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Fig. 3 does not show a stronger relationshipof noise exposure level with age-corrected PTA3,4,6 valuesin the non-user group compared to HPD users.When piding the noise exposure levels into high noiseintensities ([90 dB(A)) and moderate noise levels(between 80 and 90 dB(A)), it is shown that 84.4% of thehighly exposed workers report to use HPDs versus 53.6%of the employees exposed to moderate noise levels. Astratified regression analysis for these two groups showedthat HPD use only showed significant association withPTA3,4,6 in workers exposed to noise levels between 80 and90 dB(A) (data not shown).DiscussionThe results of this study confirm the adverse effect of noiseexposure on hearing threshold levels; the construction work-ers exposed to noise have poorer hearing thresholds comparedto their non-exposed colleagues and to an internationalreference population, especially in the 3–6 kHz region.Audiometric resultsThis study shows a maximum mean deviation of 16.5 dB at6 kHz from the ISO reference population. Compared to theinternal control group, the greatest average difference is7.0 dB, at 4 kHz.Although these differences are not as large as expected,the findings are in agreement with a study of Suter (2002).That study reports hearing threshold levels of carpentersand equipment operators that were approximately 5 dBworse than the HTLs reported in annex B of ISO-1999 inthe high frequency region. The unscreened reference pop-ulation of annex B reports HTLs, which are comparable tothe high frequency thresholds measured in our internalcontrol group.Nevertheless, the small group effects do not rule outsignificant threshold shifts in the ears of inpiduals that aremore susceptible to noise-induced hearing loss than onaverage.Study limitationsAlthough the main strength of this study was the size of thestudy population showing only a small percentage ofmissing values, some limitations in test administration anddata collection cannot be avoided.When comparing hearing threshold levels of construc-tion workers to ISO-1999 standard values, both noise-exposed workers and controls show a deviation of about10 dB HL at the lower frequencies. This deviation isreported in other studies as well, either in control groupsused to analyse hearing ability of construction employees(Hessel 2000; Hong 2005) or in a general occupationalpopulation (Dobie 2007). In this study, some aspects of testadministration may have been responsible for thisdifference.The available audiometric data are retrieved fromscreening assessments, omitting measurements of boneconduction. Therefore, we cannot correct for the presenceof possible conductive hearing losses (e.g. due to perma-nent middle ear problems or temporarily conductive lossescaused by a cold) that may be responsible for the elevatedthresholds at the lower frequencies. Moreover, audiometricmeasurements are carried out on location in a mobile unitequipped with a soundproof booth. Nevertheless, possibleexposure to background noise during the hearing test,which could produce elevated thresholds at 0.5 kHz, and toa lesser extent at 1 kHz (Suter 2002), cannot be ruled outcompletely.Furthermore, in this study no fixed noise-free periodprior to audiometric measurements is defined. However,minimal time between possible occupational noise expo-sure and hearing tests was 2–3 h. Guidelines in literaturerecommend a longer noise-free period, varying from 6 to14 h (NCvB 1999; May 2000). Consequently, the noise-free period of 2–3 h may not be sufficient to fully recoverfrom a possible temporary threshold shift (TTS) (Melnick1991; Strasser et al. 2003), and a complete absence of TTScannot be guaranteed.Moreover, collecting the appropriate data for noiseexposure in this large population appears to be anotherlimitation in this study. This study lacks inpiduallymeasured noise exposure levels. Because constructionworkers are highly mobile and perform several differenttasks, it is extremely difficult to obtain accurate estimatesof the inpidual noise exposure levels.Noise exposure estimationsAlthough regression analyses confirm a significant rela-tionship between noise intensity and PTA-values, thehearing thresholds increase only marginal with increasingnoise exposure level. This relationship follows a muchflatter curve than predicted by ISO-1999.A previous examination of Dutch industry workerscompared single frequency threshold levels to ISO pre-dictions (Passchier-Vermeer 1986) and obtained a similar pattern, suggesting that ISO underestimates hearing loss atlower exposure levels and overestimates hearing loss athigher noise levels. In a more recent study, the shiftbetween baseline and follow-up audiograms showed goodagreement