Human hearing effects by noise and hand-arm vibration on distortion products otoacoustic emissions

Epidemiological and animal studies point out the synergistic effect of noise and vibration exposure on the hearing threshold shift. The measurement of otoacoustic emissions (OAEs) could be an objective test to be involved as biomarker of early effects on human cochlear functionality. Here a study of the synergistic interaction between noise and hand arm vibration (HAV) has been performed on a group of normoacousic non-exposed volunteers (age range: 25-50 years) in three exposure conditions: 1) exposure to noise only (0-10 kHz 95 dBA white noise by means of a loudspeaker probe in the subject's right ear canal for 3 min), 2) exposure to hand-arm vibration (30 m/s2 rms acceleration level at 60 Hz for 3 min in the right forearm direction), 3) combined exposure to noise (condition 1) and hand-arm vibration (condition 2). The experimental procedure involves pre-exposure audiological test (standard broadband audiometry) and acquisition of distortion product OAEs (DPOAEs). Each exposure has been repeated for five times, and DPOAEs have been recorded during each exposure pause. After the last exposure, DPAOEs have been measured at intervals of 10 minutes for three times. A customized filtering method based on wavelet transform has been used to separate the short latency and long latency component of DPOAEs according to their different phase-gradient delay. Differences between pre-exposure and during/post-exposure DPOAEs amplitudes have been computed for each subject, and compared by means of a Student's t test in one third of octave frequency bands. The exposure to noise only is not statistically significant. On the other hand, the combined exposure to noise and HAV and the exposure to only HAV give opposite effects which are statistically significant at mid frequencies. In particular, DPOAE amplitudes decrease after the combined exposure, while they increase after the exposure to vibration only.