1. Epidemiology of occupational diseases
A.H. Mehrparvar
Occupational Medicine Department
Yazd University of Medical Sciences

2. NIHL A disease known for many years
Rammazini described deafness in coppersmiths, from constant hammering
Industrial Revolution: an abrupt rise in prevalence of deafness in a variety of trades, including textile workers and boilermakers
boilermaker's disease

3. Hazardous noise levels in a variety of work environments:
Mechanism:
Ischemia
Oxygen radicals
Hazardous noise levels in a variety of work environments:
Military service, manufacturing, construction, transportation, and communications
Leisure-time pursuits: music and hunting

4. Data from the 1977 National Health Interview Survey and the National Occupational Hazard Survey:
Approximately 3.2% had some degree of hearing loss
The proportion of those with hearing loss increased with age
within age groups, rates were consistently greater for those who worked in industries defined as noisy

5. OSHA estimation:
mild degrees of hearing loss in 17% of production workers
16% have more substantial impairments
It is estimated that more than three million workers are affected in the manufacturing sector alone

6. The Finnish Register of Occupational Diseases:
NIOSH estimation:
nearly one in four workers older than 55 years who have been exposed to high noise levels beyond 90 decibels (dB) has some degree of material impairment
The Finnish Register of Occupational Diseases:
incidence rate for occupational hearing loss: 50.3 per 100,000 workers

7. The largest numbers of occupational hearing loss cases in:
estimated annual incidence of NIHL in Great Britain: 1.9 (1.2) cases per 100,000 workers
a substantial underestimation, as many workers do not have access to occupational health services
The largest numbers of occupational hearing loss cases in:
armed forces personnel, miners, construction workers, and employees in manufacturing industries

8. Risk Factors for Hearing Loss
prolonged unprotected exposure to levels of noise above 85 dB
NIOSH estimation:
excess risk of hearing impairment after a working lifetime of exposure to an average daily noise level of 85 dB: 8%, 90dB: 25%
The decibel scale is logarithmic, a 3 dB increase represents a doubling of noise intensity

9. Other factors presbycusis or age-induced hearing loss
from 25% to 40% of people older than 65 years have some degree of hearing loss
Industrial solvent exposures
toluene, styrene, xylenes, trichloroethylene, …
lipid and cholesterol abnormalities
Diabetes
cigarette smoking
thyroid abnormalities

10. Noise and health
Acute acoustic trauma
NIHL
Extra-auditory effects

11. Acute acoustic trauma Exposure to sudden intense levels of noise
abrupt acute and subsequent permanent damage to the middle and inner ear
Mostly in military personnel
Conductive or sensorineural
traumatic rupture of the tympanic membrane
disruption of the ossicular chain
mechanical damage to the oval window
Cochlear hair cell disruption

12. Frequencies between 4,000 and 8,000 Hz
A period of weeks to months may be required for hearing to stabilize
With tinnitus

13. Chronic hearing loss
A combination of mechanical, metabolic, and vascular factors
effects in the organ of Corti, within the cochlea of the inner ear
three outer rows and one inner row of hair cells (sensory receptors of the ear)
hair cells are susceptible to the mechanical trauma of loud noise
Hair cells of the basal turn of the cochlea, which conduct sound at higher frequencies (4,000 to 6,000 Hz): more susceptible

14. Standards of noise exposure
Safe Level: 70 dB
Action level: 85 dB
REL: 85 dB
TLV: 85 dB
PEL: 90 dB
STEL: 115 dB (15 min)
Pain threshold: 120 dB

15. Permissible exposure level
OSHA:
90 dB (TWA -8h
Each 5 dB increase, the exposure time is halved
NIOSH:
85 dB (TWA – 8h)
Each 3dB increase, the exposure time is halved

16. Prevention of hearing loss in workplace
OSHA TLV-TWA for exposure to noise: 90db
NIOSH exposure limit: 85dB
Noise> 85dB
Hearing conservation program (HCP):
Noise monitoring
Engineering controls
Administrative controls
Periodic audiometric evaluations
Worker education
HPDs

17. Noise monitoring
Sound level meter
Noise dosimeter

18. Engineering controls 3 important variables: Source Path Reciever
enclosure
Path
barriers
Reciever
Increasing distance

19. Administrative controls
Purchasing suitable equipment
Reducing the exposure time:
Each 5 dB> 90 dB exposure time is halved
95dB: 4h exposure/day
100dB: 2h exposure/day
105dB: 1h exposure/day
Ceiling: 115db: 15m. Exposure/day

20. Audiometric evaluations
Pre-employment.
Prior to initial assignment in a hearing hazardous work area.
Annually as long as the employee is assigned to a noisy job (>85 dBA)
At the time of reassignment out of a hearing hazardous job.
At the Termination of employment.

21. HPDs
Ear plug (aural)
Canal cap (semiaural)
Ear muff (circumaural)

22. Ear muff