Mention o f the name o f any company or product does not constitute endorsement by the National Institute for Occupational Safety and Health. Copies o f this and other NIOSH documents are available from# G L O S S A R Y
Where possible, die definition is quoted from the appropriate American National Standards Institute (ANSI) standard, ANSI S l.1 -1 9 9 4  or ANSI S 3.20-1995 , under the tenn(s) used in that standard. E x c h a n g e r a t e : An increment o f decibels that requires the halving o f exposure time, or a decrement o f decibels that requires the doubling o f exposure time. For example, a 3-dB exchange rate requires that noise exposure tim e be halved for each 3-dB increase in noise level; likew ise, a 5-dB exchange rate requires that exposure tim e be halved for each 5-dB increase.
F e n c e : The hearing threshold level above which a material impairment o f hearing is considered to have occurred.
F r e q u e n c y : For a function periodic in time, the reciprocal o f the period. Unit, hertz (Hz) (ANSI S l.1 -1 9 9 4 : frequency).

# H e a r i n g t h r e s h o l d l e v e l ( H T L ) :
For a specified signal, amount in decibels by which the hearing threshold for a listener, for one or both ears, exceeds a specified reference equivalent threshold level. Unit, dB (ANSI S l.1 -1 9 9 4 : hearing level; hearing threshold level).

# I m m i s s i o n l e v e l :
A descriptor for noise exposure, in decibels, representing the total sound energy incident on the ear over a specified period o f tim e (e.g., months, years).
I m p a c t : Single collision o f one mass in motion with a second mass that may be in motion or at rest (ANSI S l.1 -1 9 9 4 : impact).
I m p u l s e : Product o f a force and the tim e during which the force is applied; more specifically, impulse is the time integral o f force from an initial tim e to a final time, the force being Noise Exposure I m p u l s i v e n o i s e : Impulsive noise is characterized by a sharp rise and rapid decay in sound levels and is less than 1 sec in duration. For the purposes o f this document, it refers to impact or impulse noise.
I n t e r m i t t e n t n o i s e : N oise levels that are interrupted by intervals o f relatively low sound levels.

# N o i s e :
(1) Undesired sound. B y extension, noise is any unwarranted disturbance within a useful frequency band, such as undesired electric waves in a transmission channel or device. (2) Erratic, intermittent, or statistically random oscillation (ANSI S I .1-1994: noise).

# N o i s e r e d u c t i o n r a t i n g ( N R R ) :
The NRR, which indicates a hearing protector's noise reduction capabilities, is a single-number rating that is required by law to be shown on the label o f each hearing protector sold in the United States. Unit, dB.

# P e r m a n e n t t h r e s h o l d s h i f t ( P T S ) :
Permanent increase in the threshold o f audibility for an ear. Unit, dB (ANSI S3.20-1995: permanent threshold shift; permanent hearing loss; PTS). P u l s e r a n g e : Difference in decibels between the peak level o f an impulsive signal and the root-mean-square level o f a continuous noise.

# S i g n i f i c a n t t h r e s h o l d s h i f t :
A shift in hearing threshold, outside the range o f audiometric testing variability (5 dB), that warrants followup action to prevent further hearing loss. NIOSH defines significant threshold shift as an increase in the HTL o f 15 dB or more at any frequency (5 0 0 ,1 0 0 0 , 2 0 0 0 ,3 0 0 0 ,4 0 0 0 , or 6000 Hz) in either ear that is confirmed for die same ear and frequency by a second test within 30 days o f the first te s t S o u n d : (1) Oscillation in pressure, stress, particle displacement, particle velocity, etc. in a medium with internal forces (e.g ., elastic or viscous), or the superposition o f such propagated oscillations.
(2) Auditory sensation evoked by the oscillation described above (ANSI S I .1-1994: sound).

# S o u n d i n t e n s i t y :
Average rate o f sound energy transmitted in a specified direction at a point through a unit area normal to this direction at die point considered. Unit, watt per square meter (W /nr); symbol, I (ANSI S I .1-1994: sound intensity; sound-energy flux density; sound-power density).

# S o u n d i n t e n s i t y l e v e l :
Ten tim es the logarithm to the base ten o f the ratio o f the intensity o f a given sound in a stated direction to the reference sound intensity o f 1 picoWatt per square meter (pW/m2). Unit, dB; symbol, L (ANSI S I .1-1994: sound intensity level).
S o u n d p r e s s u r e : Root-mean-square instantaneous sound pressure at a point during a given tim e interval. Unit, Pascal (Pa) (ANSI S I .1-1994: sound pressure; effective sound pressure).
tu n e -d e p e n d e n t a n d e q u a l t o z e ro b e fo re th e in it ia l tim e a n d a fte r th e f in a l tim e ( A N S I S I . 1 -1 9 9 4 : im p u ls e ) .
x rv

# Glossary

# S o u n d p r e s s u r e l e v e l :
(1) Ten times the logarithm to the base ten o f die ratio o f the time-mean-square pressure o f a sound, in a stated frequency band, to the square o f the reference sound pressure in gases o f 20 micropascals QiPa). Unit, dB; symbol, L p . (2) For sound in media other than gases, unless otherwise specified, reference sound pressure in 1 pPa (ANSI S L 1-1994: sound pressure level).
T e m p o r a r y t h r e s h o l d s h i f t : Temporary increase in die threshold o f audibility for an ear caused by exposure to high-intensity acoustic stimuli. Such a shift may be caused by other means such as use o f aspirin or other drugs. Unit, dB. (ANSI S3.20-1995: temporary threshold shift; temporary hearing loss).

# T i m e -w e i g h t e d a v e r a g e ( T W A ) :
The averaging o f different exposure levels during an exposure period. For noise, given an 85-dBA exposure lim it and a 3-dB exchange rate, the TWA is calculated according to the follow ing formula: TWA = 10.0 x Log(D/100) + 85 where D = dose.
V a r y in g n o i s e : N oise, with or without audible tones, for which the level varies substantially during the period o f observation (ANSI S3.20-1995: nonstationary noise; nonsteady noise; time-varying noise). Occupational noise exposure shall be controlled so that worker exposures are less than the combination o f exposure level ( L ) and duration (7 ), as calculated by the follow ing formula (or as shown in Table 1 -1 ).

# r(min)=^-M)/3
where 3 = die exchange rate.

# . . 2 T i m e -W e i g h t e d A v e r a g e ( T W A )
In accordance with Section 1.1.1, the REL for an 8-hr work shift is a TWA o f 85 dBA us ing a 3-decibel (dB) exchange rate.

