Occupational and Environmental Medicine 1994;51:381-385

Balance measured by head (and trunk) trackingand a force platform in chemically (PCB andTCE) exposed and referent subjects

Kaye H Kilburn, Raphael H Warshaw, Brad Hanscom

AbstractTo determine comparability of methods,postural sway was measured simultane-ously with a force platform and a devicethat registers head (and trunk) move-ments (head tracking). The effects of age,sex, height, weight, shoe area, and gripstrength on both measurements wereexamined in 162 subjects. To determinewhether either method had advantages indetecting abnormal balance 46 referencesubjects were compared with 116 peoplerandomly selected from 500 residentswhose well water was contaminated withpolychlorinated biphenyls (PCBs) andtrichloroethylene (TCE) from a metalcasting plant. Speed, mean radius, anddistance of sway were equally repro-ducible with both methods. Correlationcoefficients were 0-672 with the eyesclosed and 0*588 with the eyes open. Thebalance of those exposed to PCBs andTCE was significantly worse than that ofunexposed subjects by both head tracking(1.50 + 71 cmls v 127 + 0-36 cmls; p <0.034) and the force platform (4.93 + 1*56N (kg force) v 4-29 + 1-14 N; p < 0.013)with the eyes closed and differences werealso significant with the eyes open. Headtracking and the force platform producedequivalent results. Measurement by headtracking is recommended for field studiesbecause the device is more portable andis less influenced by weight and height.Both methods showed impaired balanceassociated with years of exposure toPCBs and TCE.

(Occup Environ Med 1994;51:381-385)

University ofSouthern CaliforniaSchool ofMedicine,EnvironmentalSciences Laboratory,2025 Zonal Avenue,CSC 201, Los Angeles,California 90033, USAand Workers DiseaseDetection ServicesInc, 112 N HarvardAve, Suite 258,Claremont, California91711, USAK H KilburnR H WarshawB HanscomCorrespondence toDr K H Kilburn

Accepted for publication23 February 1994

The objectives of this study were firstly tocompare balance by two methods, force plat-form and head tracking, simultaneously innormal subjects to see whether there were dif-ferences and secondly, to extend this compar-ison to chemically exposed subjects. Thus thedesign examined the issues of methodologicalcompatibility and also sensitivity to environ-mental exposures to polychlorinatedbiphenyls (PCBs), their contaminants, andthermal transformation products manifestedby dizziness, lightheadedness, and loss of bal-ance. Subjects in this study had used waterwells heavily contaminated with PCBs andsolvents including trichloroethylene (TCE).Their complaints led to us testing their bal-

ance compared with that of referent subjectsfrom a nearby area with a force platform and ahead tracking device simultaneously. Suchexposures have been previously associatedwith neurological complaints such as periph-eral sensory and motor nerve dysfunction, andimpaired nerve conduction velocities. 1-3

In 1851 Romberg had subjects stand withtheir feet together and arms folded for his bal-ance test.4 Breaking stance in less than aminute defined failure, so the test was insensi-tive. Many investigators52 have describedmethods to quantify balance since Goldbergin 194313 and Bormann and Jalavisto in1953'4 tracked head movements photographi-cally. In the past decade force platforms havebeen accepted as a standard method but therehas been no published comparison with headtracking. Some force platforms must be sunkinto the floor'5 and seem unsuited for field oroffice use whereas, by contrast, the headtracking apparatus is handy, lightweight, andportable.'6 Before adopting this technique, wecompared it with the force platform to answerseveral questions. Firstly, are head trackingand a force platform equivalent methods ofmeasuring balance? Secondly, does the headmove independently of the body axis?Thirdly, do the direction and velocity indicesof balance or the influence of age, height,weight, and other independent variables differin the two measurements?

MethodsA cohort comparison design was used with 62women and 54 men in the exposed group and27 women and 19 men as referents. Theexposed participants were an age stratifiedrandom selection from over 500 symptomaticresidents exposed to PCBs in hydraulic fluidtraced to a metal casting plant in ColbertCounty, Alabama. They had lived within 2A4km of this plant for at least four years between1956 and 1981 and most of them lived thereat the time of the study. The hydraulic fluid inthe die casting machines was composed ofethylene glycol containing 15% to 20% PCBs.Up to 100 000 gallons per year of PCBs hadbeen used in these machines for 17 years.Multiple analyses of the residents' well waterin the 1980s showed PCBs up to 3500 ppmand trichloroethylene (TCE) concentrationsabove the Environmental Protection Agency(EPA) action concentration of 5 ppb.Concentrations of these "test" chemicals dif-fered by one to two orders of magnitudeamong various well water samples.'7

