INDH 5131 Controls of Occupational Hazards. Noise & Hearing Conservation. Part I

Transcription

1 INDH 5131 Controls of Occupational Hazards Noise & Hearing Conservation Part I By: Magdy Akladios, PhD, PE, CSP, CPE, CSHM This Chapter in a Flash I. Historic Background II. Physics of Sound III. Physiology of Hearing IV. Hearing Loss V. Audiometric Testing VI. Sound Measurement VII. Regulations VIII.Hearing Conservation Programs IX. Control of Noise Exposure... Blindness cuts people off from things; whereas, deafness cuts people off from people.. [Helen Keller] 1

2 I. Historic Background Sounds in our lives: Transportation Day to day activities Is there something called quiet?? Some examples Sound level in db Some Examples of sound measurements Taken by NIOSH 0 Weakest sound heard by ear Ringing phone Tractor Bulldozer Chain Saw Ambulance Siren Rocket Launch Whisper Hair Dryer/Power Lawn Mower Hand Drill Spray Painter Hammer Drill Jet Engine at Takeoff Loudest Ton Normal Conversation Belt Sander Impact Wrench Miner Pneumatic Drill 12-Gauge Shotgun Type of Equipment Sound Meter Reading (db) Source: < 2

3 Some statistics 28 million US citizens have hearing loss 5-30 million workers exposed to noise or >85dBA) 10 million US workers have hearing loss Facts of Hearing Loss Hearing loss is one of 10 leading causes of occupational injury Noise Induced Hearing Loss (NIHL) is controllable and preventable Noise is very OSHA cite-able NIHL is OSHA recordable Effects of Noise on humans: Dilatation of the pupil Secretion of thyroid hormone Heart palpitation Secretion of adrenaline/adrenaline cortex hormone Movements of the stomach and intestines Muscle reaction Increased levels of Stress hormones (Cortisol) 3

4 Effects of Noise on humans... 2 Constriction of the blood vessels Causes subjective communication or >55dBA Causes nausea (disturbed balance) Headache Temporary Hearing Loss Permanent Hearing Loss Reduction in life span Historical Background 1831: Blacksmiths Deafness (mainly Tinitus) 1882: Steam boiler workers (mainly hearing loss) 1896: Hearing protection consisted of individual attempts to protect ears by wool, cotton, etc. Rejected by many Historical Background... 2 Prior to industrial revolution, Hearing loss was noted in: Battle grounds + Blacksmithing (due to impact noise) Following industrial revolution: Hearing loss became a common injury After WW-II: Returning soldiers caused swell in hearing impaired numbers Increased recognition Armed services establishes Aural Rehab Centers Field of Audiology was created 4

5 Historical Background... 3 (Compensation) Was rare in the early part of the century, since in most hands-on jobs, no loss of hearning occurred. Early 1950s: Some state courts granted compensation to a workers who had lost their hearing Few company-wide noise conservation programs Historical Background... 4 (Regulations) 1948 Air Force 1969: DOL -- Walsh-Healy: 90dBA for 8 hrs, with 5 db exchange rate, 140 db peak 1971: Walsh Healy became applicable to OSHA s General Industry Historical Background : NIOSH Criterion Document suggests reduction to 85 dba 1974: OSHA revision: An Action Limit of 85 dba Instill a noise conservation program: 1. Maintain Audiograms (baseline, aging, associated w/hearing chronic exposure) 2. Hearing protection provided and made available 3. Maintain records 4. Training 5

6 Historical Background , 1983: OSHA amendments to hearing conservation regulation 1998: NIOSH revised criteria stays with 85 dba with 3dBA exchange rate DOD: Action Limit = 82dBA Quality of life Hearing loss can: Create tension between partners Make children feel uneasy Reduce enjoying music and other enjoyments in life Hearing loss cannot be rectified Quality of life Hearing loss as related to workers: Reduces detection of changes in a machine due to poor performance of the machine Reduces on the job communication Masks warning sounds that must be heard Increases accidents/mishaps Reduces productivity and performance Increases absenteeism Reduce employee morale on the job 6

7 Quality of life... 2 Socially: People who have hearing impaired are made fun of It is fashionable to correct vision impaired by glasses, as compared to hearing aids, they are always made small to conceal them Quality of life... 3 Quiet ears: 36 million Americans suffer from Tinnitus These are sounds heard in the head in the absence of an actual sound source Can be disabling Dramatically affects the quality of life Value of Hearing Conservation On-the-job communication Extra-Auditory effects: Productivity, lost-time accidents, and related issues Health Worker attitude 7

8 II. Physics of Sound Some definitions: Sound: Vibrational Energy Noise: Any unwanted sound Auditory stimuli carrying NO INFORMATION relevant to current tasks Some basic facts Excessive exposure to noise = hearing loss Noise induced hearing loss can be avoided by: Reducing duration of exposure Hearing protectors Reducing overall exposure to noise 8

9 Physics of sound: Variation in pressure (increase and decrease) Sound Oscillations in pressure in a medium with elasticity and viscosity. Radiates as waves from the source. Sound variations 9