# . . 3 D a i l y N o i s e D o s e
When the daily noise exposure consists o f periods o f different noise lev els, the daily dose (D ) shall not equal or exceed 100, as calculated according to the follow ing formula:
Noise Exposure d = [c ,/r ,+ c j t 2 + . . . + c y r j x 100 where C^-total tim e o f exposure at a specified noise level, and r , = exposure duration for which noise at this level becom es hazardous.
The daily dose can be converted into an 8-hr TWA according to die follow ing formula (or as shown in Table 1-2    Noise Exposure 1 . 2 H e a r i n g L o s s P r e v e n t i o n P r o g r a m
The employer shall institute an effective hearing loss prevention program (HLPP) de scribed in Sections 1 3 through 1.11 when any worker's 8-hr TWA exposure equals or exceeds 85 dBA.

# . 3 N o i s e E x p o s u r e A s s e s s m e n t
The employer shall conduct a noise exposure assessment when any worker's 8-hr TWA exposure equals or exceeds 85 dBA. Exposure measurements shall conform to the A m e r i c a n N a t i o n a l S t a n d a r d M e a s u r e m e n t o f O c c u p a t io n a l N o i s e E x p o s u r e , ANSI S 12. 19-199619- . N oise exposure is to be measured without regard for the wearing o f hearing protectors.

# . 3 . I n i t i a l M o n i t o r i n g
When a new HLPP is initiated, an initial monitoring o f the worksite or o f noisy work tasks shall be conducted to determine the noise exposure levels representative o f all workers w hose 8-hr TWA noise exposures may equal or exceed 85 dBA. For workers remaining in essentially stationary, continuous noise levels, either a sound level meter or a dosimeter may be used. However, for workers who move around frequently or who perform different tasks with intermittent or varying noise levels, a task-based exposure monitoring strategy may provide a more accurate assessment o f the extent o f exposures.

# . 3 . 2 P e r i o d i c M o n i t o r i n g
I f any worker's 8-hr TWA exposure to noise equals or exceeds 85 dBA, monitoring shall be repeated at least every 2 years. Monitoring shall be repeated within 3 months o f die occurrence when there is a change in equipment, production processes or maintenance routines. It may also be prudent to assess noise exposures when work practices have changed and/or i f workers are developing significant threshold shifts (see Section 1.6.4).

# . 3 . 3 I n s t r u m e n t a t i o n
Instruments used to measure workers' noise exposures shall be calibrated to ensure measurement accuracy and, at a m inim um , they shall conform to the A m e r i c a n N a t i o n a l S t In determining TWA exposures, all continuous, varying, intermittent, and impulsive sound levels from 80 to 140 dBA shall be integrated into die noise measurements.
Chapter 1. Recommendations fa r a Noise Standard 1 A E n g i n e e r i n g a n d A d m i n i s t r a t i v e C o n t r o l s a n d W o r k Practices To the extent feasible, engineering controls, administrative controls, and work practices shall be used to ensure that workers are not exposed to noise at or above 85 dBA as an 8-hr TWA. The use o f administrative controls shall not result in exposing more workers to noise.
1 . 5 H e a r i n g P r o t e c t o r s Workers shall be required to wear hearing protectors when engaged in work that ex poses them to noise that equals or exceeds 85 dBA as an 8-hr TWA.- The employer shall provide hearing protectors at no cost to the workers.
Hearing protectors shall attenuate noise sufficiently to keep the worker's "real-world" exposure (Le., the noise exposure at die worker's ear when hearing protectors are worn) below 85 dBA as an 8-hr TWA. Workers whose 8-hr TWA exposures exceed 100 dBA should wear double hearing protection (i.e., they should wear earplugs and earmuffs si multaneously)T o compensate for known differences between laboratory-derived attenuation values and the protection obtained by a worker in the real world, the labeled noise reduction rat ings (NRRs) shall be derated as follow s: (1) earmuffs-subtract 25% from the manufac turers' labeled NRR; (2) slow-recovery formable earplugs-subtract 50%; and (3) all other earplugs-subtract 70% from the manufacturers' labeled NRR. These derating val ues shall be used until such time as manufacturers test and label their products in accor dance with a subject-fit method such as method B o f ANSI S 12.6-1997, A m e r i c a n *This recommendation should not be construed to imply that workers need not wear hearing protection unless their 8-hr TWAs equal or exceed 85 dBA For example, it would be prudent for a worker in and out of noise or habitually exposed to loud noise (e.g., 91 dBA for 1 hr and 59 min) to wear hearing pro tection while in noise-even though his or her dose was less than 100%. tThe intent of this section is not to advocate hearing protectors as die primary means of control; however, when engineering controls, administrative controls, and work practices cannot keep workers' exposures below 85 dBA as an 8-hr TWA, the use of hearing protectors shall he required For most TWA expo sures exceeding 105 dBA, hearing protectors will be necessary to supplement engineering and adminis trative controls.

# N a t i o n a l S t a n d a r d M e t h o d s f o r M e a s u r i n g t h e R e a l -E a r A t t e n u a t i o n o f H e a r i n g
Noise Exposure

# . 6 . A u d i o m e t r y
Audiometrie tests shall be performed by a physician, an audiologist, or an occupational hearing conservationist certified by the Council for Accreditation in Occupational Hear ing Conservation (CAOHC) or die equivalent, working under the supervision o f an au diologist or physician. The appropriate professional notation (e.g., licensure, certification, or CAOHC certification number) shall be recorded on each worker's audiogram.
Audiometrie testing shall consist ofair-conduction, pure-tone, hearing threshold meas ures at no less than 500, 1 0 0 0 ,2 0 0 0 ,3 0 0 0 ,4 0 0 0 , and6000 hertz ( For permanent onsite testing facilities, ambient noise levels shall be checked at least an nually. For m obile testing facilities, ambient noise levels shall be tested daily or each time the facility is moved, whichever is more often. Ambient noise measurements shall b e obtained under conditions representing the typical acoustical environment likely to b e present when audiometrie testing is performed. Ambient noise levels shall be re corded on each audiogram or made otherwise accessible to the professional reviewer o f the audiograms.

