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In addition, both groups show a slightworsening in the high frequencies in the two youngestgroups. In the older age groups, the differences betweenmedian HTLs of the exposed workers and the internalcontrols increase. These differences are greatest for hearingthresholds at 4 and 6 kHz. With increasing age, theexposed group develops a typical NIHL notching pattern inthe high frequency range, which broadens from 4 to 6 kHzto the lower frequencies.Figure 1 shows that hearing thresholds strongly dependon age. Therefore, measured HTLs are corrected for ageeffects. After these corrections, the differences between thenoise-exposed workers and controls remain statisticallysignificant for all frequencies (p\0.001). These differ-ences are relatively small at 0.5 and 1 kHz (\1 dB) butbecome more pronounced at higher frequencies, with amaximum mean difference of 7.0 dB at 4 kHz.Relationship of noise and hearing lossIn order to assess the relationship between hearing loss andnoise exposure, multivariate regression analyses are per-formed, with age as covariate.Both noise parameters and the interaction term show asignificant bivariate association with the PTA-values.However, the interaction term does not contribute signifi-cantly to both multivariate regression models and isexcluded from further analyses. For PTA1,2,4, the modelaccounts for 24.3% of the variance. The age-adjustedregression coefficient for noise level is 0.14 (99% CI0.11–0.19), for years of exposure this is 0.07 (99% CI0.05–0.09). The regression model for PTA3,4,6 accounts for32.4% of the variance. Also the age-adjusted regressioncoefficients for noise level and exposure time are higher forPTA3,4,6, 0.27 (99% CI 0.22–0.32) and 0.12 (99% CI0.09–0.15), respectively.To gain more insight into the relationship betweenhearing loss and noise exposure, the impact of bothparameters on hearing loss is further explored in separateanalyses. The age-corrected hearing thresholds enablecomparison to the noise-induced permanent threshold shift(NIPTS) predicted by ISO-1999. These NIPTS values arefunctions of audiometric frequency, exposure level andexposure time. For each inpidual construction worker, hisexpected median NIPTS is computed.PTA3,4,6 is most affected by noise, and this age-cor-rected pure-tone average is examined as function ofexposure duration. For exposure times between 10 and40 years, the median value of expected NIPTS and itsdistribution can be calculated. For exposure times shorterthan 10 years, median expected NIPTS values are interpolated from the value of NIPTS for 10 years,according to ISO-1999 (Fig. 2).Although the inter-inpidual variation in the age-cor-rected hearing thresholds is larger in the exposed con-struction workers than predicted by ISO-1999, the medianvalues of both groups follow a similar pattern for exposuretimes ranging from 10 to 40 years. However, this is not thecase in the first 10 years of exposure. Where median valuesof ISO are interpolated to a NIPTS of 0 dB HL at the startof noise exposure, the population of noise-exposed con-struction workers shows age-corrected PTA3,4,6 values thatare approximately 10 dB higher at the beginning of occu-pational noise exposure without the steep increase as ispredicted by ISO-1999.Similarly, age-corrected PTA3,4,6 values as function ofdaily noise exposure level are examined (Fig. 3).The non-exposed control group accounted for the starting point at80 dB(A). There are large differences in the distributionsof age-corrected hearing thresholds between the exposedstudy group and the ISO-1999 reference population.Hearing loss variation is, again, much greater in exposedemployees, and their PTA3,4,6 values are almost evenlydistributed over the range of noise intensities. Hearing lossincreases only slightly with increasing noise exposure levelin this population, resulting in an almost flat curve thatdeviates strongly from the NIPTS predicted by ISO-1999.Up to exposure levels of 91 dB(A), construction workersexhibit a greater hearing loss than predicted, while athigher noise levels less hearing loss is observed.Other variables of influenceData collection during periodic occupational healthexaminations also provides information about variousfactors possibly associated with NIHL, such as, the use ofhearing protection, smoking and hypertension. To investi-gate the association between these risk factors and hearingloss, bivariate and multivariate regression analyses areperformed. These analyses focus on PTA3,4,6 only and areadjusted for the confounding effect of age. Results aredisplayed for the overall population and for both HPDsubgroups separately in Table 3. The use of hearing protection shows a positive associ-ation with PTA3,4,6 values, meaning that employees usinghearing protection exhibit slightly more hearing loss thanparticipants never using HPDs. Always being employed inthe current job is associated with significantly greaterhearing loss, and there is a strong correlation between thesubjective complaints about poor hearing and the degree ofhearing loss.Hearing protectionOnly 77% of the employees exposed to daily noise levelsexceeding 80 dB(A) report to wear hearing protectiondevices at work, meaning that 23% of the exposed workersstate to never use protection.Regression analyses show that employees using HPDshave an overall increase in PTA3,4,6 of on average 1.4 dBwith regard to employees never using protection, afteradjusting for relevant covariates. To gain more insight intothe differences between participants using hearing protec-tors and participants not using protection, both groups areanalysed separately.These analyses show that HPD users are employed inconstruction for a slightly shorter period (24.0 vs. 25.4 years)and are significantly younger than non-users (43.7 and46.1 years, respectively). The percentage of HPD usersdeclines with increasing age from 83.2% in employeesyounger than 25 years to 68.5% of the workers 55 years orolder. Of the HPD users 44.8% indicated to be bothered bynoise in their jobs, which is twice asmuch as the 21.6%in thenon-user group. More importantly, the intensity of noiseexposure differs significantly between HPD users and HPDnon-users (90.6 and 89.5 dB(A), respectively). Stratified regression analyses for the subgroups of HPDusers and HPD non-users did not show any differencesbetween the results of both subgroups and of the overallpopulation, except for the insignificant contribution of jobhistory to the model for the non-users (Table 3). However,the regression coefficient found for noise intensity in thenon-user group was slightly higher than in the user group.Nevertheless,