10 Sound is characterized by: Frequency (Pitch) Amplitude (Loudness) Intensity (Power) Frequency Rate of the pressure oscillations described earlier Frequency is measured in Hz. 1 Hz = 1 cycle/sec. Frequency is subjectively determined by humans as Pitch Putting it all together: When squaring all points, then get RMS, we get Pressure 10

11 Pure Tone This is a single freq. Where the pressure is sinusoidal function of time Combined Frequencies Industry exposure is a combination of many frequencies. Combined Frequencies 11

12 Speed of Sound c c is a function of elasticity and density: Where: T = absolute temperature in Kelvin ( C ) Some Examples In air, at 21 C, c = 344 m/sec. In water, c = 1500 m/sec In steel, c = 5000 m/sec Wavelength ( ) The distance traveled by the sound wave during 1 pressure cycle is measured in m. 12

13 Freq. Vs. Wavelength c = f Where: c: speed of sound in m/sec f: frequency in Hz. : wavelength in m. Sound Pressure (P) It is the variations of pressure above and below the ambient atmospheric pressure Our ears can detect changes in atmospheric pressure <20 μpa Sound pressure is affected by distance away from source The units of sound pressure level are decibels (db). Decibels This is a dimensionless unit that is used to measure anything based on logarithmic ratio of a measured quantity to a known reference quantity, such as SOUND 13

14 Sound Intensity (I) Sound power / unit area (Watt/m 2 ) Area of a sphere (sound travels in all directions) Sound intensity diminishes by distance away from source Sound Power (W) Represents total sound energy emitted by source per unit time It describes the characteristics of a sound source Sound power is constant (affected only by source) Measured in acoustic Watts (W) Relationships There are relationships between Sound Power, Sound Pressure, and Sound Intensity 14

15 Some Rules of Thumb Double the distance, 1/4 Intensity, 1/2 Pressure, reduce by 6dB (this inverse law is called Attenuation) Lower threshold of hearing = 20 N/m 2 Threshold of Pain = 20 N/m 2 ~ 120 db Decibel Addition Addition can also be conducted by using this simple table: Difference between L P1 & L P2 Amount to add >9 0 Example Add 88dBA and 89dBA 15

16 Solution = 1, therefore, add 3 to higher Therefore, = 92dBA Reflection & Absorption When a sound source is placed in a room, 2 things happen: Absorption Reflection Reflection Absorption Reflection & Absorption... 2 An ideal point source, in the absence of reflections or absorption, will produce sound pressure levels at a distance r (in m) 16

17 Directivity Factor of sound (Q) Q describes the projection of noise Q ~ 1 for free field (a perfect sphere) Q ~ 2 for ½ sphere (source is against floor or ceiling) Q ~ 4 for 1/4 sphere (sound source is sitting on floor and against a wall) Q ~ 8 for 1/8 sphere (sound source is in a corner) As Q increases, sound effect increases Factors affecting reflection in a room Absorption abilities of surfaces Hard surface will reflect sound Soft surface will absorb sound Sound absorption coefficient ( ) All materials absorb sound (by some degree) For a material to be used for sound absorbing, it should absorb >½ of the sound In any room, each surface will have a particular = 1.0 in free field or open space = 0.0 with pure reflection (no sound absorbed) 17

18 Types of Fields Free field Near field (Reverberative field) Free Fields A free field is said to exist when sound radiated into space from a source and there is nothing that impedes the sound energy as it flows from the source. III. Physiology of Hearing Anatomy of the Ear 18

19 Frequency and the human ear Human speech: 1,000-4,000 Hz Outside this range, we lose sensitivity Young ears hear in the range of: 20-20,000Hz. Older ears lose hearing in the upper freq. Anatomy of the Ear Outer Middle Inner Outer Ear Pinna Auditory Canal Tympanic membrane (ear drum) 19

20 Functions of the Outer Ear Protects Wax (Toxic to insects) Hair (these have nothing to do with hearing) Amplifies/Collects Sound Pina Greatly amplifies in the 3,000 Hz region Ear canal amplifies in the range 2,000-4,000 Hz (area most sensitive) Amplification in the order of 10-15dB Functions of the Outer Ear... 2 Ear Drum: Moves in and out Transfers sound waves to mechanical vibrations Provides protection to middle/inner ears Area of ear drum to oval window = 17:1 (amplification is that much) Middle Ear Eustachian Tube Auditory Ossicles (bones) Malleus (Hammer) Incus (Anvil) Stapes (Stirrups) Oval Window (Fonestra Stibuli) Round Window (Fonestra Cochlea) 2 Muscles Tensor tympani Stapedius 20

21 Functions of the Middle Ear (Air-filled cavity) All mechanical vibrations Hammer Anvil Stirrup Oval window Eustachian tube connects middle ear to throat Opens periodically in synchrony w/swallowing Equalizes air pressure on each side (inside/outside) on ear drum Functions of the Middle Ear... 2 Infections: Throat warm, moist area Bones: transmitter of vibrations (act as a lever that produces further amplification of sounds) Muscles of the Middle Ear Tensor Tympani & Stapedius act opposite to each other Tensor Tympani: Pulls hammer medially >80 db; voluntary action which triggers tensor tympani to contract Stapedius: Pulls stirrups (decreases movement and dampens large sounds) Transmits vibrations about 22 s 21