# . 6 . 2 B a s e l i n e A u d i o g r a m
A baseline audiogram shall be obtained before employment or within 30 days o f em ployment for all workers who must be enrolled in the HLPP. Workers shall not be ex posed to noise levels at or above 85 dBA for a mmimum o f 12 hr before receiving a baseline audiometrie te s t Hearing protectors shall not be used in lieu o f the required quiet period.
Chapter 1. Recommendations fo r a Noise Standard
1 . 6 . 3 M o n i t o r i n g A u d i o g r a m a n d R e

# t e s t A u d i o g r a m
A ll workers enrolled in die HLPP shall have their hearing threshold levels (HTLs) meas ured annually. These audiometric tests shall be conducted during the worker's normal work sh ift This audiogram shall be referred to as the ''monitoring audiogram." The monitoring audiogram shall be examined immediately to determine whether a worker has a change in hearing relative to his or her baseline audiogram.
When the monitoring audiogram detects a change in the HTL in either ear that equals or exceeds 15 dB at 5 0 0 ,1 0 0 0 ,2 0 0 0 ,3 0 0 0 ,4 0 0 0 , or 6000 Hz, an optional retest may be conducted immediately to determine whether die significant threshold shift is persistent In most cases, die retest w ill demonstrate that the worker does n o t have a persistent threshold shift, thereby eliminating die need for a confirmation audiogram and followup action. I f a persistent threshold shift h a s occurred, the worker shall be informed that his or her hearing may have worsened and additional hearing tests w ill be necessary.
1 . 6 . 4 C o n f i r m a t i o n A u d i o g r a m , S i g n i f i c a n t T h r e s h o l d S h i f t , a n d F o l l o w u p A c t i o n
When a worker's monitoring audiogram detects a threshold shift as outlined in Section 1.6.3, he or she shall receive a confirmation audiogram within 30 days. This confirma tion test shall be conducted under the same conditions as those o f a baseline audiometric te s t I f die confirmation audiogram shows die persistence o f a threshold shift, the audio grams and other appropriate records shall be reviewed by an audiologist or physician.
I f this review validates the threshold shift, the threshold shift is considered to be a sig nificant threshold sh ift This shift shall be recorded in die worker's medical record, and die confirmation audiogram shall serve as the new baseline and shall be used to calculate any subsequent significant threshold shift. Whenever possible, the worker should re ceive immediate feedback on die results o f his or her hearing test; however, in no case shall die worker be required to wait more than 30 days.
When a significant threshold shift has been validated, die employer shall take appropri ate action to protect the worker from additional hearing loss due to occupational noise exposure. Examples o f appropriate action include explanation o f the effects o f hearing loss, reinstruction and refitting ofhearing protectors, additional training o f die worker in hearing loss prevention, and reassignment o f the worker to a quieter work area.
When die reviewing audiologist or physician suspects a hearing change is due to a nonoccupational etiology, the worker shall receive appropriate counseling, which may in clude referral to his or her physician.
Noise Exposure
1 . 6 . 5 E x i t A u d i o g r a m
The employer should obtain an exit audiogram from a worker who is leaving employ ment or w hose job no longer involves exposure to hazardous noise. The exit audiogram should be conducted under the same conditions as those o f baseline audiometry.
1 . 7 H a z a r d C o m m u n i c a t i o n 1 . 7 . 1 W a r n i n g S i g n s
A warning sign shall be clearly visible at the entrance to or die periphery o f areas where noise exposures routinely equal or exceed 85 dBA as an 8-hr TWA. A ll warning signs shall be in English and, where applicable, in the predominant language o f workers who do not read English. Workers unable to read the warning signs shall be informed ver bally about the instructions printed on signs in hazardous work areas o f the facility. The warning sign shall textually or graphically contain the follow ing information:
W A R N I N G N O I S E A R E A H E A R I N G H A Z A R D
U s e o f H e a r i n g P r o t e c t o r s R e q u i r e d
1 . 7 . 2 N o t i f i c a t i o n t o W o r k e r s
A ll workers who are exposed to noise at or above 85 dBA as an 8-hr TWA shall be in formed about the potential consequences o f noise exposure and the methods o f prevent ing noise-induced hearing lo ss (NIHL). When noise measurements are initially con ducted and confirm the presence o f hazardous noise, or when followup noise meas urements identify additional noise hazards, workers shall be notified within 30 days. N ew workers shall be alerted about the presence o f hazardous noise before they are ex posed to i t

# . 8 T r a i n i n g
The employer shall institute a training program in occupational hearing loss prevention for all workers who are exposed to noise at or above 85 dBA as an 8-hr TWA; the em ployer shall ensure worker participation in such a program. The training program shall be repeated annually for each worker included in the HLPP. Information provided shall be updated to be consistent with changes in protective equipment and work processes.

# . 9 P r o g r a m E v a l u a t i o n Criteria
The effectiveness o f the HLPP shall be evaluated at the level o f the individual worker and at die programmatic level.
The evaluation at die worker level shall take place at die tim e o f the annual audiometry. I f a worker demonstrates a significant threshold shift that is presumed to be occupation ally related, all possible steps shall be taken to ensure that die worker does not incur ad ditional occupational hearing loss.
The evaluation at the programmatic level shall take place annually. The incidence rate o f significant threshold shift for noise-exposed workers shall be compared with that for a population not exposed to occupational noise. Similar incidence rates from this com parison indicate an effective HLPP. Data for calculating an incidence rate for a popula tion not exposed to occupational noise should be drawn from Annex C in the A m e r i c a n

# N a t i o n a l S t a n d a r d D e t e r m in a t i o n o f O c c u p a t io n a l N o i s e E x p o s u r e a n d E s t i m a t i o n o f N o i s e -I n d u c e d H e a r i n g I m p a i r m e n t y ANSI S3
. 44-199644-  unless more ap propriate data are available.

# . 0 R e c o r d k e e p i n g
The employer shall establish and maintain records in accordance with die requirements in Sections 1.10.1 through 1.10.5. The employer shall establish and maintain an accurate record for each worker subject to die medical surveillance specified in Section 1.6. These records shall include, at a mini mum, die name o f die worker being tested; identification number; duties performed and job locations; m edical, employment, and noise-exposure history; dates, tim es, and types o f tests (i.e., baseline, annual, retest, confirmation); hours since last noise exposure be fore each test; HTLs at the required audiometric frequencies; tester's identification and assessment o f test reliability; the etiology o f any significant threshold shift; and the identification o f the reviewer.  NIHL is caused by exposure to sound levels or durations that damage the hair cells o f the cochlea. Initially, the noise exposure may cause a temporary threshold shift-that is, a decrease in hearing sensitivity that typically returns to its former level within a few minutes to a few hours. Repeated exposures lead to a permanent threshold shift, which is an irreversible sensorineural hearing loss.   Noise Exposure

