Noise-induced hearing loss in a longitudinal study of Norwegian railway workers

OBJECTIVE The aim of this study was to analyse longitudinal data to assess the risk of noise-induced hearing loss (NIHL) in Norwegian railway workers. DESIGN Longitudinal. SETTING A major Norwegian railway company. METHODS We examined data from the first and last audiograms for the period 1991-2014, from 9640 railway workers with varying occupational noise exposure and with an average observation period of 10 years. The course of hearing acuity in seven groups of railway workers (train drivers, conductors, bus drivers, traffic controllers, train maintenance workers, track maintenance workers and others) were compared with each other and with ISO standards (ISO 1999). RESULTS The change in hearing threshold during the observation period was 2-3 dB in the 0.5-4 kHz range and 6-7 dB in the 3-6 kHz range adjusted for age and sex, for all occupational groups, which is slightly less than expected (8 dB) according to ISO 1999. CONCLUSIONS The risk of NIHL in Norwegian railway workers during the period 1991-2014 has been negligible.