Effects of noise on hearing and “Noise-induced hearing loss” Peter R Thorne Section of Audiology, Department of Physiology and Centre for Brain Research

Introduction Purpose: to look at what we know about the incidence and prevalence of “Noise-induced Hearing Loss” in NZ what factors influence hearing loss from noise

Noise and Hearing Noise exposure causes injury to the inner ear and temporary and permanent hearing loss: clinically latter is NIHL or Noise-induced hearing loss Has always been assumed that Temporary effects are precursors to Permanent damage. This is being challenged, including our own research, suggesting some temporary loss is the ear adapting to noise. Points: Noise damages the inner ear and causes variety of changes, some of which may not be reflected in the audiogram but can cause substantial functional deficits (limiting heairng in background nosie for example) We have always believed that temporary hearing loss is reversible and is a precursor to permanent loss by reverisbly affecting the same areas of the cochlea; that continued exposure leads to trasnformation of temproary to permanent. However, good evidence from animals shows this is not true and that temporary loss may be a different phenomenon and that some injury associated woith exposures that lead to temporary loss may not recover, but leads to permanent damage that is not manifest as a change in the audiogram

Noise and Hearing Main factor defining severity and rate of progression of injury and loss of hearing is the sound energy; defined by the intensity (loudness) and duration of the sound exposure.

Time-Weighted Average =100% Dose Recommended exposure limit New Zealand Exposure Limits to produce permanent loss (and other jurisdictions too) Time-Weighted Average =100% Dose Recommended exposure limit 85 dBA 8 hours 88 dBA 4 hours 91 dBA 2 hours 94 dBA 1 hour 97 dBA 30 minutes 100 dBA 15 minutes 103 dBA 7.5 minutes 106 dBA 3.25 minutes …..3 dB exchange rate Important points are that different exposures produce different hearing loss; also this is not necessarily a safe level, just an acceptable level of exposure. Injury can still occur at lower levels or longer durations

1/2 exposure time for every 3 dB What do these levels mean? 1/2 exposure time for every 3 dB Danger level

Noise-induced Hearing Loss The audiogram is key quantitative index of the noise injury with greater loss at 4kHz. Extent of loss correlated to intensity and duration Study of weaving factory workers, (Burns 1963) deepens The hearing loss increases at the 4kHz frequency with duration of exposure but also broadens to affect other frequencies. But with prolonged exposure the loss will asymptote and can then extned into other frequencies. broadens Noise-induced Hearing Loss

Influence of Duration and Intensity For given intensity hearing loss plateau after ~10yrs exposure For a particular level of exposure the hearing loss at 4kHz reaches a plateau, normally after approximately 10 years. The hearing loss develops above a “critical” level of about 80dBA. Above this value the extent of hearing loss after 10 years is dependent on the level. Hearing loss at other frequencies has a high threshold. Note that the hearing loss at frequencies other than 4k continue to grow with exposure causing more significant handicap. Also the 85 dB is not a totally safe level but can still cause injury. It is an acceptable level to society

Increasing evidence that TTS is not totally reversible. TTS in young mice (16wks) recovers but suprathreshold changes in ABR suggest neural injury. Cochlea show loss of synapses and nerve fibres (Kujawa and Liberman, J Neurosci , 29:14077–14085 , 2009)

These data suggest that some injury with TTS may not recover although thresholds do Indicate that noise injury has profound suprathreshold functional changes Indicates that thresholds (audiogram) may not be sensitive index of noise-induced functional change Important that the audiogram does not appear to tell all the story of the injury. Even with TTS or if animals there is still functional damage if you look at suprathreshold function (ie how we hear in the real world not in a sound-proof booth). Therefore th audiogram is not a good index of injury to the ear

Noise a predominant occupational hazard Noisy industries major cause of hearing loss

Noise levels in NZ Industries Note the large range of exposure levels

1. Estimates of Incidence and Prevalence of NIHL International estimates are 10-30% of hearing loss prevalence is due to noise exposure WHO data suggest 16% of hearing loss is due to noise No previous epidemiological data for NZ. NIHL epidemiology difficult to do.

