tower is an old underground bombshelter located approximately 60 feet from thetower. An attempt was made to documenttransmitting frequencies in the vicinity of thetower, but the only frequency clearly detected wasa police band at 155.24 MHz which COEpersonnel thought was at 100–110 W. However,it is probably safe to assume that otherfrequencies were present but were not transmittingduring the time of the NIOSH evaluation, whichwas at 12:30 to 1:15 p.m. Measurements made atthe tower base and inside the building were allbelow LMIL. Data recorded from tower climb

E–Field ranges: 20.3 to 8.7 x 104

(V/m)2 during a 7 min climbMaximum body current at tower base: 3 mA

The data logged results, shown in Figure 3,indicate that E–fields were detected only near theend of the climb. Frequency measurements madeby NIOSH investigators suggest that these burstsmay have been caused by a source radiating near155 MHz.

Mt. SpokaneThis station is located on top of Mt. Spokaneapproximately 20 miles north of Spokane. Thereare many buildings and antennae located near theCOE site. The two story COE equipment buildingand tower were built in 1971. Extensivemeasurements were made on June 14 and June 20,1995, both inside the building and on the tower.

Measurements made outside the building andaround other buildings in the immediate area wereall below LMIL. The COE tower had threeradiating sources: a 1.7–1.8 GHz (COE network),163.45 MHz (COE communication system), anda 169 MHz (Federal Bureau of Investigation[FBI]) source).

Data recorded inside the equipment buildingE–field ranges (1st floor): 0 to 29.2 (V/m)2

during eight different runs E–field ranges (2nd floor): 0 to 1.06 x 104

(V/m)2 during one run for one hourH–field ranges (2nd floor): 0 to 2.3 x 10-4

(A/m)2 during one run for one hourMaximum body current on 1st floor: 0 mA Data recorded from tower climbE–field ranges: 0 to 1.49 x 105 (V/m)2 duringa 9.5 min climbH–field ranges: 1.2 x 10-3 to 0.018(A/m)2 during a 8.2 min climbMaximum contact current: 430 mA

The maximum E–field measurement inside thebuilding, 1.06 x 104 (V/m)2 (on the second floor),occurred at the end of the run; and the source ofthat field is unknown. Additional measurementswere taken immediately after the data loggedresults were obtained, but no reading greater than25 (V/m)2 was obtained. The NIOSHinvestigators were not able to explain this highinside reading.

Figure 4 indicates that there was consistentMW/RF activity associated with the climb onJune 14, 1995. This activity was probably due toeither the 162.45 or the 169 MHz sources (sharedsources) or from other, unknown sources onadjacent towers (non–shared sources). Duringthis climb, contact currents as high as 430 mAwere observed. The E–field data logged resultson June 20 were very high and support the needthat workers who climb towers at transmittingfacilities, such as found at Mt. Spokane, need tobe aware of the real potential for exposure.


Albani Falls DamSince this site had one of the highest COE towersin the network, it was decided to have the climberwear two monitors and detectors (one for theE–Field and the other for the H–Field) so thatonly one climb would be necessary.Measurements were made at this station on June15, 1995. Since there were no other towers in theimmediate area, any EMFs detected would have tobe related to sources on the towers. The sourcesidentified were the 1.7–1.8 GHz COE network,the 163.45 MHz communication system, and the109 MHz Dam communication frequency. Duringa portion of the approximate 18 min climb, theprobes and detectors were held outside of themetal tower structure to determine if it made anydifference in detecting radiation (it did not). Allradiation levels inside the equipment buildingswere below LMIL. The COE frequency of 163.45MHz was not on during the climb, but after theclimb was completed it was activated andmeasurements were made near the tower base byutilizing both the contact current meter and thefrequency counter. The contact current meterdisplayed 35 mA.

