A field research study was conducted to determine the actual noise attenuation and perceived comfort achieved by 40 noise-exposed industrial workers in 5 industrial workplaces wearing 4 different industrial hearing protection devices (HPDs) while on the job. Over 2 consecutive 3-week periods of HPD use, the study investigated the effects of 2 different HPD fitting procedures (subject-fit versus trained-fit) on the spectral field attenuation and user-rated comfort achieved with a user-molded foam earplug, a premolded, triple-flanged polymer earplug, a popular foam cushion earmuff, and an ear canal cap with compliant rubber earpods. Workers were pulled from their workplaces without prior knowledge of when they were to be tested and without re-adjusting the fit of their HPDs. Attenuation data were collected using psychophysical real-ear-attenuation-at-threshold measurement procedures as per the ANSI S12.6-1984 standard. Subjective comfort data were also obtained based on multi-dimensional bipolar rating scales. The results of statistical analyses indicated that when training for proper fitting was used, the earplugs significantly improved in noise protection (from 7.2 to 14.6 dB, depending on the frequency and the earplug) at frequencies of 125 - 1000, 6300, and 8000 Hz, whereas the earmuff and the ear canal cap were relatively resistant to the fitting effect. The training was most effective for the slow-recovery foam earplug over the 3-week period. For the comfort rating data, the foam earplug was again sensitive to the fitting effect, but the other HPDs were not. Among the 4 HPDs evaluated in the study, the canal cap protector was judged as the least comfortable HPD, while the other 3 HPDs yielded about the same perceived comfort. This research also showed that the overall field HPD protection afforded can be accurately predicted from single test band (i.e., centered at 500 or 1000 Hz) attenuation measurements. In addition, the field study demonstrated that laboratory simulation protocols designed to simulate field influences on HPD performance (used in the precursor laboratory study) may not be relied upon to yield accurate estimates of field performance of all HPDs. However, the estimates of field attenuation performance were more accurate for the earmuff than for the earplugs tested. Finally, this study demonstrated that the labeled manufacturers' single-number noise reduction ratings (NRRs) and frequency-specific data substantially overestimate the actual HPD attenuation performance achieved in the field. Consequently, on the basis of the results of this study, it appears that an appropriate, device-specific derating scheme to correct unrealistic labeled attenuation data is needed. / Ph. D.

Indiana University-Purdue University Indianapolis (IUPUI) / The inner ear contains specialized sensory epithelia that detect head movements, gravity and sound. Hearing loss and imbalance are primarily caused by degeneration of the mechanosensitive hair cells in sensory epithelia or the sensory neurons that connect the inner ear to the brain. The controlled derivation of inner ear sensory epithelia and neurons from pluripotent stem cells will be essential for generating in vitro models of inner ear disorders or developing cell-based therapies. Despite some recent success in deriving hair cells from mouse embryonic stem (ES) cells, it is currently unclear how to derive inner ear sensory cells in a fully defined and reproducible manner. Progress has likely been hindered by what is known about induction of the nonneural and preplacodal ectoderm, two critical precursors during inner ear development. The studies presented here report the step-wise differentiation of inner ear sensory epithelia from mouse ES cells in three-dimensional culture. We show that nonneural, preplacodal and pre-otic epithelia can be generated from ES cell aggregates by precise temporal control of BMP, TGFβ and FGF signaling, mimicking in vivo development. Later, in a self-guided process, vesicles containing supporting cells emerge from the presumptive otic epithelium and give rise to hair cells with stereocilia bundles and kinocilium. Remarkably, the vesicles developed into large cysts with sensory epithelia reminiscent of vestibular sense organs (i.e. the utricle, saccule and crista), which sense head movements and gravity in the animal. We have designated these stem cell-derived structures inner ear organoids. In addition, we discovered that sensory-like neurons develop alongside the organoids and form putative synapses with hair cells in a similar fashion to the hair cell-to-neuron circuit that forms in the developing embryo. Our data thus establish a novel in vitro model of inner ear organogenesis that can be used to gain deeper insight into inner ear development and disorder.