# . 6 S c o p e o f T h i s R e v i s i o n o f t h e N o i s e Criteria D o c u m e n t
The focus o f this document is on the prevention o f occupational hearing loss rather than on conservation.       *ASHA makes a distinction between the tom s "impairment" and "handicap"; however, for the purpose o f die subsequent discussion in this criteria document, only the tom "material hearing impairment" is used. The Prince et al.  paper reports the use o f a modified ASHA Task Force definition. This modification incorporates frequency-specific weights based on the articulation index for each fre quency . Negligible differences were found between excess risk estimates generated using die modified and the unmodified definitions. The excess risk estimates presented in this criteria docu ment are based on die unmodified ASHA Task Force definition. tHistorical note, ASHA did not deliberate on die definition proposed by the ASHA Task Force. *Prince et al.  found that the excess risk estimates at exposure levels below 85 dBA w oe not well defined. Insufficient data for workers with average daily exposures below 85 dBA led to considerable variability in the estimation, depending on the statistical assumptions used in the modeling.    '1997-N10SH model for the 1-2-3-4-lcHz definition of hearing impairment ^confidence interval.

# N o i s e E x p o s u r e T a b l e 3 -4 . C o m p a r i s o n o f m o d e l s f o r e s t i m a t i n g t h e e x c e s s r i s k o f m a t e r i a l h e a r i n g i m p a i r m e n t a t a g e 6 0 a f t e r a 4 0 -y e a r w o r k i n g f i f e t i m e e x p o s u r e t o o c c u p a t i o n a l n o i s e , b y d e f i n i t i o n o f m a t e r i a l h e a r i n g i m p a i r m e n t
Average cxponre fcvd(4 BA)

# .2 C e ilin g Limit
Because NIOSH is recommending a 3-dB exchange rate with an 85-dBA REL, a ceiling limit for continuous-type noise is unnecessary. For example, with an 85-dBA REL and a 3-dB exchange rate, an exposure duration o f less than 28 sec would be allowed at a 115-dBAlevel.
TTie generally accepted ceiling limit o f 140 dB peak SPL for impulsive noise is based on areportbyKryteretaL . Ward  indicated that "this number was little more than a guess when h was first proposed. To date, a proposal for a different limit has not been supported. Henderson etaL  indicated that the critical level for chinchillas is between 119 and 125 dB; and ifa20-dB adjustment is used to account for the difference in susceptibility between chinchillas and humans, the critical level extrapolated for humans would be between 139 and 145 dB. Based on the 85-dBA REL and the 3-dB ex change rate, the allowable exposure time at 140 dBA is less than 0.1 sec; thus, 140 dBA is a reasonable ceiling limit for impulsive noise.

# .E x ch a n g e R ate
Health  2. All noise exposures that produce a given TTS2 will be equally hazardous (the equal temporary effect theory).
3. Permanent threshold shift produced after many years of habitual noise exposures for 8 hr per day is about the same as the TTS2 produced in normal ears by an 8-hr exposure to the same noise.
However, these CHABA postulates were not validated. Research has been unable to demonstrate a simple relationship between temporary threshold shift, permanent thresh old shift, and cochlear damage . The CHABA criteria assumed that worker exposures could be characterized by regularly spaced noise bursts inter spersed with periods that were sufficiently quiet to allow hearing to recover. However, this assumption is not characteristic o f many typical industrial noise exposures. Workers will always develop temporary threshold shift before sustaining permanent threshold shift, barring an ototraumatic incident Temporary threshold shift is a useful metric for monitoring the effects of noise exposure; these studies do not imply otherwise.
In general, the CHABA hearing damage risk criteria proved too complicated for general use. Botsfoid  published a simplified set of criteria based on the CHABA criteria.
One o f the simplifications inherent to the Botsford  method was the assumption that interruptions would be o f "equal length and spacing so that a number o f identical ex posure cycles would be distributed uniformly throughout the day." These interruptions would occur during coffee breaks, trips to the washroom, lunch, and periods when ma chines were temporarily shut down. One response to the evidence from the animal studies and certain field studies would be to select the 3-dB exchange rate but to allow an adjustment (increase) to the PEL for cer tain intermittent noise exposures, as suggested by EPA  and Johansson et aL . This response would be in contrast to a 5-dB exchange rate, for which there is lit tle scientific justification. Ideally, if an adjustment is needed, the amount should be de termined by the temporal pattern o f die noise and the levels o f quiet between noise bursts. At this time, however, little quantitative information is available about these pa rameters in industrial environments. Therefore, the need for an adjustment should be clarified by further research. Although the 3-dB rule may be somewhat conservative in truly intermittent conditions, the 5-dB rule will be underprotective in most others. The 3-dB exchange rate is die method most firmly supported by the scientific evidence for assessing hearing impairment as a function o f noise level and duration, whether or not an adjustment is used for certain intermittent exposures.

# .4 Im p u lsiv e N o is e
The OSHA occupational noise standard  states: "Exposure to impul sive or impact noise should not exceed 140 dB peak sound pressure." Thus, in this con text, the 140-dB limit is advisory rather than mandatory. This number was first proposed by Kryter et al.  and later acknowledged by Ward  as little more than a guess. NIOSH  did not address the hazard o f impulsive (i.e., impulse or impact) noise, although NIOSH stated that the provisions o f the recommended standard in the criteria document were intended to apply for all noise. Although there is yet no unanim ity as to which criteria best describe die relationship between NIHL and exposure to im pulsive noise, either by itself or in die presence o f continuous-type (i.e., continuous, varying, or intermittent) noise, diere is an international standard that has become widely used by most industrial nations. This standard, ISO 1999, Acoustics-An Estimation o f Noise-Induced Hearing Impairment (ISO 1990], integrates both impulsive and continuous-type noise (and uses die 3-dB exchange rate of die equal-energy rule) when calculating sound exposures over any specified time period. NIOSH concurs with this

# N o i s e E x p o s u r e
approach and recommends that noise exposure levels be calculated by integrating all noises (both impulsive and continuous-type) over die duration o f die measurement Despite its simplicity, die equal-energy rule is not universally accepted as a method for characterizing exposures that consist o f both impulsive and continuous-type noises.
Another approach favors evaluating impulsive noise separate from that o f continuoustype noise. Studies that would argue for this approach will be discussed first, followed by a discussion o f studies elucidating the rationale for the NIOSH position on the equalenergy rule.

