Occupational and Environmental Medicine 1995;52:600-605

Exposure to polycyclic aromatic hydrocarbons incoal liquefaction workers: impact of a workwearpolicy on excretion of urinary 1-hydroxypyrene

R Quinlan, G Kowalczyk, K Gardiner, I Calvert

Occupational HealthService, British Coal,Leeming Lane South,Mansfield WoodhouseNGl9 9AQR QuinlanG KowalczykInstitute ofOccupational Health,University ofBirmingham,Birmingham B15 2TTK GardinerI CalvertPresent address: Zeneca,Huddersfield HD2 1FFR QuinlanCorrespondence to:Mr G Kowalczyk,Occupational HealthService, British Coal,Leeming Lane South,Mansfield Woodhouse,NottinghamshireNG19 9AQ.Accepted 15 May 1995

AbstractObjective-This study was undertaken toassess whether contaminated personalclothing worn beneath a coverall (normalworkwear) is a source of potentially sig-nificant dermal exposure to polycyclicaromatic hydrocarbons (PAHs) in coalliquefaction workers.Methods-An intervention study wasconducted over a two week period involv-ing 10 workers that reflected the range ofactivities performed at the factory. Across over design was used to examinethe influence of normal workwear (per-sonal clothing worn beneath a coverall)and intervention workwear (new coverall,shirt, trousers, underwear, socks, andboots) upon excretion of urinary 1-hydroxypyrene (1-OHP) and skin paddeposition ofpyrene.Results-The impact of intervention wasnoted in three ways:(1) A notable reduction (55%) in themass of 1-OHP excreted on the first dayof the intervention phase was found. Themedian reduction in mass excreted (22.7nmol) was significant from zero at the 5%level; (95% confidence interval (95% CI)9*5-40-8 nmol).(2) A notable reduction (82%) in skin paddeposition of pyrene on the first day ofthe intervention phase was found. Themedian reduction of 13.20 ng.cm 2 wassignificant from zero at the 5% level;(95% CI 7-3-26-4 ng.cm-2).(3) About a 50% reduction in 1-OHPconcentration over the working weekoccurred during the intervention phase;an increase of 2-07 ,umollmol creatininewas found from the start to the end of thework period during the interventionphase compared with an increase of 4-06umol/mol creatinine during the normalphase. This reduction was not significantat the 5% level.Conclusion-The results indicate that onthe first day of the working week investi-gated, significant reductions in absorb-tion (as measured by excretion of urinary1-OHP) and deposition ofPAHs (as mea-sured by skin pad deposition of pyrene)can be effected by improvements inworkwear policy. The impact of theimproved workwear regimen was alsodetected by reduction in spot urinaryl-OHP concentrations, although thiseffect was less pronounced. One implica-tion of the findings is that exposure to

PAHs may arise from workers' own con-taminated personal clothing. As a con-sequence of this study an improvedworkwear policy has been implementedat the factory.

(Occup Environ Med 1995;52:600-605)

Keywords: polycyclic aromatic hydocarbons, 1-hydroxypyrene, workwear

Coal liquefaction involves the transformationof coal into hydrocarbon fractions that can beused as transport fuels. The process has beenpreviously described' and only a brief sum-mary is included here. In the extraction stagecoal is digested in anthracene oil, which pos-sesses hydrogen donation properties. About85% by weight of coal passes into solution.The digest is filtered to remove undissolvedcoal and mineral matter, and clean coal solu-tion is catalytically hydrocracked in a tricklebed reactor. The main products are naphtha(initial boiling point up to 1 700C), mid-distillates (170'C-450'C), and a smallamount of heavy products (>450'C). Thenaphtha and mid-distillates need to be chemi-cally refined to make quality petrol and dieselfuels. The pilot plant converts 2'5 tonnes ofcoal a day into light distillates (35%-47% dis-tillate/tonne of coal received).The process stream consists of complex

mixtures that are mainly polycyclic aromatichydrocarbons (PAHs) of which some areknown carcinogens.2 Previous studies at thecoal liquefaction factory have concluded that:(1) Measurement of 1-hydroxypyrene(1-OHP) in urine specimens taken at the endof the work shift is a valid measure of totalexposure to and absorption of PAHs in coalliquefaction workers.'(2) Group median 1-OHP concentrations canbe used as an effective indicator of hygieneperformance at the factory.3(3) The potential for exposure to the processstream materials occurs during both routine andnon-routine tasks and is primarily dermal.'

