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HPD usage and smoking (Mizoueet al. 2003; Wild et al. 2005).In this study, a large audiometric dataset of 29,216construction workers is used to describe their hearing sta-tus. The effect of noise exposure on hearing is observed bycomparing hearing threshold levels of noise-exposedworkers to thresholds of references. The relationshipbetween hearing and noise intensity and noise exposuretime is examined, with particular interest in the hearingloss established during the first 10 years of noise exposure.The measured relationships are compared to ISO-1999predictions. In addition, the influence of wearing hearingprotection and other factors collected in periodic occupa-tional health surveys on NIHL is considered. MethodsThis cross-sectional study is based on data collected byArbouw, the Dutch national institute on occupationalhealth and safety in the construction industry. These dataare derived from medical records of periodic occupationalhealth examinations (POHE), performed between 1November 2005 and 20 July 2006 throughout TheNetherlands.A POHE consists of an extensive self-administeredquestionnaire and a physical examination, including stan-dardized audiometric testing. POHEs are provided for allemployees in the construction industry, irrespective ofoccupational noise exposure. The right to participate is laiddown in the collective labour agreement, and participationis completely voluntary.Demographic, occupational and health-related data areextracted anonymously from the medical records. Thisincludes information regarding job title, use of HPDs (yes/no), self-reported hearing complaints, noise disturbance atwork and the number of years employed in both the con-struction industry and the current occupation. Cigarettesmoking status (non-/ex-/current smoker) alcohol intake(gl/wk) and blood pressure are also recorded. Hypertension isdefined as systolic blood pressure C 140 mmHg combinedwith diastolic blood pressure C 90 mmHg (De MoraesMarchiori 2006). Independent ethical approval is not neededfor this type of retrospective analyses in the Netherlands.ParticipantsThe eligible study population contains all 29,216 con-struction workers who had undergone a POHE in the givenperiod. Hearing threshold levels of the noise-exposedconstruction workers are compared to different referencegroups, in order to separate the effects of occupationalnoise from those due to ageing and other non-occupationalcauses of hearing loss. The ISO-1999 standard providestwo reference databases: database A, based on a highlyscreened non-noise-exposed population free from otologicdisease, which is used in this study to correct for medianage-related hearing loss; and annex B, an alternativedatabase representing a typical otologically unscreenedpopulation of an industrialized country, not occupationallyexposed to noise. This database derived from representa-tive population-based samples can serve as an appropriatecomparison group (Dobie 2006).The participants of the study population currentlyexposed to daily noise exposure levels below 80 dB(A),such as office workers, can be considered as a comparisongroup as well. These non-noise-exposed employees arerecruited from the same companies and are examined in thesame period and according to the same protocol as theexposed subjects.However, almost two-third of these currently unexposedworkers (65.8%) reported prior employment in the con-struction industry. Their past job titles, and correspondingexposure history, are unknown, but past occupational noiseexposure cannot be excluded for each of these workers.Since an unscreened industrialized population should notbe occupationally exposed, only the 1.016 non-exposedemployees without prior employment are considered as anappropriate control group.These controls show hearing threshold levels (HTLs)very similar to ISO database B, especially in the highfrequency region (3–6 kHz). Since these non-exposedemployees match the workers under consideration, theyform an ideal comparison group (Prince 2002; Prince et al.2003). Thus, this internal comparison group is preferredover the unscreened ISO annex B to be used as controlgroup in this study.Audiometric measurementHearing ability is assessed by a qualified medical assistantusing standardized audiometric examination proceduresaccording to ISO-6189 (ISO 1983). Pure-tone audiometryis conducted at the workplaces in a mobile unit equippedwith a soundproof booth, using a manual audiometer(Madsen Electronics, Taastrup, Denmark) coupled withTDH-39 headphones. Audiometers are annually calibratedaccording to the ISO-389 standard (ISO 1991). Testing isdone during the work shift, but subjects had at least anoise-free period of approximately 2–3 h prior to testing.Pure-tone air-conduction thresholds are determined at fre-quencies 0.5, 1, 2, 3, 4, 6 and 8 kHz in both ears, in 5-dBincrements.A hearing threshold level of 90 dB is the upper limit ofthe equipment and hearing threshold is marked as 95 dB ifthe participant does not respond to this maximum soundsignal. Because of this ceiling effect, only HTLs up to90 dB HL or better are preserved in this analysis.Noise exposure estimationYears of exposure is defined as the years employed inconstruction industry, as is reported in the questionnaire. Ifthe number of years employed in construction sectorexceeds the number of years in the current job, it isassumed that the former job had equivalent exposurelevels.Sound levels are expected to vary more from day to dayfor the inpidual workers than between different workersin the same trade. Therefore, workers are classified by the time weighted average (TWA) noise exposure levels esti-mated for standardized job titles.These daily noise exposure levels were extracted from adatabase of Arbouw. Most of the estimates reported in thisdatabase are retrieved from findings of Passchier-Vermeeret al. (1991). Their findings were based upon a collection ofaudiometric hearing thresholds of a large population ofconstruction workers. For each profession, the noise levelswere derived from the observed HTLs, using a maximum-likelihood fitting procedure in conjunction with the algo-rithm given in ISO-1999. A comparable approach is usedmore recently in a military population (Tufts et al. 2009).This way, hearing thresholds can be predicted for popula-tions, even when noise exposure levels are not preciselyknown. The calculated noise level estimates are a result ofall unknown aspects that may have influenced the workers’noise exposure, such as HPD use, non-occupational noiseexposure, inpidual susceptibility and other factors.Therefore, these predictions were verified by noise mea-surements in 1983, 1991, 2002 and 2007. These measure-ments are generated by Arbouw and include full-shiftpersonal dosimetry and sound level measurements duringspecified job-related tasks. Sound level measurements arecombined logarithmically in order to calculate an 8-hequivalent noise level, using the duration and frequency ofeach task. The daily noise exposure levels obtained bydosimetry are arithmetically averaged to obtain job-specificexposure estimations. Table 1 provides an overview of theavailable data on noise exposure estimates for the twentymost prevalent jobs in the current dataset.The results of the noise measurements showed goodagreement with the noise level calculations for the majorityof job titles (Table 1). In case of a deviation, the result ofthe noise measurements was considered the appropriatenoise exposure level to be used in this study. Also, thedifferent measurements performed in different periodsshowed great similarity.Exclusion criteriaOf the 29,216 participants included in this study, all 951female workers are discarded because of their concentra-tion in non-noise-exposed jobs.