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

2. 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

3. 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

4. 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.

5. 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

6. 1/2 exposure time for every 3 dB
What do these levels mean?
1/2 exposure time for every 3 dB
Danger level

7. 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

8. 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

9. 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)

10. 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

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

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

13. 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.

14. 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 ISO ;
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

15. 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 ( )
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

16. 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.

17. 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

18. 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
Some NIHL Population
90699
2.25
104088
2.58
*Assuming a prevalence of hearing loss in NZ of 10% (Greville, 2005)

19. 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
Some NIHL Population
90699
2.25
104088
2.58
*Assuming a prevalence of hearing loss in NZ of 10% (Greville, 2005)

20. 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
Some NIHL Population
90699
2.25
104088
2.58
*Assuming a prevalence of hearing loss in NZ of 10% (Greville, 2005)

21. 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
Some NIHL Population
90699
2.25
104088
2.58
*Assuming a prevalence of hearing loss in NZ of 10% (Greville, 2005)

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

23. Estimates show similarity to level of hearing loss in different industries

24. 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.

25. 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

26. 65 year old males exposed > 10 years
Factors Affecting Risk of NIHL in Humans
65 year old males exposed > 10 years
ISO 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

27. Conclusions and Implications
Prevalence of occupational NIHL in NZ is around % of workforce and % 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