# . 4 . 1 E v i d e n c e T h a t I m p u l s i v e N o i s e E f f e c t s D o N o t C o n f o r m t o t h e E q u a l -E n e r g y R u l e
In her evaluation o f the effects o f continuous and varying noises on hearing, Passchier-Vermeer  found that the HTLs o f workers in steel construction works did not con form to the equal-energy hypothesis; that is, the hearing losses in these workers, who were exposed to noise levels with impulsive components, were higher than predicted.  . The ob served and predicted values differed in that the observed hearing loss was smaller than predicted at the lower audiometric frequencies, but the observed hearing loss was greater than predicted at the higher audiometric frequencies. In their study of hearing loss in weavers, who were exposed to continuous noise, and drop-forge hammer men,

# C h a p t e r S . B a s i s f o r t h e E x p o s u r e S t a n d a r d
who were exposed to impact noise o f equivalent energy, Sulkowski et al.  found that the hammer men had substantially worse hearing than the weavers.
Thiery and Meyer-Bisch  conducted a cross-sectional epidemiologic study at an automobile manufacturing plant The automotive workers were exposed to continuous and impulsive noises at Acq^ ranging from 87 to 90 dBA. When their HTLs were com pared with those o f workers exposed to continuous noise at o f 95 dBA for the same exposure time, the automotive workers showed greater hearing losses at the 6000-Hz audiometric frequency than the reference population after 9 years o f exposure.  compared at the4000-Hz audiometric frequency the HTLs of forest workers using chain saws and shipyard workers using hammers and chippers. The forest workers were exposed to continuous-type noise, whereas the shipyard workers were ex posed to impact noise. Starck et al.  also used the immission model developed by Bums and Robinson  to predict the HTLs for both groups. They found that the Bums and Robinson model was accurate at 4000 Hz for the forest workers; however, it substantially underestimated die HTLs at 4000 Hz for the shipyard workers.

# Staxck et aL
The studies described here provide evidence that the effects o f combined exposure to impulsive and continuous-type noises are synergistic rather than additive, as die equal- energy hypothesis would support One measure for protecting a worker from such syn ergistic effects would be to require that a correction factor be added to a measured TWA noise exposure level when impulsive components are present in the noise. The magni tude o f such a correction has not been quantified. The matter becomes more complicated when other parameters o f impulsive noise are considered. Noise energy does not appear to be the only factor that affects hearing. The amplitude, duration, rise time, number of impulses, repetition rate, and crest factor also appear to be involved . The criteria for expo sure to impulsive noise based on die interrelationships o f these parameters await the re sults o f further research.

# . 4 . 2 E v i d e n c e T h a t I m p u l s i v e N o i s e E f f e c t s C o n f o r m t o t h e E q u a l -
E n e r g y R u l e

# . 4 . C o m b i n e d E x p o s u r e t o I m p u l s i v e a n d C o n t i n u o u s -T y p e N o i s e s

# Instrumentation for Noise Measurement
No single method or process exists for measuring occupational noise. Hearing safety and health professionals can use a variety o f instruments to measure noise and can choose from a variety o f instruments and software to analyze their measurements. The choice o f a particular instrument and approach for measuring and analyzing occupa tional noise depends on many factors, not die least of which will be die purpose for the measurement and the environment in which die measurement will be made.

# . 1 . 1 F r e q u e n c y W e i g h t i n g N e t w o r k s
The human ear is not equally responsive to all frequencies; it is most sensitive around 4000Hz and least sensitive in the low frequencies. The responses o f die sound level me ter are modified with frequency-weighting networks that represent some responses of the human ear. These empirically derived networks approximate die equal loudnessweighting networks or scales; some also have a B-scale. The A-scale, which approxi mates the ear's response to moderate-level sounds, is commonly used in measuring noise to evaluate its effect on humans and has been incorporated in many occupational noise standards. Table 4-1 shows die characteristics o f these scales.

# . 1 . 2 E x p o n e n t i a l T i m e W e i g h t i n g
A sound level meter's response is generally based on either a FAST or SLOW expo nential averaging. FAST corresponds to a 125-millisecond (ms) time constant; SLOW   The correct use o f die microphone is extremely important in obtaining accurate meas urements. Microphones come in many types and sizes. A microphone is typically de signed for use in a particular environment across a specific range o f SPLs and frequencies. In addition, microphones differ in their directionality. For example, some are intended to be pointed directly at the sound; and others are designed to measure sound from a "grazing" angle o f incidence. Thus users should follow the sound level meter manufacturer's instructions regarding the type and size o f microphone and its ori entation toward a sound. Also, care should be taken to avoid shielding the microphone by persons or objects . When measuring a diffuse sound field, the person conducting the measurement should hold the microphone as far from his or her body as practical .
"Meters that are set to integrate or avenge sound do not use either the FAST or SLOW time constant; they will sample many times each second. For a more detailed description o f exponential time weighting, re fer to Yeager and March .   "Whenever the unit dB is used in sudiometric testing, it actually refers to dB HTL.

# C h a p t e r 5 . H e a r i n g L o s s P r e v e n t i o n P r o g r a m s ( H L P P s ) T a b l e 5 -1 . C l a s s i f i a b l e f i r s t t a g s - a c r o s s 1 5 d a t a b a s e s - a n d f i r s t t a g s c l a s s i f i e d a s t r u e p o s i t i v e f o r e a c h o f t h e 8 -s h i f t c r i t e r i a *

# No criterion evaluated is best in every respect
The relative merits o f each are tabu lated in

# A v e r a g e s t o g e t h e r f r e q u e n c i e s t h a t v a r y in s u s c e p t i b i l i t y t o n o i s e X X U s e s a s h i f t m a g n i t u d e w i t h i n t h e r a n g e o f n o r m a l a u d i o m e t r i c v a r i a b i l i t y X
Chapter 5. Hearing Loss Prevention Programs (HLPPs)

# N o i s e E x p o s u r e
unlikely to be affected by NIHL, but it may be useful as an indicator o f excess ambient noise in the audiometrie test booth and as an indicator o f the presence o f medical ear conditions sudi as conductive ear pathologies. The 6000-Hz audiometrie frequency is one o f the three high frequencies (3000,4000, and 6000 Hz) at which hearing is most likely to be affected soonest and to die greatest degree by NIHL. This audiometrie fre quency is more susceptible than others to measurement variability if there is inconsis tent earphone placement If the immediate retest is not performed, NIOSH recommends that the significant threshold shift be confirmed by a followup test within 30 days o f the testing that showed the significant threshold shift This followup test is called the confirmation test and is preceded by a 12-hr quiet period. If the significant threshold shift is confirmed and later validated by an audiologist or physician, the confirmation audiogram should be the one with which all subsequent audiograms are compared.
To comply with this recommendation and to provide m a x i m u m protection for workers and maximum documentation for employers, NIOSH advocates that audiograms be per formed on the following occasions:
1. Before employment or before initial assignment into a hearing hazard work area.