Modelling Estimates of Prevalence and Incidence Using Global Burden of Disease model for occupational NIHL (WHO, Concha-Barrientos et al., 2004) Proportion of working population in economic sectors exposed to noise (>85dBA) estimated from international and NZ data; Relative risk of occupational noise-induced hearing loss above background (age) estimated from ISO1999-1990; NZ data obtained from field measurements of noise levels in different industries and personal dosimetry International data obtained from NIOSH (1998), WHO (2004) and Prince et al., (1997) Our approach to the epidemiology is targetted and iterative. Starting at high level

Modelling Estimates of Prevalence and Incidence Modelled prevalence and incidence calculated (DISMOD II software) using NZ hearing loss prevalence data (eg Greville, 2005) and census data (NZ Statistics retrospective and prospective estimates); Estimated for 2006 as anchor year (last census) and backward and forward (1986-2030) Using international data and NZ data provides a range of estimates Important that these are seen as estimates only Our approach to the epidemiology is targetted and iterative. Starting at high level

Estimated number of new cases of NIHL (>25dBHL 0 Estimated number of new cases of NIHL (>25dBHL 0.5-4kHz) in the workforce has increased since 1986, but incidence (rate) has decreased.

Estimated number of new cases of NIHL (>25dBHL 0 Estimated number of new cases of NIHL (>25dBHL 0.5-4kHz) in the workforce has increased since 1986, but incidence (rate) has decreased. This we assume to be due to decline in workforce in noisy industries

Prevalence Hearing Impaired Population Comparison of prevalence estimates with International (WHO) and NZ data Measure WHO Data NZ Data Prevalence Hearing Impaired Population N % NIHL only, Workforce 29242 1.47 42497 2.14   Some NIHL Workforce 42309 2.13 47026 2.37 NIHL only, Population 62169 1.54 69613 1.73 15.4-17.3 Some NIHL Population 90699 2.25 104088 2.58 22.5-25.8 *Assuming a prevalence of hearing loss in NZ of 10% (Greville, 2005)

Prevalence Hearing Impaired Population Comparison of prevalence estimates with International (WHO) and NZ data Measure WHO Data NZ Data Prevalence Hearing Impaired Population N % NIHL only, Workforce 29242 1.47 42497 2.14   Some NIHL Workforce 42309 2.13 47026 2.37 NIHL only, Population 62169 1.54 69613 1.73 15.4-17.3 Some NIHL Population 90699 2.25 104088 2.58 22.5-25.8 *Assuming a prevalence of hearing loss in NZ of 10% (Greville, 2005)

Prevalence Hearing Impaired Population Comparison of prevalence estimates with International (WHO) and NZ data Measure WHO Data NZ Data Prevalence Hearing Impaired Population N % NIHL only, Workforce 29242 1.47 42497 2.14   Some NIHL Workforce 42309 2.13 47026 2.37 NIHL only, Population 62169 1.54 69613 1.73 15.4-17.3 Some NIHL Population 90699 2.25 104088 2.58 22.5-25.8 *Assuming a prevalence of hearing loss in NZ of 10% (Greville, 2005)

Prevalence Hearing Impaired Population Comparison of prevalence estimates with International (WHO) and NZ data Measure WHO Data NZ Data Prevalence Hearing Impaired Population N % NIHL only, Workforce 29242 1.47 42497 2.14   Some NIHL Workforce 42309 2.13 47026 2.37 NIHL only, Population 62169 1.54 69613 1.73 15.4-17.3 Some NIHL Population 90699 2.25 104088 2.58 22.5-25.8 *Assuming a prevalence of hearing loss in NZ of 10% (Greville, 2005)

Estimates of prevalence of NIHL by occupation/industry Similar to claims except more in Construction and less in agriculture

Estimates show similarity to level of hearing loss in different industries

2. Variation in Individual Susceptibility to Noise Very clear that individuals show different sensitivities to the same noise exposure, indicating that there are other factors which define the response to noise.

Factors Affecting Risk of NIHL Genetic differences (Gates et al., 2000, Rosenhall et al., 2003) Solvents and ototoxic drugs/compounds, smoking (eg. Uchida et al., 2005; Wild et al., 2005) Sex/gender Age? Exposure variances = Large variation in susceptibility

65 year old males exposed > 10 years Factors Affecting Risk of NIHL in Humans 65 year old males exposed > 10 years ISO 1999-1990 10 yr exposure at 100dBLAeq Big differences in the hearing loss across individuals, therefore very difficult to determine the level of hearing loss in an individual without serial audiograms or functional measures. Wherther by age or noise, big differences in suscepribiloity and may also be interactions

Conclusions and Implications Prevalence of occupational NIHL in NZ is around 1.5- 2.4% of workforce and 1.5-2.6% of the population (15- 25% of the hearing impaired population) Estimate around 1800 new cases each year Effects of noise on the ear affected by many factors. Genetic and environmental influences significant Thresholds (Audiogram) may be insensitive to injury and do not tell the full story