Occupational Noise Exposure

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Occupational Noise Exposure

• Industrial Hygiene
• IENG 341
• Carter J. Kerk
• Industrial Engineering Department
• SD Tech
• Spring 2008

2
Reading Assignment

• Nims, Chapter 9
• Critical Thinking Questions
• p. 225-226
• 1-5
• Due ?

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Outline Occupational Noise Exposure

• Physics of sound
• Anatomy of the ear
• Evaluating hearing ability and hearing loss
• Standards for occupational noise exposure
• Measuring noise in the occupational setting
• Controlling noise

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Introduction

• High levels of noise cause hearing loss
• Hearing loss is mostly irreversible and usually
  preventable
• Noise can also
• produce stress
• reduce productivity
• cause communication problems

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Physics of Sound

• Noise unwanted sound
• Energy in the form of pressure waves
• Waves can be described by frequency (f), speed
  (c), and wavelength (?)
• c f ?
• Sound moves at 344 m/sec in air, 6100 m/sec in
  steel
• Some materials will amplify or reflect sound
• Frequency (f) is related to pitch
• Healthy, young person can detect 20 to 20,000 Hz
  (cycles/sec)
• This declines with age and exposure history

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Sound Pressure Level

• We can measure sound pressure
• Force per unit area
• SI units, pascal, Pa (N/m2)
• All sound pressures are related to a reference
  sound pressure of 20 ?Pa (approximate lower
  threshold for human hearing at 1000 Hz)
• Lp 20 log10 (P / 20 ?Pa)
• Where Lp is the sound pressure, in decibels (dB)
• P is the measured sound pressure, in Pa
• The decibel is a dimensionless quantity based on
  the logarithm of a ratio and gives a more
  convenient range of values than would Pa

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Weighting Scales A, B, C

• Each scale approximates the response of the human
  ear at different ranges of pressure
• Because the human ear does not hear sound as if a
  machine. The human ear is more sensitive to
  higher frequencies
• Derived from comparison experiments
• Example a noise of 1000 Hz frequency and an SPL
  of 20 dB sounds as loud as a noise of 25 dB at
  500 Hz
• A-Scale is most common and referenced by OSHA
  regs
• B-Scale rarely used (medium sound pressure
  levels)
• C-Scale common for evaluating explosions and
  impact noise

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Anatomy of the Ear

• Outer ear and ear canal directs and amplifies the
  sound by 10-15 dB
• Sound pressure waves impact on the ear drum and
  vibrate the three tiny bones in the middle ear
• Malleus (hammer)
• Incus (anvil)
• Stapes (stirrup)
• Which vibrates against the oval window leading to
  the inner ear

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Inner Ear

• Cochlea (inner ear)
• Basilar membrane (lining of the cochlea)
• Supports 25,000 specialized hair cells
• Which send characterizing nerve impulses to the
  brain
• Three semicircular canals (in orthogonal planes)
• Filled with fluid
• Provides sense of balance and relative body
  position
• Have you ever felt dizzy?
• Eustachian tube
• Connects middle ear to throat
• Equalizes pressure
• Have your ears ever popped?

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Hearing Loss

• Conductive hearing loss
• Interruptions along the pathway reducing hair
  stimulation
• Excessive earwax, otitis media (fluid in middle
  ear), ruptured eardrum
• Sensory hearing loss
• Presbycusis (loss due to age)
• Noise-induced hearing loss
• Sociacusis (loss from everyday life)
• Nosacusis (loss from disease, heredity, drugs,
  sudden and severe pressure changes, traumatic
  head injuries)
• Tinnitus (follows traumatic exposure to loud
  noise perceived ringing, roaring, hissing may
  be permanent)

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Evaluating Hearing Ability and Hearing Loss

• Audiograms
• A hearing evaluation exam, called audiometry,
  produces a report called an audiogram
• OSHA requires all workers exposed to an 8-hour
  TWA of at least 85 dBA (Action Level) receive a
  baseline audiogram and annual follow-up exam
• Employee sits in soundproof booth with headphones
  and control button to produce HTL (Hearing
  Threshold Level)
• Method of Limits at the following test
  frequencies 500, 1000, 2000, 3000, 4000, 6000
  Hz, the range most detectable by the human ear
• Speech range 1000 4000 Hz

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Reasons for Variation in Audiometric Testing

• Ear wax buildup
• Head cold, congestion
• Confusion about response procedure
• Incorrect placement of headphones
• Hair under headphones
• Audiometer malfunction

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OSHA Occupational Noise Exposure Standard

• 29 CFR 1910.95
• Requirements for maintaining and calibrating
  audiometric equipment and technician training
• Text Appendix 5 Table G-16A of Appendix A
• Relates A-weighted sound level to allowed
  duration
• Text Appendix 6 Table A-1 of Appendix A
• Converts Noise Exposure (Dose) to 8-hour TWA

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Quantifying Hearing Loss

• Watch for changes in the HTL (Hearing Threshold
  Level)
• STS (Standard Threshold Shift) decrease of 10
  db or more at 2000, 3000, or 4000 in either ear
• Represents permanent hearing loss
• Call for a re-test (after at least 14 hours of
  relative quiet)
• TST (Temporary Threshold Shift) a shift in HTL
  that disappears after the person has been in a
  quiet environment for a few hours

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Standard Threshold Shift (STS)

• If an STS is identified
• Notify the employee in writing
• Provide additional training
• Provide adequate hearing protection
• Workers Compensation
• Realize that WC laws for identifying and
  compensating STS will vary across the states