Control of health risks at the factory iseffected by a detailed occupational health pro-gramme that involves routine medical surveil-lance, annual biological and atmosphericmonitoring, and adherence to good occupa-tional hygiene practices. As well as showeringfacilities and skin luminoscope examinations,hygiene practices at the factory rely in part onprovision of protective clothing to minimiseany possibility of dermal exposure to coal

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liquefaction materials. Workers are issuedwith protective coveralls, protective gloves,and boots; beneath these items workers weartheir own personal clothing and underwear.Workwear permeability studies (Walton,

personal communication) have shown thatArco Coroguard TP Coveralls (Proban-100%cotton drill; 305 g.m2) provide the best pro-tection against penetration by process liquids.Each worker at the factory is provided withtwo coveralls for routine everyday use.Although these items provided by the factoryare laundered regularly, responsibility forlaundering personal clothing worn beneathcoveralls is left to each worker.

Previous studies of occupational hygienepractices at the factory have highlighted thepotential for dermal exposure and absorptionof PAHs, and measures have been takenthrough improved working practices toincrease worker awareness of skin contamina-tion. These activities have resulted in succes-sive annual reductions in excretion of 1-OHPat the factory. This provides a quantitativemeasure of improvements in occupationalhygiene. Nevertheless, despite these improve-ments, relatively high concentrations of 1-OHP are still being detected in workers andfurther action to reduce these exposures istherefore still warranted.Van Rooij and co-workers have shown that a

substantial amount of dermal contaminationby PAHs can arise on areas of skin covered byworkwear, which suggests that when contami-nated this may be an important source ofdermal exposure.45 Our observations ofworking practices have indicated that contam-inated workwear may also be a potentialsource of exposure to PAHs in coal liquefac-tion workers.To determine whether the workwear

practices (personal clothing worn beneath acoverall) in the liquefaction factory could con-tribute to total exposure of PAHs, the impactof an improved workwear regimen (new cov-erall, shirt, trousers, underwear, socks, andboots) was assessed principally by monitoringchanges in excretion of 1-OHP. Should thenormal workwear be a source of exposure toPAHs, then reductions in 1-OHP would beanticipated after improvements in the work-wear regimen. Also skin pads were used inthis study to obtain evidence of direct deposi-tion on the skin at sites covered by workwear.

Several methods are available for measur-ing dermal deposition; garment samplers, sur-rogate skin samples, fluorescent tracertechniques, skin washing, tape stripping, andsurface wipes. The appropriateness and limi-tations of these techniques have been recentlyreviewed by Fenske.6 We chose to use skinpads as a surrogate skin sampler, as these hadbeen successfully applied in previous studiesof dermal exposure to PAHs.4578 Althoughskin pads have been used as measures of totaldermal deposition of substances and possiblyalso of the quantity of dermally absorbedmaterial, our application of the technique wasof a more limited nature and was aimed atfinding whether a difference in pad deposition

was obvious between the two workwearphases. The skin pads were located on sitescovered by protective clothing and weredesigned to detect any transfer of PAHs thatmay arise either through the protective clothingor from PAHs already deposited on the cloth-ing. Two skin pads per person were used onthe first day back at work when the maximumbenefit of the intervention strategy was likely.

Materials and methodsDESIGN AND SAMPLE SIZEAn intervention study9 with a crossoverdesign'0 was used in which the differences inexcretion of 1-OHP and skin pad depositionof pyrene in a group of exposed workers werecompared during a normal workwear phaseand an intervention workwear phase (table 1).The advantages of the crossover design werethreefold; firstly the design allowed moreobservations for the given sample size, sec-ondly every worker acted as his own controlthus eliminating variations between workers,and finally the design aimed to reduce thepotential bias from variability in exposure overtwo consecutive work periods (weeks).

