Background.

Occupational noise-induced hearing loss (ONIHL) is a major cause of preventable morbidity and one of the most prevalent work-related health hazards globally. Occupational hearing conservation programs (HCPs) rely on a hierarchy of control measures for prevention, early detection, and diagnosis of ONIHL. Typically, the goal of HCPs is not complete prevention of ONIHL so much as minimising risk of ONIHL to some acceptable level. Despite HCPs, ONIHL remains a leading cause of morbidity implying potential ineffectiveness.

Our main aims were to: (i) Assess evidence on commonly used noise-exposure limits, associated NIHL risk levels among workers, and predicted excess risk at these levels. (ii) Develop an understanding of the incidence, compensation costs, and trends of ONIHL in New Zealand and other high-income countries. (iii) Investigate practices, perceived barriers, facilitators, and challenges with existing HCPs in high-income countries, using New Zealand as a case study. (iv) Examine the accuracy and precision of noise monitoring and surveillance methods. (v) Investigate the susceptibility of different audiometric test frequencies in early ONIHL stages and evaluate commonly used early flag/shift criteria.

Methods

The thesis used a variety of methodological approaches and types of data to address the aims, including secondary data via systematic reviews, and primary data via analysis of ONIHL compensation databases, and garnering expert input via the Delphi technique and cross-sectional surveys.

More specifically: i) Three systematic reviews addressed five research questions on accuracy and precision of different noise-exposure assessment methods, susceptibility of audiometric test frequencies to early ONIHL changes, sensitivity of early detection metrics, ONIHL levels at various noise exposures, and differences between observed and predicted excess risk. ii) A retrospective analysis of ONIHL compensation data from five high-income countries (Australia, Germany, New Zealand, Hong Kong, and Canada) examined incidence rates, trends, and costs over a decade. iii) A cross-sectional online survey with quantitative and qualitative questions was conducted with stakeholders (employees, employers, occupational health specialists) to assess practices, barriers, strengths, and challenges of existing HCPs. A validated questionnaire covering key HCP components was adapted, piloted, and administered in English via Qualtrics from July 2023 to April 2024.

Results

The first systematic review assessing occupational noise exposure and the associated excess risk at different noise limits found that, as expected, higher noise levels were linked to an increased risk of ONIHL. Commonly used prediction models estimated an excess risk of 8% to 22% for exposure to 85 dBLAeq, 8hr, 8hr over 40 years. Variability in estimates was attributed to differences in ONIHL definitions and inconsistencies in hearing conservation practices.

Across the five high-income countries where complete data was available, 131,433 newly compensated ONIHL cases were identified, with males comprising 96.3% of cases. The construction and manufacturing sectors were primary contributors to ONIHL. Across all study countries, ONIHL ranked among the top three work-related conditions, with incidence rates ranging from 5 to 148 per 100,000 employed workers. Rehabilitation compensation costs exceeded USD 800 million during the study period, with Germany accounting for the highest expenses. Overall, nearly USD 2 billion was spent on ONIHL compensation during this period, including pension costs associated with ONIHL cases in Germany. The analysis of compensation data showed that in New Zealand, male workers comprised over 90% of new ONIHL cases, with more than 50% occurring among workers aged 65 years and older. The mining, manufacturing, agriculture, forestry, and fishing sectors had the highest ONIHL incidence rates. Submitted and accepted compensation claims rose until 2016, followed by a decline. Despite fewer new cases post-2016, the economic burden persisted, marked by increasing annual costs.

The survey involved 58 participants, including 40 occupational health specialists, 9 employees in noisy environments, and 9 employers/supervisors. Results indicated a lack of understanding among employees regarding ONIHL and proper Hearing Protection Devices (HPD) use, signalling ineffective training. Baseline hearing tests were often delayed, and noise measurements were deemed costly. Current HPDs are uncomfortable, leading to frequent removal. Despite this, occupational health specialists reported a gradual increase in awareness, with audiometry and ONIHL education becoming common during annual health checks. Notably, 67% of employers/supervisors overlook noise ratings when purchasing equipment.

Financial barriers to implementing HCPs, particularly in lower-income countries, were also considered. A systematic review comparing various noise measurement methods various methods found that a hybrid approach (combining task-based, subjective rating, and trade mean measurements) performed with negligible bias, high precision, and accuracy comparable to gold-standard personal dosimetry. Given the variation in HCP assessment strategies, namely in relation to audiometric monitoring, systematic review-2 explored the relative importance of each audiometric test frequency. The results suggested that those frequencies above 8 kHz, particularly 12-16 kHz, were most susceptible to early ONIHL, while conventional frequencies of 3-6 kHz were also susceptible. Therefore, depending on which frequencies are used in early flag metrics, audiometric monitoring processes might have low sensitivity and specificity, but modifications can improve performance. Another feature of HCPs that can vary are the criterion noise exposure levels that trigger action.

Conclusions

The thesis provides valuable insights into high-risk groups for ONIHL. The findings emphasise the need for sector-specific preventive measures. The findings also emphasise the need for intensified ONIHL prevention efforts, particularly within the manufacturing and construction industries across high-income countries. The thesis highlights the disproportionate representation of males among the new ONIHL cases, whether due to their concentration in noisy settings or inherent susceptibility to noise-induced hearing changes. The economic burden is evident, as nearly USD two billion was allocated for compensation of ONIHL costs. The significant burden of ONIHL as a leading occupational issue, and the present work also offers fresh insights for areas to target towards intensified prevention and intervention efforts for accelerated reduction in new ONIHL cases.

Improving awareness and training programs, as well as expanding the variety of HPDs, are crucial for enhancing compliance and HPD utilisation. Consistent and timely audiometric monitoring, along with comprehensive noise assessments, aligns with evidence-based practices. Addressing these gaps can tailor HCPs to effectively prevent ONIHL, promoting better health outcomes for workers in noisy work environments.

A variety of relatively lower-cost noise exposure assessment methods had relatively high accuracy and precision levels comparable to personal dosimetry. These findings are particularly relevant for low-income countries where occupational noise measurements should be obtained with minimal work disruptions and costs. There is a need to refine ONIHL monitoring metrics and study extended high frequencies for early detection. In addition, improved noise exposure guidelines are required, based on long-term multinational data to generate more precise risk estimates.