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The inpidual fitting of HPDs, theconsistency of HPD usage and exposure level during useand non-use are crucial elements in determining the actualnoise dose (Seixas et al. 2005).In addition, HPD data are based on employees’ self-report, which can be subject to reporting bias and socialdesirability (Griffin et al. 2009). These uncertainties canlead to misclassification, thereby overestimating HPD usage and underestimating the true effect of hearing pro-tection (Davies et al. 2008).Unfortunately, data about the effectiveness of the HPDsand about the consistency of usage were unavailable.Effects of noise exposure timeThe relationship of hearing loss and exposure time, definedas years of employment in construction, is also explored.Exposure time is positively related to hearing thresholdlevels; longer exposure times are associated with higherPTA3,4,6 values.This effect was about 0.09 dB loss in PTA3,4,6 for eachyear of exposure, after adjustment for age, noise intensity,and other risk factors. This increase is similar as reported inISO-1999, which predicts an average increase in medianPTA3,4,6 values of 1 dB/decade for exposure levels of90 dB(A) (ISO 1990). Also a review by Ro ¨sler (1994)reports the same amount of increase in age-corrected HTLsat 4 kHz, after the first 10 years of exposure.When comparing the age-corrected PTA3,4,6 values ofthe study population and the ISO predicted NIPTS as afunction of exposure time, the greater inter-inpidualvariation in the distribution of NIHL in exposed con-struction workers is remarkable. This suggests a high var-iation in factors influencing the susceptibility to hearingloss in each exposure year interval of the study group, suchas HPD use, prior employment, non-occupational noiseexposure, hearing disorders, and variability in noiseintensity.However, the median values of both the noise-exposedworkers and the ISO predictions have a similar slope, atleast for exposure times between 10 and 40 years.An interesting aspect is the relationship during the first10 years of noise exposure. Construction workersemployed for less than 10 years show greater hearinglosses than expected based on the interpolation of ISO-1999. In addition, observed hearing loss increases over thefirst 10 years of exposure at the same rate as in the fol-lowing 10–40 years of exposure duration, where a patternof strongly increasing thresholds would have been expected(ISO 1990;Ro ¨sler 1994; Prince 2002).To investigate the role of job history in this group withshort exposure duration, this relationship is determinedonly for construction workers younger than 30 years of agethat reported no prior employment. This selection of 2,190employees shows the same pattern of median age-correctedPTA3,4,6 values that is about 10 dB higher than predictedby ISO.A number of previous studies also found a discrepancybetween ISO predictions and measured hearing loss duringthe first years of exposure. Analyses based on serial au-diograms of railway workers showed that hearingthresholds exceed model predictions in the very beginningof noise exposure, showing age-corrected hearing loss atjob entrance of 9 dB averaged over 2 and 4 kHz (Hen-derson and Saunders 1998). Another study, monitoring acohort of newly enrolled construction apprentices, showedHTLs of 12.2 dB HL at 4 kHz at baseline (Seixas et al.2004) without any change in audiometric hearing thresh-olds over the first 3 years of employment (Seixas et al.2005). The reported hearing threshold levels at job entrancein these studies are all higher than 0 dB HL and correspondto the median age-corrected PTA3,4,6 of 10.9 dB HL foundhere.The ISO-1999 model depends on the interpolation ofpredicted hearing thresholds after 10 years of exposure andthe assumed hearing thresholds of 0 dB HL at the begin-ning of employment. Our findings suggest that this may notcorrectly represent the true development of NIHL over thisperiod of exposure. The interpolation of the ISO formulacould either be less applicable to the population of interest,or the starting point of 0 dB HL is set too low, possibly dueto the fact that the amount of early hearing damage in thispopulation is underestimated.NIHL in young employeesA Dutch survey of health-related and occupational prob-lems among construction workers shows that 7.6% ofconstruction workers younger than 25 years are diagnosedwith NIHL (Arbouw 2009). Reported prevalence of hear-ing loss among young adults entering the constructionindustry in literature is even higher, ranging from 14.4 to16% (Rabinowitz et al. 2006; Seixas 2005). This suggeststhat the starting point of 0 dB HL defined in ISO-1999 isset too low in this population, because NIHL is alreadypresent in workers even before employment. Possibly, thisis caused by noise exposure in recreational settings,underlining that non-occupational noise is another com-plicating factor in the relationship of occupational noiseexposure and hearing impairment. Neitzel et al. (2004)demonstrated that approximately one-third of apprenticesin the construction industry experience equivalent noiselevels higher than 80 dB(A) from recreational noiseexposure, placing them at risk for NIHL even beforeconsidering occupational exposure. Effects of both occu-pational and non-occupational noise exposure will accu-mulate and exposure to non-occupational noise preventsworkers to recover from occupational noise exposure.Since the current study was conducted during audiometricscreening in an occupational health setting, no informationconcerning exposure to leisure noise is available.Information about non-occupational noise exposure and abaseline audiometric measurement would be highlyadvisable for medico-legal purposes. Effects of confounding factorsThe influence of other possible confounding factors mustbe considered when interpreting the presented relationshipsbetween hearing loss and noise exposure. Despite con-founding factors such as job history and use of hearingprotection, the multiple linear regression analysis still showa significant contribution of noise exposure to the regres-sion model. Lifestyle factors, such as smoking, alcoholintake and hypertension, do not show a relationship withNIHL in this population.