Data recorded from tower climb E–field ranges: 142 to 3600 (V/m)2 during a18:00 min climbH–field ranges: 9 x 10-4 to 4.9 x 10-3 (A/m)2

during a 18:00 min climbMaximum body current at tower base duringclimbs: 5 mA

The E and H fields were very low. The bulk ofthe E–field activity, as shown in Figure 5,occurred in 6 peaks, each lasting one to 2 seconds,while the H–field activity was limited to just onepeak. The most dominant frequencies observedwere either the 109 or 153.45 MHz signal. Sincethe climbing tower rungs became compressed withheight, the climber had extreme difficulty inclimbing in tight quarters with a backpack havingprobes protruding from the pack. The video tapesshow that the probes were trapped in metalstructures on the tower resulting in hot–spotindications.

Black MountainMeasurements were made at this site on June 16,1995. This site, located at the top of a mountain,had several towers located close to the COE towerwith approximately 10 transmitting sources. Thesources identified as being shared on the COEtower (in addition to the 1.7–1.8 GHz networksource) were a 160 MHz source operated by theUnion Pacific Railroad, 172.5 MHz Border Patrol(BPA) source, and several State of Idahofrequencies.

Data recorded from tower climb E–field ranges: 10 to 1190 (V/m)2 during a9:25 min climbH–field ranges: 0.0 to 0.36 (A/m)2 during a9:20 min climbMaximum body current near tower base for atleast 2 minutes during climbs: 70 mA

Figure 6 shows the time-intensity plot for theE–field. Both the electric and magnetic field timeintensity plots were quite active during the climbtime. It was estimated, using the frequencycounter, that the 160 and 172.5 MHz sources wereprobably responsible for this activity during theclimb time. However, other unrecognizedfrequencies, probably from the nearby towers,were also present at the same time interval.Measurements in and around several of theequipment buildings produced radiation levelsbelow LMIL.

Libby DamThis site was unique in that the COE antennaewere not located on a metal tower. Rather, thesources were mounted on top of the dam’s walland access to the sources, as well as theequipment area, was through a door on the dam’sroadways. Measurements were made at thisstation on June 17, 1995, in front and back of theCOE sources since they were directly accessibleto NIOSH investigators. Available sources werethe 171 MHz source for the Water Work, the6GHz BPA sources mounted on the Dam’s front,


the 1.7–1.8 GHz COE network, local COEbroadcast system at 530 MHz or 1610 MHz, andthe COE communication at 163.45 MHz. Nodata–logging was done at this site, insteadmeasurements were recorded manually.

Data recorded manuallyRecorded E–field: All antennae measure-ments taken in front and on sides were belowLMILRecorded H–field: range 0.0 to 19.4 (A/m)2

along back, front center, and perimeter of theantenna. This maximum result was confirmed(twice) by turning off the 1.7–1.8 GHznetwork and measuring zero. Maximum body current near antenna base:6 mA

COE workers normally do not work on towers insuch a way that they are in front of the antennae.At Libby Dam, the location of the sourcespresents a potential situation that could result inworker exposure to the COE network. TheHoladay meters do not respond to H–fields above300 MHz, the high fields that were recorded wereartifacts (this last statement has been verified bythe manufacturer). Therefore, measurementsmade at this station at extremely close distances tothe antennae clearly showed that the outputs ofconcern for tower climbers were not associatedwith the microwave system of 1.7 to 1.8 GHz, butrather were due to frequencies other than 1.7 to1.8 GHz.

King MountainThis site had only one tower but there were sixdifferent sources located on it. These included the1.7–1.8 GHz COE network, Northern Light47 MHz source operating at 60 to 75 watts, anunknown source operating around 170 MHz at110 W, COE communication at 164 MHz at 55W, a ham radio digital feed at 223 MHz at 10 W,2–meter digital feeder at 145 MHz at 35 W, and aBPA source at 169 MHz operating around 5 W.On June 19, 1995, only H–field data–loggedresults were made at the station as shown inFigure 7. Since the H–field cannot be recorded

above 300 MHz, then the fields measured weredue to the presence of the other sources on thetowers. Magnetic field measurements were alsomade at the tower base, 20 feet above the base,and 30 feet above the base to see how the levelmight change with distance above ground.