# Annually for any worker whose noise exposure equals or exceeds 85 dB A as an 8-hr TWA (monitoring audiometry)
. Annual testing may lead to a number o f retests if a significant threshold shill occurs. In addition, it may be a good practice to provide audiometry twice per year to workers exposed to more than 100 dBA, because die most susceptible 10% o f a population exposed to daily average noise levels of 100 dBA with inadequate hearing protectors could develop significant hearing loss well before the end o f 1 year .

# At the

# M o n i t o r i n g A u d i o g r a m s
Monitoring audiometry shall be conducted no less than annually. Unlike baseline audio metry, these annual tests should be scheduled at the end of, or well into, the work shift so that temporary changes in hearing due to insufficient noise controls or inadequate use of hearing protection will be noted. The results should be compared immediately with the baseline audiogram to check for any change in hearing sensitivity. The collection of au diograms for later batch comparison with baseline audiograms in another location is an unacceptable practice because it does not afford the opportunity to conduct retests or to discuss the findings with workers in a timely manner.

# . . 1 . 3 R e t e s t A u d i o g r a m s
As good practice, NIOSH suggests that audiometry be repeated immediately after any monitoring audiogram that indicates a threshold shift o f 15 dB or more at 500, 1000, 2000,3000,4000, or 6000 Hz in either ear. The worker should be reinstructed and the headphones refitted before conducting the retest Those who employ the retest strategy  The audiometric manager should be responsible for making whatever recommen dations he or she deems necessary and for seeing that they are carried out

# . . 1 . E x i t A u d i o g r a m
Audiometry should be conducted when a worker leaves employment or is permanently rotated out o f an occupational noise exposure at or above 85 dBA as an 8-hr TWA. This exit audiogram, like the baseline, should be performed after a minimum o f 12 hr of quiet The use ofhearing protectors as a substitute for quiet is not acceptable.
NIOSH suggests that hearing tests be offered as a health benefit to workers who are not exposed to hazardous noise levels. The tests in these workers can be conducted early in the day^when it is not recommended that noise-exposed employees be tested for changes in hearing thresholds. In addition to providing a valuable internal control group for comparison to the noise-exposed workers, this policy elevates the perceived impor tance o f the HLPP for management and workers .  1. Requirements o fa n d rationale fo r the occupational noise standard.

# 5 . 2 A u d i o m e t e r s
Effects o f noise on hearing. This should cover both die audiometrie effects (i.e., how noise effects show up on an audiogram) and die functional effects (i.e., the impact of NIHL on everyday life).
Company policy fo r the elimination o f noise as a hazard, including noise controls al ready implemented or plannedfor thefuture. This topic is very important and helps ensure that workers do not accidentally interfere with control measures.

# Hazardous noise

# C h a p t e r 5 . H e a r i n g L o s s P r e v e n t i o n P r o g r a m s ( H L P P s )
In addition to these episodic training sessions, an ongoing educational process should be offered. HLPP personnel, especially die program implementor, should visit the work ers' jobsites to see how they are doing. They should talk to workers about the program when they meet them in the halls, at lunch, etc. Posters, bulletin boards, informational pamphlets, etc., can be used as a constant reminder of the importance that the company places on hearing conservation. Contests or awards for effective hearing conservation practices can be used to promote safe behavior Royster 1986,1990]; how ever, incentive programs should be planned and implemented with full worker partici pation or they may be perceived by the workers as manipulative attempts by management to control worker behavior .

# .8 R e c o r d k e e p in g (C o m p o n e n t 7 )
Recordkeeping

# .1 0 A g e C o rr ectio n
NIOSH does not recommend that age correction be applied to an individual's audio gram for significant threshold shift calculations. Although many people experience some decrease in hearing sensitivity with age, some do not It is not possible to know who will and who will not have an age-related hearing loss. Thus, applying age correc tions to a person's hearing thresholds for calculation o f significant threshold shift will overestimate the expected hearing loss for some and underestimate it for others, because The adjustment o f audiometric thresholds for aging has become a common practice in woriceis- compensation litigation. In this application, age corrections reduce the amount ofhearing loss attributable to noise exposure, with a consequent reduction in the amount o f compensation paid to workers for their hearing losses. However common "age cor recting" is and regardless o f the extent to which it is applied, it is technically inappropri ate to apply population statistics to an individuaL Each age correctionnuinber is nothing more than a median value from a population distribution. In age-correcting an audio gram, the underlying assumption is that the individual value is given the 50th percentile, when in feet the 10th or 90th percentile may be the correct value. Thus age-correction formulas cannot be applied to determine with certainty how much o f an individual's hearing loss is due to age and how much is due to noise exposure.
Age-correcting audiograms obtained as part o f an occupational HLPP are even less ap propriate. This is not a compensation issue. The purpose o f the program is to prevent hearing loss. If an audiogram is age corrected, regardless o f the source o f the correction values, the time required for a significant threshold shift to be noted will be prolonged. Delaying the identification o f a worker with a significant threshold shift is completely contrary to the purpose o f an HLPP.

# Hearing Protectors
A personal hearing protection device (or hearing protector) is any device designed to re duce the level o f sound reaching the eardrum. Earmufls, earplugs, and ear canal caps (also called semi-inserts) are the mam types ofhearing protectors. A wide range ofhearing protectors exists within each of these categories. For example, earplugs maybe sub categorized into foam, user-formable (such as silicon or spun mineral fiber), premolded, and custom-molded earplugs. In addition, some *The OSHA methods arc a simplification o f NIOSH methods #2 and #3 .
One problem inherent to using single-number descriptors of sound attenuation is the need to ensure that the resulting value does not sacrifice the estimated protection for the sake o f simplicity. Thus these calculations will typically underestimate laboratoryderived "long methods" for estimating sound attenuation. To get around some o f the limitations associated with NRR calculations, other methods have been developed for estimating hearing protector performance. The single-number rating method and the high-middle-low method may be used when a person needs to estimate performance more accurately than possible with the NRR but does not want to resort to octave-band  To summarize, the best hearing protection for any worker is die removal o f hazardous noise from the workplace. Until that happens, die best hearing protector for a worker is die one he or she will wear willingly and consistently. The following factors are ex tremely important determinants ofworker acceptance ofhearing protectors and die like lihood that workers will wear them consistently:
- Convenience and availability - Belief that the device can be worn correctly - Belief that the device will prevent hearing loss
- Belief that the device will not impair a worker's ability to hear important sounds  Foam: Crawfotd and Nozza  Hachey and Roberts  Lempert and Edwards  Edwards and Green  Edwards and Green  Abel et aL  A beletal.  Behar  Pfeiffer et aL  Casali and Part  Casali and Raric  Berger and Kieper  Premolded: Ultra-Fit Casali and Park  Casali and Park  Royster et aL  V- 51R Royster et aL  Abel et al.  Edwards et al.  Fleming  Padilla [1976         Management procedures for workers identified with substantial hearing impairment need to be studied. They would include training in listening strategies, speech reading, and optimal utilization ofhearing aids. Research also needs to be directed at developing bearing instruments designed to help workers continue to function in noise while pro tecting bearing and enhancing communication.
E-A-R
Rehabilitation communication strategies need to be studied. Currently, if hearingloss-prevention service providers were to suggest that noise-exposed workers with NIHL could benefit from amplification, they would be fired. In such a hostile environ ment, it is very difficult to define, develop, deliver, and evaluate a rehabilitation program.
Taylor