At the liquefaction factory workers whomay be exposed to the process material areengineers, responsible for monitoring plantequipment and sampling process streams, andmaintenance workers who are required toattend routine maintenance procedures andunexpected occurrences, for example processstream blockages. The study took place dur-ing two consecutive working weeks. For theengineers each working week consisted of four12 hour shifts and consecutive weeks wereseparated by four days off. For the mainte-nance staff each working week consisted offive eight hour shifts and consecutive weekswere separated by two days off.

There was a total population of 35 workersavailable for investigation. Ten workers (fiveengineers and five technicians) were selectedas follows; of the 20 engineers on site (fourshift teams, each of five workers) one shiftteam (E2, E9, E10, E11, and E12) wasrandomly selected and consisted entirely ofnon-smoking Caucasian men aged 36 to 53

Table 1 Study design

Weeks:OneNormal workwearIntervention workwear

TwoNormal workwearIntervention workwear

M2, M6, E2, ElO, ElIMl, M4, M8, E9, E12

Ml, M4, M8, E9, E12M2, M6, E2, ElO, Eli

Workwear:Normal Coverall, gloves, boots; workers' own

clothing and undergarments.Intervention New coveralls, new boots, new gloves,

new shirts, new trousers, and newundergarments.

Measurements:1-OHP:

Skin pads

Urine collection 24 h on day 1 of eachstudy week plus five spot samples(four taken on days 1 and 2 and thefifth at the end of the working week).Groin and ankle sites on day 1 of eachstudy week.

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years (mean 43 6). Their job experience at thefactory was between three and five years(mean 3-8) and all were clinically in goodhealth. Of the 15 technicians on site five wereselected to give a stratified sample thatreflected the range of activities performed bymaintenance workers and consisted of threemechanical technicians (M4, M6, and M8),one instrument technician (M2), and onemanual technician (Ml). All were Caucasianmen aged 31 to 52 years (mean 44.6 years)with between one and five years (mean 2 4)job experience at the factory. All were clini-cally in good health. Three of them weresmokers, but smoking was strictly prohibitedat the workplace. The 10 workers were ran-domly assigned to one of two study groups;intervention workwear phase followed by nor-mal workwear phase for workers Ml, M4,M8, E9, and E12, and normal workwearphase followed by intervention workwearphase for workers M2, M6, E2, EI0, andEli. An information sheet was distributed tothe participants to brief them of all the rele-vant facets of the study. A self administereddiary card was used for each week andincluded information on age, smoking habits,diet, medication, and nature of workwear.Informed consent was obtained and eachworker had the right to withdraw from thestudy at any time. After analysis and interpre-tation of the data, workers were informed oftheir own individual results and the employerwas provided with group data.

STRATEGY OF URINARY 1-OHP SAMPLINGSix urine samples were taken from eachperson during each of the two consecutiveworking weeks. These were one 24 hour urinecollection on the first day of each workingweek, four spot samples during the first 24hours after the start of each working week(start of shift on day 1, end of shift on day 1,midway between shifts on days 1 and 2, andstart of shift on day 2) and a further spot sam-ple at the end of each working week. A total of120 samples were taken for analysis.The impact of intervention workwear was

assessed by changes in: (a) the mass of1-OHP excreted in the first 24 hours of eachsampling period-that is, on return to workafter a break-and by (b) the changes betweenconcentrations of 1-OHP at the start and theend of the working week. Taking additionalspot samples-that is, at the end of the firstshift, midway between shifts, and at the startof the second shift-allowed for a comparisonto be made between the mass of excreted 1-OHP derived from a 24 hour urine collection,with the value obtained by calculating areaunder the excretion rate/time curves.