Data recorded on towerH–field results: 0.0 to 0.85 (A/m)2 nearantenna edge during a 10:25 min climb1 x 10-4 to 1.4 x 10-4 (A/m)2 at tower base over5 min samples9 x 10-4 to 2.5 x 10-3 (A/m)2 20 feet above tower base over 6 min sample9 x 10-4 to 1.6 x 10-3 (A/m)2 30 feet above tower base over 5 min sample

Due to time constraints, no E–field, body orcontact current data was taken at the station.

Tony MountainThis site contained a 70–foot tower constructed in1982, that was difficult to reach due to numerousfallen trees that had to be cleared using a handsaw. There were three sources at this tower; 1.7to 1.8 GHz COE network, a 171–172 MHz sourceoperating at 35–50 W, and the Forest Servicecommunication system. Due to an impeding rainstorm in the afternoon of June 19, 1995, a climbwas not performed. One H–Field measurementwas performed at the tower base and very littleactivity was seen. Levels measured were 1 x 10-4

to 4 x 10-4 (A/m)2. No frequencies were observedduring this measurement and all E– and H–fieldstrength measurements in and around theequipment building were below LMIL. Bodycurrent levels at the tower base were zero.

DISCUSSION

Tower MeasurementsThe maximum six–minute averages for both theE– and H–field were determined for each climb(Table 1). While measurements of the frequencies


were made on the incident radiation fieldexposing the tower with a frequency counter, itwas impossible in a multi–source environment,without using a frequency analyzer, to determinefor how long a given frequency signal was on. Inthis evaluation, a number of communicationsources operate in the same IEEE frequency bandsas shown in Table 2. Unfortunately, not all of theMW/RF sources at a given station were emittingduring the measurement periods. It is possible,however, to assume a worst case for workerexposure to human resonance frequencies (i.e., 30to 300 MHZ) or the maximal absorbing range.Almost all of the frequencies encountered in thisevaluation were in this region, except for the1.7–1.8 GHz area. Using this assumption theE–field measured at Mt. Spokane and the H–fieldsmeasured at Black and King Mountains exceededthe MPE for these frequency regions. While theassumptions made for this type of an analysis hasits limitations, it does suggest that at certain timesthe MW/RF levels encountered by COEmaintenance workers can exceed occupationalexposure guidelines. Although COE maintenancepersonnel climb towers very infrequently, theystill need to be informed that exposures aboveoccupational guidelines are possible.

The dominant exposures associated with climbingtower are not associated with the 1.7 to 1.8 GHzsystem but are due to frequencies in the 30 to300 MHZ range produced by COE shared andnon–shared tower sources. The NIOSHinvestigators found that there is more occupationalexposure from MW/RF sources located onnon–shared towers than from COE shared towers.These findings may make controlling MW/RFoccupational exposure even more difficult forCOE safety personnel.

While contact currents as high as 430 mA weremeasured (when the meter was placed in directcontact with the tower), those levels were not ona worker. In contrast, worker’s body currents nearthe ground were much lower. There is no easyway to measure actual body currents to workersduring climbing activities. The presence of the

contact currents only indicates that currents arebeing induced on towers from some nearby fields,as well as from sources located on the tower itself.Moreover, the strengths of the contact currentsvary from station to station depending on anumber of parameters, such as power level beingabsorbed by the tower, construction and locationof the tower, and the manner in which the meterwas applied to the metal tower surface.

Site MeasurementsAll MW/RF levels were below the LMIL either inthe equipment buildings, the ground around thetower structure, outside the equipment buildings,and in the immediate vicinity of parked vehicles.All body current measurements taken atnon–tower locations were below 1 mA.