22 Muscles of the Middle Ear... 2 Aural reflex; acoustic reflex Flex up to 15 min; can t sustain contraction Delay 10 th sec. to contract Won t protect against a gunshot and/or sudden impacts Inner Ear (Fluid-filled cavity) Bony Labyrinth Vestibule Cochlea Organ of Corti Semicircular Canals Membranous Labyrinth Functions of the Inner Ear... 2 Transmits mechanical vibrations to liquid (cochlea) 22

23 Functions of the Inner Ear Cochlea: Very small Oval window (moves in and out) = entrance to cochlea Cochlea Oval Window Stapes Scala Vestibuli & Scala Media Helicotrema Round Window High Frequency Scala Tympani Basilar Membrane Organ of corti Hair cells Low Frequency Organ of Corti: Hearing organ (scala media) Membrane/hair cells Sensory neural hearing Nerve damage 23

24 Hair Cells 4,000 inner hair cells Assisted by 12,000 hair cells in each cochlea Hair Cells... 2 A hair cell responds only to the deformation of its cilia Transform mechanical movements by Shear Force into neural stimuli Respond to insults of chemicals including neurotransmitters, antibiotics and diuretics When damaged, the sensations of sound, movement, equilibrium and orientation are affected Hair Cells... 3 (Balance and movement) Hair cells are organized in a such manner as to permit detection of movement in any direction. Hair cells in some areas are turned ON whereas adjacent cells are turned OFF by the same movement 24

25 Semicircular Canals Each pair of semicircular canals (one in each ear) is responsible for a different plane of rotational movement of your head These detect head movements All of the balance-information parts of both of your inner ears combined are called the vestibular system. Gel Vibration Up-snail, comes out to round window = exit Oval window (Gel) Cochlea Pulsing Scale Vestibuli (move vestibular membrane) Cochlea round window Liquid pushes on vestibular membrane and organ of corti Auditory Nerve Nerve = auditory nerve Brain Bending of hair cells releases a chemical transmitter onto the nerve fiber Nerve (electrical signal) brain via auditory nerve Signal becomes perception of signal Language Pitch Loudness 25

26 Pathway of Hearing Pinna Auditory Canal Tympanic Membrane Ossicles Malleus (hammer) Incus (anvil) Stapes (stirrup) Ligaments Muscles Amplitude reduction Pressure amplification Attenuation reflex (protection, low frequency masking) Oval Window Cochlea Auditory Nerve Auditory Cortex IV. Hearing Loss Types of hearing loss Types of Hearing Loss 1. Presbycusis (due to the aging process) 2. Noise Induced Hearing loss: Occupational Hearing Loss Sociacusis: due to social activities 3. Nosoacusis (due all other causes) Disease (Rubella, Mumps, etc) Chemicals (drugs, ototoxic chemicals, etc) Trauma (blow to the head, injury, etc) 26

27 Hearing loss in humans Hearing is lost in high frequencies first (range of hearing in humans is 20-20K Hz) Causes of hearing loss: 1. Sensory neural 2. Conductive 3. Mix 4. Central 1. Sensory-neural Occurs due to high exposure to noise Occurs at 2-6K Hz Damage to hair cells Hearing aid won t improve transmission because it only amplifies > sound waves GONE... FOREVER...! Damage to hair cells 27

28 Damage to hair cells... 2 The way we perceive frequencies Place theory (high freq): Each frequency stimulates a particular place on organ of corti Temporal Theory (code): Vestibular/nerve fiber line at a rate (<500) Combination: 1,000-5,000Hz (not only one area, but also nerve fibers) More sensitivity 2. Conductive hearing loss This is mechanical hearing loss Loss of hearing occurs equally at all frequencies. Can use a hearing aid to correct Can also be corrected surgically Occurs due to: Mechanical trauma, Sounds not being, conducted, Wax build up, Scar tissue, Busted tympanic membrane, Otosclerosis (abnormal growth of bones) 28

29 2. Conductive hearing loss... 2 Bones are not transmitting Arthritis - joints don t move properly Muscle fatigue (won t protect as well) Infections: Leaves scar tissue (hence, loss of elasticity) Eustachian tube plugged: increases by increase in pressure, causing poor transmission 3. Mix This is a combination of conductive and sensory neural hearing loss Occurs due to a combination of reasons as explained earlier Hearing aid may help 4. Central Loss This is caused by an obstacle in the brain 29

30 Temporary Hearing Loss High noise levels lead to temporary hearing loss (Tinnitis) Temporary threshold shift at 2 min (TTS 2 ) dba : no TTS dba: TTS 2 proportional to exposure Permanent Hearing Loss Continuous noise may lead to permanent hearing loss Begins at 4,000 Hz Generally restricted to 3,000 6,000 Hz Hearing Loss (Gender Factor) 30

31 Noise-induced Hearing Loss 31