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The referents were from Russellville,Alabama (in Franklin County, 30 km distant)or surrounding towns that were selected tomatch the exposed residents for sex, age, andyears of educational attainment (highestschool grade attained). They were interviewedto ascertain their willingness to be tested, toinsure matching by age and sex to the exposedgroup and absence of employment at the diecasting plant, or of living within 8 km of it.Examiners were blinded to the exposure stateof the subjects. All subjects gave theirinformed written consent to the protocol,which was approved by the University humansubjects review committee. Questionnaireswere self administered for occupational expo-sures including to neurotoxins, hours of gen-eral anaesthesia, and for past neurological andmedical diseases. They were checked for com-pleteness by trained interviewers.

Grip strength was measured in kg by com-pressing the calibrated spring of a dynamome-ter. Weight and height were measured insubjects without shoes by means of a clinicalplatform scale and extendable metal metricrod. Shoe area was measured by computerisedplanimetry.Body balance was measured by both tech-

niques simultaneously with the subject stand-ing erect on the platform with feet togetherand arms folded, both with eyes open andwith eyes closed, while wearing a sound emit-ting headband. For measurements with eyesopen subjects focused on a target 6 feet awayfor 30 seconds. After a short rest period dur-ing which subjects were encouraged to relaxand move they assumed the same position,focused on the target, and stood with eyesclosed for 30 seconds. Three sequences ofeyes open then eyes closed were separated by30 to 45 seconds. Sway data were collectedwith a Neuro-Test, Inc Postural SwayAnalyser with both force platform and posi-tional tracking modules (NTI, Claremont,California, USA).The force platform module is 8 cm high by

110 cm square with a 45 cm2 measurementarea. Force is measured by eight foil deflec-tion strain gauges placed two at each corner ofthe active area. The module was calibrated byplacing a 20 kg load on each pair of straingauges and adding increments of 1, 4, and 8kg. A sampling rate of 4000/s was reduced byprogramming to 60/s, which was 10 timesfaster than the expected frequency of sway.The reproducibility was within 2% of thegreatest mass applied. The four unfiltered sig-nals from each pair of gauges were integratedin the computer with software of our design.The head tracking module consists of twomicrophones mounted horizontally 36 cmapart on a tripod, which permits adjustmentfor the subject's height, and a sound emitterwith a barely audible (4/s) click, which wassecured by a headband behind the subject'sear.16 Its sampling rate was 16 67/s; threetimes the expected frequency. The headtracking module was calibrated by moving theemitter along the boundaries of a 21-59 by27-94 cm rectangle placed horizontally at the

same height as a line connecting the mid-points of the microphones. The head trackingdigitiser was connected directly without filtra-tion to an IBM compatible computer.Repeatability was within 2%. The force mod-ules were connected to an IBM compatiblecomputer with a controller for the positiontracking module and an eight channel, 16 bitanalog to digital converter. Force and positionplots did not interact (bias each other) so theywere displayed simultaneously in real time on asplit screen and all coordinate pairs wererecorded.

Results are reported for head tracking asmean radius of sway, eyes open (ro) and eyesclosed (rc), and mean speed of sway eyes open(so) and eyes closed (sc). Mean radius wasdefined as the average distance from the centrepoint of all positions sampled during 60 sec-onds and could be regarded as the averagedeviation from this hypothetical centre. Asmall "fT in front of each value identifies therate of change measured by the force platformdata (fro, etc). Units of measurement are cmfor radius and cm/s for speed with head track-ing and kg force or Newtons (N) for the plat-form.

Comparison of head-trunk tracking mea-surements on 10 subjects tested while stand-ing on a level floor and repeated whilestanding on the platform were not different,indicating that the size of the platform wassufficient to overcome possible confoundingeffects from the 8 cm height of the platformabove the floor. This absence of effect makesfield use of this platform practical where itwould be difficult to level its top surface to thesurrounding floor.

Distributions ofmean radius of sway and ofspeed with eyes open and with eyes closed byhead tracking and by the force platform wereGaussian. Of the eight transformations (tosquare, cube, square root, logarithm, l/squareroot, inverse, 1/square, and 1/cube) only theinverse improved the untransformed (iden-tity) data slightly for force. There were noimprovements for head tracking in regressionmodelling. Differences were insignificant sowe used untransformed data. Weight andheight x weight were used to express bodysize.