CONCLUSIONSAnalysis of the data–logged measurements madeon selected towers suggests that COEmaintenance workers are not exposed to MWradiation emitted by their microwave system inthe 1.7 to 1.8 GHz region. However, occupationalexposure above occupational guideline limits canoccur to workers from RF sources operatingmainly in the 100 to 300 MHZ region, locatedeither on the COE towers or on adjacentnon–COE towers. Exposure to MW/RF fromsources located in and around the equipmentbuildings appears to be quite low. COEmaintenance workers who need to climb towers toperform their tasks will need to be informed(trained) about the potential for exposure andtechniques to reduce that exposure. While thisevaluation did show a potential to exceed theMW/RF occupational guidelines, it should benoted that due to the manner in which the datawas collected, these measurements should not beused to predict future exposures.


RECOMMENDATIONS

THE FOLLOWING RECOMMENDATIONS AREOFFERED TO REDUCE OCCUPATIONAL EXPOSURESAND SAFETY RISKS TO WORKERS WHO PERFORMMAINTENANCE OF THE COE MICROWAVEFACILITIES:

1. THE USE OF A TOWER CLIMBING LOG FOREACH STATION WOULD BE USEFUL IN BETTERESTIMATING EXPOSURE POTENTIAL TO RFSOURCES. THIS LOG WOULD CONTAIN CLIMBINGDATES, REASON FOR THE CLIMB, APPROXIMATECLIMB TIME, AND A DESCRIPTION OF THE ACTUALTASK PERFORMED.

2. DISCUSSIONS WITH THE COE WORKERSINDICATE THAT THEY ARE NOT REQUIRED TO HAVEANNUAL PHYSICALS. IT SEEMS REASONABLE THATCLIMBING 30 TO 90 FT TOWERS, CARRYING HEAVYELECTRONIC EQUIPMENT AND CUTTING ANDMOVING TREES THAT FALL ON ROADS, ARE JOBTASKS THAT REQUIRE WORKERS BE IN GOODPHYSICAL SHAPE. WORKERS INVOLVED WITHSUCH ACTIVITIES SHOULD HAVE SOME FORM OFMEDICAL SURVEILLANCE (SUCH AS ANNUALPHYSICALS).

3. IT WAS OBVIOUS THAT THE COE WORKERSARE OFTEN AT REMOTE LOCATIONS BYTHEMSELVES. CONSIDERATIONS SHOULD BE GIVENTO HAVING AT LEAST TWO WORKERS AT EACHLOCATION AS PART OF A “BUDDY” SYSTEM. THISIS ESPECIALLY IMPORTANT IN CASES WHERESOMEONE FALLS DOWN A LADDER, BREAKS BONESTRIPPING OVER LOGS, OR CUTS THEMSELVES WITHA SAW. THE USE OF A GLOBAL POSITIONINGSYSTEM (GPS) TO HELP LOCATE A WORKER TOWITHIN A FEW METERS WOULD ALSO BE USEFUL.FURTHERMORE, THE USE OF A “BUDDY” SYSTEMWOULD IMPROVE THE FIRST AID PROGRAM, SINCEIT PROVIDES A WAY FOR TIMELY FIRST AID TO BERENDERED THAT THE CURRENT WORKARRANGEMENT DOES NOT PROVIDE.

4. COE SHOULD TAKE STEPS TO DEVELOP AMW/RF SOURCE INVENTORY FOR EACH COETOWER. IT MAY ALSO BE USEFUL TO HAVEDETAILS ON ALL SOURCE CHARACTERISTICS ATALL THE TOWERS INDEPENDENTLY WHETHER THEYARE COE OR NON–COE SOURCES. IN THIS WAYWORKERS WILL HAVE MORE KNOWLEDGE OF THEIREXPOSURE POTENTIAL.

