Peter R ThorneSection of Audiology, Department of Physiology and Centre for Brain Research

Effects of noise on hearing and “Noise-induced hearing loss”

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

Introduction

• 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.

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.

Noise and Hearing

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

New Zealand Exposure Limits to produce permanent loss (and other jurisdictions too)

What do these levels mean?

Noise-induced Hearing Loss 7

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

broadens

8

Influence of Duration and Intensity

For given intensity hearing loss plateau after ~10yrs exposure

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

• Noisy industries major cause of hearing loss

Noise a predominant occupational hazard

12

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.

• 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)

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

Modelling Estimates of Prevalence and Incidence

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 0.5-4kHz) 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

Comparison of prevalence estimates with International (WHO) and NZ data

*Assuming a prevalence of hearing loss in NZ of 10% (Greville, 2005)

Measure WHO Data NZ Data Prevalence Hearing Impaired

Population

N % 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

Comparison of prevalence estimates with International (WHO) and NZ data

*Assuming a prevalence of hearing loss in NZ of 10% (Greville, 2005)

Measure WHO Data NZ Data Prevalence Hearing Impaired

Population

N % 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

Comparison of prevalence estimates with International (WHO) and NZ data

*Assuming a prevalence of hearing loss in NZ of 10% (Greville, 2005)

Measure WHO Data NZ Data Prevalence Hearing Impaired

Population

N % 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

Comparison of prevalence estimates with International (WHO) and NZ data

*Assuming a prevalence of hearing loss in NZ of 10% (Greville, 2005)

Measure WHO Data NZ Data Prevalence Hearing Impaired

Population

N % 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

Estimates of prevalence of NIHL by occupation/industry

Estimates show similarity to level of hearing loss in different industries

• Very clear that individuals show different sensitivities to the same noise exposure, indicating that there are other factors which define the response to noise.

2. Variation in Individual Susceptibility 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 yearsISO 1999-1990 10 yr exposure at 100dBLAeq

Factors Affecting Risk of NIHL in Humans

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