STRATEGY FOR SKIN PAD SAMPLINGAs the impact of intervention workwear waslikely to be most significant on the first day ofeach working week (the potential benefits ofnew clothing would be likely to be diminishedby contamination with process material onsubsequent days) skin pads were worn only onthese days. Exposure pads consisted of a soft

polypropylene adsorbing material surroundedby self adhesive tape for fixation to the skin.7Each worker had two exposure pads to moni-tor dermal contamination during the entirework shift at body sites most likely to be cont-aminated by penetration and permeationthrough clothing. The groin (junctionalregion between thigh and abdomen) and theforefoot on the dominant side were used.

ANALYSESUrine samplesAfter collection, urine samples were stored at- 20'C. All samples were coded and the per-son who undertook the analysis was blind tothe sample sources. Urinary 1-OHP concen-trations were measured by high performanceliquid chromatography (HPLC). The methodwas based on those described by Jongeneelen etal,II Tolos et al,12 and Gardiner et al13 butincorporated modifications as previouslyreported in detail.'

Exposure padsAfter removal exposure pads were packed inaluminium foil and kept in the dark at 4°Cuntil subsequent analysis for pyrene andbenz(a)pyrene. The method involved extrac-tion by dichloromethane followed by highperformance liquid chromatographic separa-tion and fluorescence detection.7

Treatment ofdataDifferences between the effects of normalworkwear and intervention workwear onexcretion of 1-OHP and skin pad depositionof pyrene were examined. Non-parametricstatistical methods (Wilcoxon signed ranktest), with Minitab software, were used as thenumber of subjects was small and the obser-vations were not expected to be normallydistributed.

ResultsThe study investigated two workwear phases,(normal workwear v intervention workwear),two consecutive work periods (each periodcontaining a mixture of normal and interven-tion workwear) and two exposure groups (onein the first instance wearing normal workwearfollowed by intervention workwear, and theother wearing intervention workwear followedby normal workwear).

Only the first comparison, normal work-wear phase v intervention workwear phase,showed a significant difference in excretion of1-OHP and skin pad deposition of pyrene. Nosignificant difference in excretion of 1-OHPor skin pad deposition of pyrene was foundwhen the two consecutive working weeks orthe two exposure groups were compared.

Tables 2 and 3 relate to informationobtained on day 1 of each working week.Table 2 shows the difference between theeffects of normal workwear and interventionworkwear on mass of 1-OHP excreted overone day as determined from a complete 24hour urine collection. A notable reduction(55%) was achieved during the intervention

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Table 2 Mass of 1-OHP excreted (nmol; 24 h urine collection) on day 1 of each workingweek (normal v intervention workwear) for engineers and maintenance workers (n = 10)

Normal Intervention Change in massWorker number workwear day 1 workwear day I excreted

E2 37-92 12-35 -25-57E9 125-78 60-48 -65-30ElO 42-41 22-57 -19-84Ell 43-64 14-98 -28-66E12 8-43 15-76 + 7-33Ml 69- 12 34-84 -34-28M2 65-70 24-78 -40-92M4 69-31 52-73 -16-58M6 67-77 51-57 -16-20M8 67-55 65-14 - 2-41Median (n= 10) 66-62 29-81 -22-70

phase; normal workwear phase median 66-62(range 8-43-125 78) nmol v interventionworkwear phase median 29.81 (range12-35-65-14) nmol. The median reduction inmass excreted (22-7 nmol) was significantlydifferent from zero at the 5% level (95% CI9-5-40-8 nmol). Only one person (E12) didnot show a decrease in mass of 1-OHPexcreted during the intervention strategy, butthis may be due to an abnormally low result(median 8-43 v 66-2 nmol) recorded duringthe normal workwear period. Review of thediary cards of work activities undertaken didnot provide an explanation for this finding:the low urine collection over 24 hours (0.68 1)may have been a factor, but the total creati-nine output of 7-8 mmol over 24 hours,although low was not dissimilar from findingsduring the intervention phase (10 mmolexcreted in 2-22 1 collected). This 24 hourexcretion data for this person is also inconsis-tent with the skin pad deposition data (seenext paragraph and table 3) and remainsunexplained, although a discrepancy in the 24hour urine collections is a possible explana-tion as is excretion of 1-OHP arising fromdietary and environmental sources during theintervention period that were not adequatelyrecorded in the diary card.The effects of the intervention phase upon