5. IT IS STRONGLY SUGGESTED THAT THEPRACTICE OF USING HAND SAWS TO CUT TREESWHICH HAVE FALLEN ACROSS ROADS BEIMMEDIATELY TERMINATED, AND PORTABLE GASSAWS BE PURCHASED. IN ADDITION TO THEHAZARDS ASSOCIATED WITH CUTTING LARGETREES, COE NEEDS TO CONSIDER THE ERGONOMICCONCERNS ASSOCIATED WITH MOVING THE PARTSOF THE CUT TREES. THERE MAY BE A NEED TO USEWINCHES ON TRUCKS TO ASSIST IN THE MOVEMENTOF LARGE TREE PARTS. IT IS REALIZED THAT THEUSE OF GAS SAWS MAY REQUIRE THE NEED TOHAVE A HEARING CONSERVATION PROGRAM, INADDITION TO WEARING OF GLOVES ANDPROTECTIVE EYEWEAR; HOWEVER, IN THE OPINIONOF THE INVESTIGATORS THE ADVANTAGES OF SUCHAN ACTION FAR OUTWEIGH THE DISADVANTAGES.

6. ANY MAINTENANCE OPERATIONS AT FIELDSTATIONS THAT REQUIRE WORKERS TO SPENDLONG TIMES (SUCH AS SEVERAL HOURS PER DAY)ON TOWERS MAY NEED TO BE STUDIED BY COE TOMINIMIZE POTENTIAL OCCUPATIONAL MW/RFEXPOSURES.

7. COE SHOULD DEVELOP OTHER TECHNIQUESTO IDENTIFY MAINTENANCE PROBLEMS ONTOWERS BESIDES CLIMBING. SUCH MONITORINGTECHNIQUES COULD INCLUDE THE USE OF TVCAMERAS OR SPECIAL BINOCULARS.

8. COE SHOULD DEVELOP INDUCED ANDCONTACT CURRENT MONITORING PROGRAMS FORWORKERS WHO ARE REQUIRED TO CLIMB TOWERS.THIS DATA CAN BE USED TO TRAIN WORKERS ANDD E V E L O P P R O G R A M S T H AT M I N I M I Z EOCCUPATIONAL EXPOSURE TO MW/RF SOURCES.


9. AFTER 1998, OSHA WILL REQUIRECONSTRUCTION INDUSTRIES TO USE FULL BODYHARNESSES FOR WORKERS ON TOWERS. COESAFETY PERSONNEL MAY WISH TO INVESTIGATETHE INCORPORATION OF HARNESSES IN THEIR FALLPROTECTION PROGRAMS. EXPERIENCE HAS SHOWNTHAT HARNESSES OFFER BETTER WORKERPROTECTION AGAINST FALLS THAN SAFETY BELTS.

REFERENCES

1. IEEE [1991]. SAFETY LEVELS WITH RESPECTTO HUMAN EXPOSURE TO RADIO FREQUENCYELECTROMAGNETIC FIELDS, 3 KHZ TO 300 GHZ.INSTITUTE OF ELECTRICAL AND ELECTRONICSENGINEERS STANDARD C95.1–1991.


TABLE 1SUMMARY OF E– AND H–FIELD MEASUREMENTS AT EIGHT COE MW/RF STATIONS ON JUNE 12–20, 1995

NATIONAL INSTITUTE FOR OCCUPATIONAL SAFETY AND HEALTHHETA 95–0177

TOWERLOCATION

TOWER TYPE

MW/RF SOURCES AT SITEDURATION OF

TOWER CLIMB(MINUTES)

DATA RANGE FROM TOWERCLIMB MAXIMUM BODY (B)

OR CONTACT (C)CURRENT (MA)

HIGHEST 6 MINAVERAGE

ELECTRICFIELD(V/M)2

MAGNETICFIELD (A/M)2

E(V/M)2

H(A/M)2

ADAMS RIDGENON–SHARED

1.7–1.8GHZ @ 1 W (COE)2 TO 30 MHZ @ 1000 W (DOD)

6.58.0 21.2 TO 3250 2X10-3 TO 0.05 1 (B)