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29 CFR 1910.95

• Enacted in 1971
• Hearing Conservation Program is required whenever
  employee exposures exceed 85 dBA 8-hr TWA
• This is half the allowable noise exposure for an
  8 hour day or 50 Daily Noise Dose (DND)
• Note 90 dBA for an 8-hr TWA is 100 DND

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Hearing Conservation Program Elements

• Exposure monitoring
• Audiometric testing
• Hearing protective devices
• Training program
• Access to the written standard
• Recordkeeping

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Hearing Conservation Program Appendices

• Appendix A Methods for computing employee noise
  exposure
• Appendix B Methods for determining attenuation
  by hearing protection devices
• Appendix C Performance of audiometric testing
  equipment
• Appendix D Maximum levels of noise allowable in
  testing rooms and booths
• Appendix E Calibration requirements for
  audiometric testing equipment
• Appendix F Age-correction of audiograms
• Appendix G Information on noise monitoring

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Measuring Occupational Noise

• Sound Level Meters (SLM)
• Used for area surveys
• Settings for average, peak, impulse, ABC scales
• Noise Dosimeters
• Used for individual monitoring
• Clip microphone near the ear
• Wear all day
• Calibrate before and after

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Adding Decibels

• Often there is a need to combine two or more
  noise sources
• Because decibels are logarithms, they cannot be
  added directly
• 80 dB 85 dB ? 165 dB
• 80 dB 85 db 86 dB

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Example

• Given three machines in a room measured at 80,
  85, and 87 dB, respectively
• SPLtotal 10 log (1080/101085/101087/10)
• SPLtotal 89.6 dB

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Estimation Method

• Easier and relatively accurate

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Example

• Add 80, 85, and 87 dB using the estimation method
• Start with 85 and 87, difference is 2 dB, so
  select 2 dB from the table
• 87 2 89 dB
• Add 89 and 80, difference is 9 dB, so select 1 dB
• 89 1 90 dB (compare to 89.6 dB)

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Computing Daily Noise Doseand Calculating 8-hr
TWAs

• OSHA limits workers to 100 of the daily dose or
  90 dBA for 8-hr TWA
• D 100 (C1/T1 C2/T2 Cn/Tn)
• D daily nose dose, in percent
• C total time of exposure at the measured noise
  level
• T reference allowed duration for that noise
  level from Table G-16a of Appendix A of 29 CFR
  1910.95 (see Text Appendix 5)

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Example

• A workers exposure was monitored for 2 hrs at 80
  dBA, 2 hr at 85 dBA, and 4 hr at 87 dBA. What is
  the DND (Daily Noise Dose)?
• D 100 (2/32 2/16 4/12.1) 51.8
• The worker received 51.8 of their DND. (This is
  OK.)
• Given a DND 51.8, find the 8-hr TWA.
• Go to Table A-1 (Text Appendix 6). Round to 55
  (conservative). Yields 85.7 dB TWA. The Hearing
  Conservation Program is required.

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Controlling Noise

1. Engineering Controls
2. Administrative Controls
3. PPE

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Engineering Controls

• You can make a career out of engineering
  controls for controlling noise
• Devices insulative curtains coverings for
  noise-reflective floors, ceilings, walls
  vibration isolation devices
• Remember sound is a wave and cannot turn around
  corners this is the concept of directivity
• Reflection sound waves can bounce back and
  add sound pressure at the source
• Resonance a material vibrates at the same
  frequency as the emitted sound use an
  vibration isolator or rubber mounting

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Engineering Controls

• Some surfaces absorb the sound energy, or do
  not allow it to reflect effectively
• Noise control curtains fiber-filled cloth office
  partitions
• Proper preventative maintenance (PM) on
  machines parts such as motors, bearings, drive
  belts, pumps, etc.
• Adjustments, lubrication, replacement, vibration
  isolators
• Think outside the box for new designs and work
  with suppliers
• One of the best engineering controls is
  distance
• The relationship between noise and distance
  follows the inverse square law
• Doubling the distance reduces the noise by ¼
• Tripling the distance reduces the noise by 1/9

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Administrative Controls

• Used when engineering controls are exhausted or
  infeasible
• Limiting time in exposed areas worker rotation
  limiting the number of workers in exposed areas
  (limited access)

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Hearing Protective Devices (HPD)

• After engineering and administrative controls are
  exhausted and infeasible
• All workers exposed at 85 dBA for 8-hr TWA must
  be provided HPD at no cost
• Employers must ensure workers actually wear the
  HPD
• Employers must provide a variety of HPD

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HPD Continued

• Employers must train workers to use HPD and how
  to care for them
• HPD attenuation must effectively reduce noise
  exposure to below the OSHA action level (85 dBA)
• Noise Reduction Rating (NRR)
• Typically 22 30 dB NRR
• Numerical attenuation value determined in a
  laboratory
• When using the A-Scale, you must deduct 7 dB from
  the NRR
• When using the C-Scale, no deduction is necessary

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Example

• Worker is exposed to 98 dBA for 8-hr TWA.
  Earplugs are available with a 29 NRR and earmuffs
  are available with a 25 NRR.
• Since A-Scale, Earplugs (NRR 29-7 22) and
  Earmuffs (NRR 25-7 18)
• Earplugs 98 22 76 dBA
• Earmuffs 98 18 80 dBA
• Both are below the 85 dBA 8-hr TWA Action Limit

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Extreme Exposures

• For extreme exposures with 8-hr TWA in excess of
  100 dBA, it may be necessary to use both earplugs
  and earmuffs
• Their NRRs are not additive
• Tests show an additional 3 10 dB NRR is
  achieved with the second device

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Reading Assignment

• Nims, Chapter 9
• Critical Thinking Questions
• p. 225-226
• 1-5
• Due ?