with model predictions (ANSI 3.44 1996)atlower noise exposure levels, while at higher noise inten-sities less hearing loss than predicted was observed(Rabinowitz et al. 2007).In the current study, inpidual noise exposure intensi-ties are assigned based on job titles. This may have beentoo simplistic. It does not take into account that exposuremay vary extensively between workers and over time. Thepersity in specific tasks and the variety of equipment usedat different workplaces introduces uncertainty in the cal-culations of noise exposure (Passchier-Vermeer 1986;Rabinowitz et al. 2007). As a consequence, the resultingestimates are not accurate enough to obtain a reliable dose–effect relationship.Although the majority of the noise level estimates usedin this study are mainly based upon carefully conductedsound level measurements and/or on personal dosimetry,noise levels are determined during a limited period of time.Therefore, the noise estimations are only samples and thislimited sampling in complex and variable job situations,may have resulted in less accurate estimations.Finally, the present noise exposure levels are also usedas estimations of past exposure. Noise exposure levels ofthe construction workers may have varied considerablyover their career. Regression analyses show only a smalleffect of prior employment on hearing, but the changeswithin jobs overtime may have limited the validity of thenoise intensity estimations.All these uncertainties in noise level estimations mayhave obscured a clear dose–effect relationship for theinpidual construction worker. However, for groups ofworkers with a sufficient number of employees, we mayassume that most of the uncertainties mentioned above, e.g.the day-to-day variability and variations between inpid-ual workers, will be averaged out. Although the relationsfound in such an approach may be prone to some bias, wedid not expect to find such a weak dose–effect relationship.Attenuation of noise exposure from the use of hearingprotection might partly explain the lack of the typicaldose–response effect between noise level and hearing lossas well (Rabinowitz et al. 2007). The use of HPDs cancause inaccuracy in inpidual noise exposure estimation.This may have resulted in an overestimation of hearing lossfor HPD users at noise intensities exceeding 90 dB(A), atwhich a higher percentage of usage is reported. For thisreason, stratified analysis for subgroups of HPD users areperformed. The interpretation the results of the HPD usersis difficult because data on the effectiveness of hearingprotection and the consistency of wearing are unknown.But also for the non-users the results do not show theexpected relationship of noise intensity and hearing loss(Fig. 3).Apparently, the variability between inpidual workerscombined with confounding factors such as the use ofhearing protection, differences in past exposure, slightTTS-effects, and the inaccuracy of the noise exposureestimations prevent us from making accurate predictions ofthe effects of noise intensity on hearing, even in a popu-lation of this large size.Effects of hearing protectionHearing protection may have its greatest effect at highambient noise levels. Workersexposedtohighernoiseintensities are obliged to wear hearing protection and aremore bothered by ambient noise, making them moreconsistent in wearing their protection (Rabinowitz et al.2007). In lower ambient noise levels HPDs may interferewith communication, jeopardizing the consistency ofusage (Suter 2002). Current analysis shows that 84.4% ofthe employees exposed to noise levels exceeding90 dB(A) indicated to use HPDs versus 53.6% of theemployees exposed to noise levels between 80 and90 dB(A).Regression analysis shows a positive association ofhearing loss and HPD use; employees using HPDs had onaverage 1.4 dB higher PTA3,4,6 values than non-users.Bauer et al. (1991) also found a positive associationbetween of the usage of HPDs and hearing loss by ana-lysing a very large population of workers exposed tooccupational noise. This can be explained by the sugges-tion that workers with beginning hearing problems arebetter motivated to use HPDs more consistently than theircolleagues without hearing problems.When workers are pided into highly exposedemployees and employees exposed to moderate noise lev-els (80–90 dB(A)), HPD usage only shows a significantassociation with hearing in the moderately exposed group(data not shown). HPD use does not contribute significantlyto the multivariate regression model for PTA3,4,6 in thehighly exposed group, despite the assumption that these aremore consistent users.In this study, HPD usage was scored as a binary vari-able, while the actual consistency of usage would be amore suitable predictor.