19 (C) 5200 .003

CRESTON BUTTESHARED AND NON–SHARED

1.7–1.8GHZ @ 1 W (COE)155.24 MHZ @ 110 W (POLICE) 7.0 20.3 TO

8.7X104 NOT TAKEN 3 (B) 2530 —

MT. SPOKANESHARED ANDNON–SHARED

1.7–1.8GHZ @ 1 W (COE)163.45 MHZ @ ? W (COE)169 MHZ @ ? W (FBI)

9.58.2 0 TO 1.49X105 1.2X10-3 TO 0.018 482 (C) 4900 .006

ALBANI FALLSDAM

SHARED1.7–1.8GHZ @ 1 W (COE)163.45 MHZ @ ? W (COE)109 MHZ @ ? W (COE)

18.0 142 TO 3600 9X10-4 TO 4.9X10-3

5 (B)35 (C) 190 .001

BLACKMOUNTAIN

SHARED ANDNON–SHARED

1.7–1.8GHZ @ 1 W (COE)160 MHZ @ ? W (UPR)172.5 MHZ @ 5 W (BPA)UNKNOWN IDAHO STATESOURCES

9.39.2 10 TO 1190 0 TO 0.36 70 (B) 110 .09

LIBBY DAM SHARED

1.7–1.8GHZ @ 1 W (COE)171 MHZ @ ? W (WATERWORKS)6GHZ @ ? W (BPA)163.45 MHZ @ ? W (COE)530 AND 1610 MHZ

NO CLIMB LMIL 0 TO 19.4 6 (B) — —

KING MOUNTAIN SHARED

1.7–1.8GHZ @ 1 W (COE)47 MHZ @ 75 W (NORTHL)170 MHZ @ 110 W ?163.45 MHZ @ 55 W (COE)223 MHZ @ 10 W (HAM)145 MHZ @ 35 W (HAM)169 MHZ @ 5 W (BPA)

10.25 NOT TAKEN 0 TO 0.85 NOT TAKEN — .048

TABLE 1SUMMARY OF E– AND H–FIELD MEASUREMENTS AT EIGHT COE MW/RF STATIONS ON JUNE 12–20, 1995

NATIONAL INSTITUTE FOR OCCUPATIONAL SAFETY AND HEALTHHETA 95–0177

TOWERLOCATION

TOWER TYPE

MW/RF SOURCES AT SITEDURATION OF

TOWER CLIMB(MINUTES)

DATA RANGE FROM TOWERCLIMB MAXIMUM BODY (B)

OR CONTACT (C)CURRENT (MA)

HIGHEST 6 MINAVERAGE

ELECTRICFIELD(V/M)2

MAGNETICFIELD (A/M)2

E(V/M)2

H(A/M)2


TONY MOUNTAIN SHARED1.7–1.8GHZ @ 1 W (COE)171 MHZ @ 50 W ?U. S. FOREST SERVICE ?

NO CLIMB NOT TAKEN 1X10-4 TO 4X10-4 0 (B) — —

? = UNKNOWN OR ESTIMATED


TABLE 2. RF AND MICROWAVE IEEE OCCUPATIONAL EXPOSURE GUIDELINES

FREQUENCY RANGE(MHZ)

ELECTRIC FIELDSTRENGTH

(V/M)2

MAGNETIC FIELDSTRENGTH

(A/M)2

POWER DENSITYE–FIELD/H–FIELD

(MW/CM2)

0.003 – 0.1 377,000 26,600

0.1 – 3 377,000 (16.3/F)2

3 – 30 (1842/F)2 (16.3/F)2

30 – 100 3770 (16.3/F)2

100 – 300 3770 0.027 1.0

300 – 3,000 F/300

3,000 – 15,000 10

15,000 – 300,000 10

F = FREQUENCY IN MHZMHZ = MEGAHERTZV/M = VOLTS PER METERA/M = AMPS PER METERMW/CM2 = MILLIWATTS PER SQUARE CENTIMETER

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