All scores and computed data for sway wereentered into a Tristar 486 computer anddescriptive and analytical computationsincluding t tests and linear regression model-ling used were carried out with Stata statisti-cal software (Computing Resource Center,10801 National Blvd, Los Angeles,California, 90064, USA, 1987). Differenceswith a p value of <0.05 were considered sta-tistically significant.

ResultsThe ages, educational level (last year of schoolcompleted), average high school grades,heights, and weights of exposed and referentmen and women were similar (table 1). Theexposed group had significantly higher familyincomes for the past three years than did the

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Balance measured by head (and trunk) tracking and a force platform in chemically (PCB and TCE) exposed and referent subjects

Table 1 Descriptive data for exposed and referent groups

Exposed Referent(n = 116) (n = 46)Mean (SD) Mean (SD) p Value

Age (y):Women 42-7 (12-9) 37-0 (16-2) NSMen 39-1 (13-5) 418 (18-0) NSOverall 41-1 (13-2) 39-0 (16-9) NS

Education (y) 12-4 (2 3) 12-5 (2-0) NSGrades (A = 1, F = 5) 2-5 (0 6) 2-4 (0-6) NSIncome (x $1000) 31-18 (23 57) 16-92 (13-30) 0 0003

Alveolar alcohol (i/dl):Women 0-0001 (0-0004) 0-0003 (0 0006) NSMen 0 0004 (0 0008) 0-0001 (0 0003) NS

Height (cm):Women 161-8 (6 2) 163-6 (4 5) NSMen 174-7 (6 0) 173-7 (7-2) NS

Weight (kg):Women 79 7 (18-0) 74-2 (16-0) NSMen 87-7 (16-7) 85-3 (19-5) NS

Grip strength (kg); women:Right 30-1 (8-8) 34-7 (6-1) 0-015Left 28-0 (7 6) 33-7 (6-1) 0-0007

Grip strength (kg); men:Right 56-1 (13-3) 53-5 (14-1) NSLeft 55-4 (11 9) 50-1 (14-5) NS

p Values by t test (two sided).

referents, which reflects an urban and ruraldifference. Income had no statistically signifi-cant coefficients separate from age and educa-tional level in regression models. No subjectin either group had an alveolar air alcoholconcentration above 0 03 ul/dl.

Occupational exposure to 15 known orpotentially neurotoxic chemicals had occurredin 83% of the referent group, more than the69% in the exposed group obtained by ques-tionnaire.'7 Subjects who shared exposure tosingle neurotoxicants made groups that weretoo small for separate analysis. Taken togetherthey accounted for most of the cases, so theycould not be eliminated from analysis.Because a greater proportion of the referentgroup had these exposures than did theexposed group, however, the overall effectwould narrow the group differences.'7

Sex was not a determinant of sway speed orradius in regression models, nor did it affectgroup comparisons of exposed and controlsubjects by t test, so the sexes were analysedtogether. For a comparison of correlationcoefficients mean values for three 30 secondtrials were averaged for each group.

Sway speed by tracking with the eyes openhad a correlation coefficient (r) with the plat-form of 0x588 and with the eyes closed r was

Table 2 Correlation coefficient* matix for head tracking andforce platform for speedand mean distance in 162 subjects measured simultaneously (average of three trials)

Sway speed Sway mean radius

Head tracking Force platform Head tracking Force platform

Open Closed Open Closed Open Open Open Closed(so) (sc) (fso) (fsc) (ro) (rc) (fro) Mrc)

so 1-000sc 0-853 1-000fso 0 588 0-542 1-000fsc 0534 0-672 0-853 1-000ro 0-851 0-726 0 407 0-338 1-000rc 0-755 0-885 0-424 0-522 0-770 1-00fro 0-736 0-679 0-684 0-668 0 755 0 705 1-000frc 0-601 0-763 0-664 0-825 0-522 0-760 0-850 1-000

*Pearson correlation coefficient; all are statistically significant (p < 0 000).

0-672. Both were statistically significant(p < 0-0001; table 2). For sway radius the rvalues, both tracking and platform, were0-755 with the eyes open and 0-760 with theeyes closed (p < 00001). Sway speed, radius,or distance on the x or y axis separately didnot improve discrimination. The patterns ofdisplacement on forward-backward (y) axisand side to side (x) axis were similar in formfor both methods in all subjects. Figure 1 (Aand B) shows this similarity. Sway speed byhead tracking with eyes open and with eyesclosed had an r value of 0-853 (p < 0-0001).The correlations for sway radius with the twodevices were 0756 with the eyes open and0-760 with the eyes closed.

