For these subjects, only the latter ear is preserved in the dataset. Data are excluded for 447 workers with insufficient noise exposure data; they miss either information on job title (n = 19) or duration of employment (n = 428). Finally, the 1,958 currently exposed workers

that reported prior employment in construction are excluded from the internal control group. The excluded participants do not differ significantly from the included subjects, except for younger age (−3.3 ± 0.5 years) and shorter employment duration (−6.0 ± 2.9 years). However, age-corrected hearing loss is similar in both groups (p = 0.908). The study population thus comprises 27,644 EX 527 solubility dmso men and 54,931 ears. Data analysis All statistical analyses are performed using SPSS for windows selleck products software, version 15.0. Binaural average thresholds are computed for each test frequency and for all

subjects. If threshold levels of only one ear are available, these are regarded as the binaural thresholds and are used for further analyses. Audiogram data usually have a positively skewed distribution. However, the tested sample is assumed to be large enough to approach a normal distribution and parametric tests are used (Dawson-Saunders and Trapp 1994). The mean binaural hearing threshold levels of exposed workers are compared to age-matched. ISO-standard values using a paired Student’s t test, and to HTLs of the non-exposed control group using an independent Student’s t test. In order to compare hearing thresholds of the noise-exposed workers to those of controls and to NIHL predictions by ISO, HTLs of each participant are corrected for age

effects by subtraction of the Phosphatidylinositol diacylglycerol-lyase age-matched median HTL predicted by annex A of ISO-1999. This ISO model assumes that noise-induced permanent threshold shift (NIPTS) and age-related hearing loss (ARHL) are additive, according to the following empirical formula: $$ \textHTL = \textARHL + \textNIPTS-(\textARHL*\textNIPTS)/120 $$The correction term (ARHL * NIPTS)/120 starts to modify the result significantly when NIPTS + ARHL is more than approximately 40 dB HL. To avoid underestimation of NIPTS in this study, this correction term was taken into account in calculating the age-corrected thresholds for measured HTLs exceeding 40 dB HL. To simplify the results, hearing loss is also evaluated using pure-tone averages calculated for 1, 2 and 4 kHz (PTA1,2,4) and for the noise-sensitive frequencies 3, 4 and 6 kHz (PTA3,4,6). These parameters are used in multiple linear regression analyses, to investigate the dependence of hearing threshold levels on noise intensity and exposure time. Since there is an Smoothened Agonist in vitro important dependence between age and hearing loss, age is also considered as an explanatory variable.