skin pad deposition (table 3) supports thefindings already discussed. A notable reduc-tion in deposition (82%) was achieved withintervention; normal workwear median 16(range 6 90-44 50) ng.cm-2 v intervention

workwear median 2.80 (range 2 80-7 05)ng.cm 2. The median reduction, 13.20ng.cm-2 was significant at the 5% level (95%CI 7-3-26-4 ng.cm-2). The skin pads werealso analysed for benzo(a)pyrene but allresults were below the limit of detection (0 05ng). Considering that the pyrene to benzo(a)-pyrene ratio is greater than 450 in bothairborne and liquid phases3 a much higher(about 10-fold) pyrene deposition, in theorder of 150 ng.cm.2 per pad would havebeen required for benzo(a)pyrene to reach thelevel of detection.The impact of intervention over the whole

working week was also assessed (table 4). Areduction in the concentration of 1-OHP atthe end of the working week was found;intervention v normal workwear median 5-73 v7-46 ,umol/mol creatinine. Also, the increasein concentration of 1-OHP was on average50% lower during intervention 2-07 v 4-06pumol/mol creatinine. Neither result, however,achieved significance at the 5% level.The results were also analysed after exclud-

ing values for workers M4 and M6 as theyworked through the weekend before the nor-mal workwear phase and therefore their sam-ples from the start of the work period couldnot be regarded as true baselines. The value atthe end of the work period for E2 during thenormal workwear phase was also excludedon the basis of a low creatinine value(<4mmol/1),8 which might possibly haveresulted in a high 1-OHP reading. With theseexclusions there was still a substantial impactof intervention upon excretion of 1-OHPalthough significance was not reached.

DiscussionA large and significant effect was seen on thefirst day of intervention, as measured by 24hour mass of 1-OHP and skin pad depositionof pyrene (tables 2 and 3). The magnitude ofthis effect, however, was not maintained at theend of the study period where the impactupon the concentrations of 1-OHP at the endof the working week was smaller and notsignificant (table 4). A possible explanationmight be that as intervention clothingbecomes contaminated during the working

Table 3 Deposition ofpyrene (ngcm-2) on skin pads on day 1 of each working week (normal v intervention workwear)for engineers and maintenance workers (n = 10)

Normal workwear day 1 Intervention workwear day I

Skin pad sites Skin pad sites ChangesMean Mean in pad

Worker number Foot Groin value Foot Groin value deposition

E2 24-00 14 10 17 50 ND ND 2 80 -14-70E9 41-60 47 70 44 50 11-30 ND 7 05 -37 00

E10 14-10 14-90 14-50 ND ND 2 80 -11 70

Ell 15-10 21-00 18-05 ND ND 2 80 -15 25E12 ND 22-40 12-60 ND ND 2-80 - 9 80Ml ND 16-80 9-80 ND ND 2 80 - 7 00

M2 7 50 29-80 18 65 ND ND 2-80 -15-85M4 10 40 78-30 44-35 ND ND 2-80 -41-55

M6 ND 11-30 6-90 ND ND 2-80 - 4-10

M8 11-30 16-80 9 05 ND 9 30 6-05 - 3 00

Median (n 10) 16-00 2.80 -13-20

ND = not detectable (<5.6 ng.cm-2), to show average pad concentrations, 50% of the detection limit = 2.8 ng.cm-' was used m

calculations.

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Table 4 1-OHP results (mol/mol creatinine) for normal v intervention workwearfor engineers over afour day workingweek (96 h) and maintenance workers over afive day working week (104 h) (n = 10)

Normal workwear Intervention workwear

Start of End of Change over Start of End of Change overWorker number week week working week week week working week