Multiple linear regression with stepwiseelimination of non-significant independentvariables was used to model the influence ofheight, weight, height x weight (interaction),age, shoe area, and grip strength on swayspeed and radius. Body weight, height, andage did not contribute significantly to headtracking with the eyes open or with the eyesclosed (table 3) but with the force platformheight and weight had significant coefficientsand age was marginal, which accounted for0-2640 of the variance (adjusted r2) with theeyes open and 02972 with the eyes closed.The age and weight covaried. Thus age wasnot a major predictor of sway speed by headtracking in these subjects between 17 and 71years.

Linear models showed that height x weightincreased head tracking sway speed andradius and accounted for more variance(adjusted r2) than did height or weight alone.There was a 10-fold to 20-fold greater effectof height x weight on force platform speedand radius (adjusted r2 0-22 to 0 25) com-pared with the corresponding tracking mea-surements (adjusted r' 0-019 to 0-025).Greater grip strength decreased sway speedand radius but these coefficients, althoughsignificant, were only 10% to 20% of the coef-ficients of height x weight. Furthermore, griphad significant coefficients for height(adjusted r' 0-3749) and height x weight(adjusted r2 0-1177). Shoe area alone had asignificant coefficient for sway speed and forradius but it became insignificant whenthe more powerful coefficient, that forheight x weight, was put in the model. Swayspeed with eyes open (trial 3) had a small sig-nificant positive coefficient with age (r 0 007adjusted r2 0- 1061) but with eyes closed, r wassmaller and not significant.

Sway speed was greater in subjects exposedto PCBs and other chemicals for four to 17years, than for referents as measured by track-ing and the platform for each trial (table 4).Speed and radius for exposed subjects weresignificantly different from referents by bothmethods for all trials (p from < 0-034 to< 0 000) and were slightly smaller for theplatform. Confounding influences were mini-mal. More referents than exposed subjectshad occupational exposures to other possibleneurotoxins.'7 Alcohol concentrations werenot raised in any subject. Neurological dis-

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o Force platform (kg)Head tracking (cm)

Table 3 Regression modelsfor speed ofsway by headtracking with eyes open (SO) and closed (SCU) andforceplatform for eyes open (FSO) and with eyes closed(FSC) including grip strength and height x weight

Standard AdjustedCoefficient enror p Value r?

Xd

a

200

Samples (16.67/s)

01 1~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

200

Samples (16.67/s)(A) Force platform and head tracking positions ony axis (forard and backward) otypical unexposed subject during a 30 second trial (16-67 observation points/s (B). Fiplatform and head tracking positions on x axis (side to side) of typical unexposed subduring a 30 second trial (16-67 observations/s.

400

SO,:AgeHeightWeightConstant

SC,:AgeHeightWeightConstant

FSO,:AgeHeightWeightConstant

FSC3:AgeHeightWeightConstant

v SO:Grip

1/ SO:Height x WeightConstantHeight x WeightConstant

SC:Grip

0 Height x WeightConstantHeight x WeightConstant

FSO:GripHeight x WeightConstantHeight x WeightConstant

FSC:I1--~- Grip400 Height x Weight

ConstantHeight x WeightConstant

Force Grip:Sect Height x Weight

Constant

eases were absent in both groups. The meanhours of general anaesthesia were not differ-ent between the groups and as an indepen-dent variable in regression models of swayspeed or radius, total hours of anaesthesia hadno significant coefficients.

Table 4 Comparison ofsimultaneous head tracking andforce platform measurementswith eyes open and closed during three 30 s trials for exposed subjects and referents

Referents (n =46) Exposed (n = 116)Method Eyes Mean (SD) Mean (SD) p Value

Trial 1:Head tracking Open 0-92 (0-19) 1-05 (0-37) 0-026Force platform Open 3-64 (0 64) 4-08 (0-60) 0 000Head tracking Closed 1-32 (0-33) 1-49 (0-51) 0-034Force platform Closed 4-46 (1-08) 5-06 (1-45) 0-013

Trial 2Head tracking Open 0-90 (0-21) 1-05 (0-39) 0-013Force platform Open 3-53 (0-64) 3-98 (0-62) 0-000Head tracking Closed 1-24 (0-33) 1-45 (0-53) 0-015Force platform Closed 4-26 (0-95) 4-85 (1-30) 0-006

