Change in Hearing Conservation and Legislation and Prevention of Permanent Threshold Shift

Abstract

Background: Between 1985 and 2006 there were improvements in hearing conservation at Nyrstar Hobart Smelter, including engineering controls and the introduction of mandatory hearing protection in noisy areas. In 1996 there was a change to Tasmanian legislation regarding compensation for hearing loss, so that compensation was based on audiometric criteria. Objectives: To examine the effect of changes to hearing conservation programmes and legislation.on the rate of hearing deterioration in employees of Nyrstar Hobart Smelter. Methodology: 1. The audiograms of 224 Employees of the Nyrstar Zinc Smelter were compared between two 10-year time periods (1986-1996 and 1996-2006) to determine if there is a difference in the incidence of NIHL diagnoses. Null hypothesis 1: That there is no difference in the incidence of NIHL diagnoses between the first and second 10 year periods 2. The audiograms were further analysed to determine the incidence of NIHL diagnoses at different plants (Roast/Acid/Wharf, Leach/Purification, Electrolysis, Casting) within the site. Null hypothesis 2 – That there is no difference in the incidence of NIHL diagnoses between the areas of the site Results: Overall 22.6% of smelter workers met the criteria for NIHL in the 10 year period before 1996. 28.5% of workers met the criteria NIHL in the 10 year period post 1996. Therefore there was no significant difference in the rate of NIHL between the two time periods (p=0.4864). The proportion of employees in the Roast and Leach plants meeting the criteria for NIHL actually was higher in the latter time period, but the difference was not statistically significant (p= 0.494, p=0.0983). The proportion of employees diagnosed with NIHL showed a mild improvement in the Casting and Electrolysis plants and other miscellaneous staff, but again was not statistically significant (p=0.89, p=0.09, p=0.99). No part of the plant showed a significant difference in the proportion of employees diagnosed with NIHL compared with the population as a whole. Discussion: Noise Induced Hearing Loss is a major cause of occupational morbidity. This study found no significant difference in the frequency of diagnosis of NIHL between two subsequent decades. This suggests no significant impact of two known significant changes over the period - a combination of improved hearing conservation programmes at the worksite and changes to legislation on noise-induced hearing loss.