E2 0-74 48-47 +47-73 0-38 8-33 +7-95E9 5-08 36-33 +31-25 3-62 8-32 +4-70ElO 1-40 3-83 + 2-43 1-15 7-33 +6-18Ell 0-83 7-96 + 7-13 0 37 4-48 +4-11E12 0-73 2-79 + 2-06 1-04 4-41 +3-10Ml 2-19 4-33 + 2-14 1-94 3-22 +1-28M2 3-30 8-85 + 5-55 2-78 3-88 +1-11M4 9-59 5-48 - 4-11 599 4-81 -1-18M6 15-43 8-07 - 7-36 3-79 6-65 +2-86M8 3-25 7-04 + 3-79 4-57 7-65 + 3-08Median (n= 10) 2-72 7-46 + 4-62 2-36 5-73 +2-07

week, the benefit of thediminished over time;intervention would therearise on the first day.On review of the dat;

each study phase, there N

association between masand the deposition of pyiduring both the intersworkwear phases (figure)lation is not unexpected,1-OHP is a reflection ofabsorbed, the depositionselectively placed skin pdirect index of depositiowhole body, nor more ibody uptake of pyrene.ever, was steeper for tUphase, which may sugge,the skin contributes mc

during the normal workintervention workwear p

reinforced by the findingcontamination was founiworkwear phase and cohighest mass of 1-OH]concentration at the end(engineer E9; normal we

45 _ o Normal 0

40 0 InterventionE Normal Workwearm ~35 _ y = 0-253x + 4-4702

2R = 0.318230 Intervention Workwear

y = 0.0549x + 1-6006OQ 25 _R2 0-4828Ca(D

20

0)~~~~15

._ 5C/) { OE: -O El I

OII0 20 40 60 80

1-OHP (nmol)Relation between mass of 1-OHP excreted and skin pad depositand intervention workwear.

new clothing may be 2, 3, and 4). This implies that interventionmaximum effect of prevented deposition on the skin and that thefore be anticipated to excretion of 1-OHP for the intervention group

may be largely attributable to inhalation.a for the first day of The mass of 1-OHP excreted during thewas evidence of some normal workwear period was consistent with;s of 1-OHP excreted previously estimated values at the factory3 andrene on the skin pads was similar in magnitude to results in othermention and normal studies.4 5 7 8 14 15). The lack of a corre- A comparison was made between the mass,for although mass of of 1-OHP excreted from 24 hour urine collec-F total dose of pyrene tions and calculation from area under excre-n of pyrene on two tion rate/time curves derived from data oftads is not of itself a 1-OHP concentration as previously reportedon of pyrene over the In summary this involved the calculation ofimportantly of whole excretion rates of 1-OHP of individual peopleThe gradient, how- (umol/h) and subsequent determination of thehe normal workwear area under excretion rate/time curves to esti-st that deposition on mate total mass excreted over a given time-)re to total exposure that is, in 24 hours. With only four datawear phase than the points; start of the working week, end of the)hase. This is further first shift, midway between shifts, and at thethat the highest pad start of the second shift, area under the curved during the normal calculations tended to overestimate the massrresponded with the excreted by about threefold. This findingP excreted and the shows the limitation inherent in attempting toof the working week use a few spot urine results to estimate massorking period, tables excreted. From our findings many more than

four data points would be needed to convertspot sample data into a reasonable estimate oftotal mass excreted.

There has been much criticism of the use of0 deposition pads as quantitative measures of

dermal contamination, but the use in thisstudy was of a much more limited nature,because the magnitude of deposition was usedas a comparative indicator of deposition overthe phases of the study under investigation.No prediction of the total amount of dermallydeposited material, nor of the materialabsorbed by the skin, was attempted from ourdeposition data. We consider that the use ofskin deposition pads in this limited way isentirely justified, and can provide an impor-tant source of information.The study further shows the usefulness of

urinary 1-OHP in securing improvements inworkplace control and protection for groups

100 120 140 of workers who may be exposed to PAHs.Although this study investigated the effect

ion ofpyrene for normal of changes in workwear policy upon theexcretion profile of 1-OHP, the biological

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monitoring technique lends itself to theassessment of other controls that may influ-ence exposure in the workplace-for example,improved ventilation, washing facilities, or

changes to working practices, such as shiftpatterns. Also, a crossover design allowsworkers to act as their own controls andremoves many of the confounding factors insuch investigations.