Trial 3:Head tracking Open 0-92 (0-25) 1-08 (0-40) 0-014Force platform Open 3-53 (0-50) 3-97 (0-55) 0-000Head tracking Closed 1-27( 0-36) 1-50 (0-71) 0 034Force platform Closed 4-29 (1-14) 4-93 (1-56) 0-013

Grip:Height x WeightConstant

0-001919-0286

0-2640

0-2972

0- 1314

0-0250

0- 1053

0-019

0-2968

0-2214

0-3123

0-2503

0-3749

0-0004 0-0005-0211 0-000 0-1177

DiscussionSway speed and distance (mean radius) wereeasily measured simultaneously by head track-ing and the force platform in 46 referencesubjects and 116 chemically exposed subjects.The important finding is that they were equiv-alent methods to measure balance. Thefidelity of the head tracking device equals thatof the platform at the working frequencies.Neither signal required filtering. There was noevidence of effects of head movement inde-pendent ofmovements of the trunk while sub-jects stood still. Head tracking is effectivelyindependent of body size (height x weight)but otherwise balance was measured equallywell by either method. Both devices areportable but the head tracking instrument islightweight and more easily carried and setup. The platform requires no headband somay be accepted slightly better by subjects.The force platform is much more sensitive tobody size expressed as height x weight and to

A133 -

2

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0co'amWen

-181 -

300 -

Ea)0Cu

0.Ca

0

0-1061

0-00210-00920-00080-5854

0-00360-01610-00141-019

0-00300-01350-00120-8567

0-00730-03270-00282-0733

0-0000-2300-0710-038

0-0850-0650-0080-016

0-0310-0350-0000-000

0-0530-0200-0000-008

0-0070-0-01110-00151-2231

0-0062-0-02990-00372-4771

0-0066-0-02880 00754-1364

0-01420-07670-02025-5928

0-00720-000030-92100-000020-7837

0-01020-000051-28600 000031-0906

-0-01110-00012-93410-00012-7219

-0-02230-000272-47030-00022-0427

2-965854-2865

-300 -

0

0-0016 0-0008-74 0-0000-1054 0-0008-69-06 0-0251-069 0-000

0-0025 0-0000-00001 0-0000-1681 0-0000-00001 0-0440-1686 0-000

0-0026 0-0000-00001 0-0000-1736 0-0000-00001 0-0000-1750 0-000

0-00570-000030-37970-000030-3797

0-0000-0000-0000-0000-000

0-3238 0-00020-0536 0-000

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weight alone than is head tracking. Althoughthis sensitivity would be of no consequence incomparing populations with the same weightdistributions, it may be important for groupsthat differ and could bias results in longitudinalstudies when weights of people vary. Gripstrength was negatively correlated with swayspeed and radius but this may be an artifact,as exposed women had significantly less gripstrength than referents.

Force platforms are influenced materiallyby body size.'5 1819 The head trackingmethod's independence of body size or weightmay be an advantage, especially when agroup's weights are not equally distributed orchanged during the study. Age did not have asignificant coefficient for balance in ambula-tory subjects from ages 17 to 71, although ithas been shown that balance deterioratesbeyond 80 years of age." '4 Other variablessuch as overall distance, x and y axis dis-tances, and acceleration can be derived fromthe multisample per second recordings ofeither head tracking or the force platform.These had no advantages in our comparisons.

During the past decade force platformshave been preferred with body (belt line)magnetic tracking used occasionally.6 Becausenone of the current devices is lightweight andeasily adaptable for field studies we haveshown that a head tracking device that is easilyportable produces similar data. Both devicesshowed significant impairment of balance in apopulation exposed to PCBs, dibenzofurans,TCE, and other solvents, which also showedother neurophysiological and neuropsycho-logical abnormalities.17The chemically exposed group had abnor-

mally greater sway speeds and radii than refer-ents, which were associated with four to 17years of exposures to PCBs, dibenzofurans,and solvents including TCE from well water.Also, the metal casting operation may haveadded some airborne chemical exposure par-ticularly to TCE. Presumably tricyclic poly-chlorinated aromatic compounds, includingdibenzofurans and dibenzo-p-dioxins, conta-minated the commercial grade PCBs andwere transformed from PCBs as temperaturesin metal casting reached the 270-620'Cneeded to create these products in the labora-tory.20 21 Also, these compounds have beenfound after fires of transformers containingPCBs.22 Firefighters exposed to thermal trans-formation products of PCBs, who were stud-ied in 1988, had abnormal sway radius andspeed with the eyes open23 and, when com-

pared with recent referent subjects, abnormalsway speed with the eyes closed as well. Thepresent paper confirms previous findings ofabnormal balance in subjects exposed toPCBs using two sensitive devices. These newfindings supplement the evidence for reducedperipheral sensory and motor function andnerve conduction velocity.' 2

1 Chen RC, Tang SY, Miyata H, et al. Polychlorinatedbiphenyl poisoning: correlation of sensory and motornerve conduction, neurologic symptoms and blood levelsof polychlorinated biphenyls, quaterphenyls and diben-zofurans. Environ Res 1985;37:340-8.