# MTRODUCTION
The most common goal for protecting workers from the auditory effects o f occupational noise has historically been the preservation o f hearing for speech discrimination. With tins protection goal in mind, the National Institute for Occu pational Safety and Health (NIOSH) defined hearing handi cap as a b ia u ra l avera g e o f hearing levels exceeding 25 dB at the ancfiometric test frequencies o f 1, 2, and 3 kHz and 0 5 , 1. and 2 kHz (NIOSH, 1972). Here, the term " bianral average" is used to identify die mean value for the left and right ears. Using these definitions, NIOSH (1972) estimated die excess risk o f hearing handicap as a function o f age, sound levels and duration o f occupational noise exposure. Excess risk, also known as percentage risk, is defined as the percentage o f individuals with hearing handicap among indi viduals exposed to daily 8^iour occupational noise exposure after subtracting die percentage o f individuals who would typically incur such a handicap doe to aging in an unexposed population. For a 40-year lifetime exposure to average daily (8-hour) noise levels o f 8 0 ,8 5 , and 90 dB in the workplace, NIOSH (1972) estimated the excess risk to be 3%, 15%, and 29%, respectively for the biaural average over 1, 2, and 3 kHz.  Table I com pares die NIOSH (1972) excess risk estimates fo r die biaural average over 0.5, 1, and 2 kHz to those developed by other organizations at approximately the same time.
Since the publication o f the 1972 NIOSH Criteria Docu ment, statistical methods for analyzing categorical data out comes have been improved to assess risk o f disease (Breslow and Day, 1980a). The aim o f this paper is to reevaluate the models used to generate excess risk estimates from data col lected for the NIOSH 1968-72 Occupational Noise and Hearing Survey (ONHS) (Lempert and Henderson, 1973). Using these newer statistical methods, the paper examines die relationship between exposure to noise and risk o f noiseinduced hearing handicap (NIHH) and highlights s e a s o f uncertainty in estimating risks. These results will be com pared to the 1972 NIOSH analysis (NIOSH, 1972) and to the ANSI S3.44 (ANSI, 1996) standard, which adopted the methods developed by die In te rn a tio n a l Standards Organiza tion (ISO 1971(ISO , 1990. The data collected in the NIOSH survey are o f continuing interest since they were obtained before hearing protection devices were widely used in the U.S. Observations by NIOSH investigators during sound level surveys and management's impressions o f their respec tive plants did not indicate that participating companies had policies req u irin g hearing protection use. Use o f protectors, if available at all, were left to the discretion o f the workers. No mass use o f hearing protectors was noted in any o f the companies surveyed (Cohen, personal communications, 1996).

# L RELEVANCE TO COMPARABLE STUDIES OF NCMSE-MDUCED HEARING LOSS
Several investigators (Robinson and Sutton, 1975;Royster and Thomas, 1979;NCHS, 1965;Robinson, 1970;Yexg e t a l-, 1978) have examined the relationship o f noiseinduced permanent threshold shift (NIPTS) and occupational noise exposure. Studies similar to the NIOSH 1968-72 Noise Survey with respect to time period and methods of data collection include Baughn (1973), Passchier-Vermeer (1968 and Bums and Robinson (1970). These studies will be the main focus o f our review o f die relevant noise and hearing surveys from this period. These studies have been used by ISO 1999(1971) and ANSI S3.44 (ANSI, 1996 to estimate the risk o f N1HH or NIPTS. Table II presents major study characteristics o f each o f these studies.
As shown in Table D, only die Baughn (1973) study did not screen their workers for otologic abnormalities. These studies report that their populations were restricted to work ers with daily constant levels o f steady state noise exposure for the entire length o f employment A review o f these stud ies' limitations has been addressed by Ward and Glorig (1975) and Yeig e t aL (1975). They include possible con tamination o f non-steady state noise exposure in the popula tion and small sample sizes for subjects exposed to continu ous steady state for daily sound levels below 90 dB. The Passchier-Vermeer report (1968) reviewed published studies and was not specifically designed to address criteria for a noise standard. The NIOSH study (Lempert and Henderson, 1973) was specifically designed to examine risk o f noise-  NIOSH in 1972. The aim o f the survey was " to characterize noise exposure levels in a variety o f industries, to describe die hearing status o f workers exposed to such noise condi tions, and to establish a relationship between occupational noise exposure and hearing handicap that would be appli cable to general industry." Subjects for the study were re cruited through notices at industrial hygiene conferences and through the regional offices o f the UJS. Public Health Ser vice. All companies interested in participating were consid ered if certain priority considerations applied. These in cluded (1) existence o f a factory o r occupational noise conditions having noise levels relevant to developing noise standards and criteria, and (2) a work force with a wide range o f years o f exposure to sudi noise levels.
The data collected in the survey included noise measure ments, personal background information, medical and oto logica! data and audiometrie examinations. Noise level mea surements (using Bruel-Kjaer Sound Level Meters) were taken at different areas o f each plant and tape recordings were used for laboratory analysis o f noise characteristics. A questionnaire was used to obtain information on each work er's jo b history, military service, hobbies, and medical his tory pertinent to ear abnormalities and hearing difficulty. An otoscopie inspection o f the ears was also made, usually after die completion o f the questionnaire. Measurements o f hear ing levels (using a Rndmose RA-108 audiometer) for pure tone frequencies o f 0 .5 ,1 , 2, 3 ,4 , and 6 kHz in die right and left ears o f the workers were conducted in a Rndmose audio metrie travel van (model RA-113). Workers from noisy workplaces were always tested at die beginning o f their work s h ift For plants with less than 500 employees, the entire work force was tested. For larger plants, a random sample was selected. Individuals from each plant who worked in offices or other quiet work areas were also included in the survey to provide control data.