Successive studies1 3 at the factory haveshown small reductions in median urinaryconcentrations of 1-OHP at the end of thework period as working practices and hygieneawareness have improved. The effect of theintervention workwear policy upon this valuehas so far provided the highest impact, andhas reduced the previously lowest medianvalue of 7 pmol/mol creatinine to 5-46. This isa clear indication that a further improvementin hygiene at the factory can be achieved bychanges in workwear policy. Ideally the work-wear policy should be daily provision of cleanclothing and frequent chemical cleaning ofthis clothing would be desirable; however, thisideal was precluded on cost grounds. As a

result, the following recommendations were

made to management; no personal clothingshould be worn and workers should be pro-

vided with two sets of workwear, includingunderclothes. During normal working activi-ties, workwear should be changed at leastweekly. Outer clothing should preferably belaundered by a dry cleaning method, as nor-

mal washing procedures are not effective inremoving contamination of PAHs (Van Rooij,personal communication). Additional work-wear should be available for workers acciden-tally contaminated by process material.

It is anticipated that the impact of thesechanges will further improve the occupationalhygiene practices at the factory and willreduce even further potential exposure to andabsorption of process material.

Mr K Hale, Regional Laboratory for Toxicology, CityHospital NHS Trust, Birmingham, and Drs F Jongeneelen

and J Van Rooij, IndusTox, University Business Park,Nijmegen, Netherlands are gratefully acknowledged for carryingout analyses of urinary 1-hydroxypyrene and skin pad pyreneand benz(a)pyrene, respectively.

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5 Van Rooij JGM, Bodelier-Bade MM, Jongeneelen FJ.Estimation of individual dermal and respiratory uptakeof polycyclic aromatic hydrocarbons in 12 coke ovenworkers. BrJ Ind Med 1993;50:623-32.

6 Fenske RA. Dermal exposure assessment techniques. AnnOccup Hyg 1993;37:687-706.

7 Jongeneelen E;, Scheepers PTJ, Groenendijk A, Van AertsL, Anzion RBM, Bos RP, et al. Airborne concentrations,skin contamination and urinary metabolite excretion ofpolycyclic aromatic hydrocarbons among paving workersexposed to coal tar derived road tars. Am Ind HygAssocJ1988;49:600-7.

8 Van Rooij JGM, Bodelier-Bade MM, De Looff AJA,Dijkmous APG, Jongeneelen El. Dermal exposure topolycyclic aromatic hydrocarbons among primaryaluminium workers. Med Lav 1992;83:519-29.

9 Goldenhar LM, Schulte PA. Intervention research inoccupational health and safety. Occup Med 1994;36:763-75.

10 Senn S. Cross-over trials in clinical research. Chichester: JohnWiley, 1993:1-16.

11 Jongeneelen El, Anzion RB, Henderson PT.Determination of hydroxylated metabolites of polycyclicaromatic hydrocarbons in urine. J Chromatogr 1987;413:227-32.

12 Tolos WP, Shaw PB, Lowry LK, MacKenzie BA, DengJF, Markel HL. I-Pyrenol in urine: a biological monitor-ing method to assess exposure to polynuclear aromatichydrocarbons. Appl Occup Environ Hyg 1990;5:303-9.

13 Gardiner K, Hale KA, Calvert IA, Rice C, Harrington JM.The suitability of the urinary metabolite I-hydroxy-pyrene as an index of polynuclear aromatic bioavailabilityfrom workers exposed to carbon black. Ann Occup Hyg1992;46:681-8.

14 Jongeneelen El, Anzion RBM, Scheepers PTJ, Bos RP,Henderson PTh, Nijenhuis EH, et al. 1-Hydroxypyrenein urine as a biological indicator of exposure topolycyclic aromatic hydrocarbons in several workenvironments. Ann Occup Hyg 1988;32:35-43.

15 Boogard PJ, van Sittert NJ. Exposure to polycyclicaromatic hydrocarbons in petrochemical industries bymeasurement of urinary 1-hydroxypyrene. OccupEnviron Med 1994;51:250-8.

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