2 Murai Y, Kuroiwa Y. Peripheral neuropathy in chloro-biphenyl poisoning. Neurology 197 1;21:1173-6.

3 Rogan WJ, Gladen BC. Neurotoxicology of PCBs andrelated compounds. Neurotoxicology 1992;13:27-36.

4 Romberg MH. Lehrbuch der Nervenkrankheiten des men-schen. 2nd. Berlin: A Hirschward, 1851.

5 Bhattacharya A, Morgan R, Shukla R. RamakrishananHK, Wang L. Non-invasive estimation of afferent inputsfor postural stability under low levels of alcohol. AnnBiomed Eng 1987;15:533-50.

6 Dean EM, Griffiths CJ, Murray A. Stability of the humanbody investigated by sway magnetometry. J Med EngTechnol 1986;10: 126-30.

7 Edwards AS. Body sway and vision. J Exp Psychol Gen1946;36:526-35.

8 Glorig A, Davis H. Age, noise and hearing loss. Ann OtolRhinol Laryngol 1961;70:556-71.

9 Jansen EC, Larsen RE, Olesen MB. QuantitativeRomberg's test. Acta Neurol Scand 1982;66:93-9.

10 Savolainen K. Combined effects of xylene and alcohol onthe central nervous system. Acta Pharmacologica etToxicologica 1980;46:366-72.

11 Sheldon JH. The effect of age on the control of sway.Gerontology 1963;5:129-38.

12 Thyssen HH, Brynskov J, Jansen EC, Munster-SwendsenJ. Normal ranges and reproducibility for the quantitativeRomberg's test. Acta Neurol Scand 1982;66:100-4.

13 Goldberg L. Quantitative studies on alcohol tolerance inman. Acta Physiol Scand 1943;5:28-55.

14 Bormann K, Jalavisto E. Standing steadiness in old andyoung persons. Am Med Exp Biol Fem 1953;31:446-55.

15 Dick RB, Bhattacharya A, Shukla R. Use of a computer-ized postural sway measurement system for neurobehav-ioral toxicology. Neurotoxicol Teratol 1990;12: 1-6.

16 Kilburn KH, Warshaw RH, Hanscom B. Are hearing lossand balance dysfunction linked in construction ironworkers? BrJInd Med 1992;49:138-41.

17 Kilburn KH, Warshaw RH. Neurobehavioral testing ofsubjects exposed residentially to ground water contami-nated from an aluminum die casting plant and local ref-erents. J Toxicol Environ Health 1993;39:101-14.

18 Goldie PA, Bach TM, Evans OM. Force platform mea-sures for evaluating postural control: reliability andvalidity. Arch Phys Med Rehabil 1989;70:510-7.

19 Murray MP, Seireg AA, Sepic SB. Normal postural stabil-ity and steadiness: quantitative assessment. Jf Bone JointSurg (Am) 1975;57:510-6.

20 Buser HR. Formation, occurrence and analysis of poly-chlorinated dibenzofurans, dioxins and related com-pounds. Environ Health Perspect 1985;60:259-67.

21 Hutzinger Choudhry GG, Chittim GB, Jonston LE.Formation of polychlorinated dibenzofurans and dioxinsduring combustion, electrical equipment fires and PCBincineration. Environ Health Perspect 1985;60:3-9.

22 Fitzgerald EF, Stanafast SJ, Youngblood LG, Melius JM,Janerich DT. Assessing the health effects of potentialexposure to PCBs, dioxins and furans from electricaltransformer fires: The Binghamton state office buildingmedical surveillance program. Arch Environ Health1986;41:368-76.

23 Kilburn KH, Warshaw RH, Shields MG. Neurobehavioraldysfunction in firemen exposed to polychlorinatedbiphenyls (PCBs): possible improvement after detoxifi-cation. Arch Environ Health 1989;44:345-50.

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