# B. S c re e n e d popu latio n fo r a n a ly sis
The survey population was " screened" to exclude indi viduals with prior noise exposure (from occupational and non-occupational sources) and medical or otologie condi tions that might affect a person's risk o f hearing loss, inde pendent o f occupational noise levels at the time o f the sur vey. Criteria for data exclusion included (I ) uncertainty in the noise exposure history or validity o f audiometrie tests and (2) evidence that hearing loss might have b e a i caused by factors other than occupational noise exposure (e^., military history, other non-occupational noise exposures, head trauma, other audiological/otologic medical conditions). Workers exposed to noise that was not continuous (e.g., dis crete impact sounds or noise with highly variable and unpre dictable levels) and all maintenance workers were also ex cluded. Due to die relatively small number o f females in the survey population, all analyses were limited to 1172 males (792 noise-exposed and 380 controls).

# C. V ariable d efin itio n s

# D e f i n i t i o n o f h e a r i n g h a n d i c a p
The major outcome o f interest is hearing handicap, de fined as a biaural average hearing threshold level o f greater than 25 dB for a selected set o f frequencies. In tins analysis, the set o f frequencies includes (a) 0 .5 ,1 , and 2 kHz, (b) 1,2, and 3 kHz and (c) 1 ,2 , 3, and 4 kHz (heron denoted as 1 -4 kHz). The 1 -4 kHz frequency average was recommended by an American Speech-Language-Hearing Association (ASHA) Task Force (ASHA, 1981), which focused on die need to indude frequencies most affected by noise exposure. The ASHA Task Force recommended that percentage formu las should include hearing threshold levels for 1, 2, 3, and 4 kHz, with low and high fences o f 25 and 75 dB, representing 0 percent and 100 percent hearing handicap boundaries, re spectively (ASHA, 1981). In this analysis, the ASHA recom mendation was modified by calculating a weighted average across frequencies rather than an arithmetic average over the test frequencies o f 1 ,2 ,3 , and 4 kHz. Weights were assigned according to frequency specific articulation indexes (ANSI, 1969). The articulation index (AI) is a weighted fraction rep resenting (for a given listening situation) the effective pro portion o f the speech signal that is available (above a mask ing noise level o r hearing threshold) to a listener for conveying speech intelligibility (ANSI, 1969).
Average hearing threshold levels (HTL^g) using the ar ticulation indexes as weights were calculated  and then averaged over both ears:
____ h T L |  W i K ~f~ I f l l^t W a H r L^t = W i + W2+ W i +W 4 
(1) where, W, = 0.24, W2= 0 3 8 , W3= 0 3 4 , and WA= 0 2 4 are die weights at 1, 2, 3, and 4 kHz, respectively. This defini tion will be referred to as the 1-4 kHz AI average" defi nition o f NIHH.

# . M e a s u r e m e n t o f n o i s e e x p o s u r e
Daily 8-hour time-weighted average (TWA) noise exposure was estimated for each worker or worker group using (1) area survey samples, (2) interviews with workmen and supervisors to establish typical workday patterns and (3) time-study charts. These charts segmented the workday into a succession o f exposures at specific noise levels and for specified durations. Discussions with both management and workmen were necessary to determine changes in noise ex posure over the course o f many years. Consideration was given to variations in occupational noise conditions due to placement or relocation o f machinery and as well as changes in workers' work routine and locations. The reported noise *111 die 1972 NIOSH analysis, those exposed to noise for less d m 6 months were coded as "0" for duntioa of exposure. In die current analysis, con trols were coded as "0 for duration of exposure and exposed individuals with less than 6 months of exposure were given a value of 0-25. levels for the study population represent A-weighted eight hour TWA sound levels calculated assuming a 5 dB ex change rate (i.e., 5 dB increase in sound level is exchanged against a factor o f 2 in duration within the workday). All levels were measured with sound level meters set to " slow" response. The A-weighted daily noise levels were available on the 792 noise-exposed individuals but not available for the 380 controls. Although sound levels for the control popu lation were not recorded, they were reported to be below 80 dB (Lempert and Henderson, 1973).

# . O t h e r c o v a r i a t e s
Other covariates o f interest in this paper were age and duration o f exposure in years. Tbe risk o f hearing handicap was examined in relation to die covariates defined in Table m . For models that included categorical variables for age (reference: 17-27 years) and duration (reference: 0 -1 years), four indicator variables were created for different levels o f age and duration exposed (Table III). For models that in cluded continuous variables for duration exposed, all con trols were reassigned a duration value o f zero because it was assumed that duration has no effect on the bearing o f the controls. Exposed individuals with less than six months were coded as 0.25 years (midpoint between 0 and 0.5 years).
Logistic regression models were used to analyze hearing handicap, defined as the proportion o f individuals whose biaural hearing level is greater than 25 dB for averages over selected frequencies. These logistic regression models were fit using the SAS LOGISTIC procedure (SAS Institute, Inc., 1989) and the nonlinear minimization (NLMINB) routine in S-PLUS (Statistical Sciences, Inc., 1993).
Stratified contingency table analyses (Breslow and Day, 1980a) w o e performed to assess these data for qualitative evidence o f hearing handicap due to exposure to noise after controlling for age. The 2X2 contingency tables were strati fied by one year age groups and the prevalence o f bearing handicap among tbe three noise-exposed categories o f 80-84 dB, 85-89 dB, and 90-102 dB were contrasted to the preva lence among controls. One-sided tests for detecting increased rides were computed nsing Mantel-Haenszel methods. Fur ther details o f this method are found in Breslow and Day (1980a).
The quantitative relationship between hearing handicap and the covariates (defined below) was modeled using logis tic regression methods (Breslow and Day, 1980b). These models can be expressed as e F i X ,a ,f i .- .L 0 ) P = MY=\\X)= l+eFtX:a^).
where, p -the expected proportion with average hearing level greater than 25 dB (indicated by Y= I), given X. (Y = 0 indicates an average hearing level is less than or equal to 25 dB); X = a vector o f explanatory variables containing infor mation on age, sound level, and duration o f exposure; F(X;a;fi;^h0)=a+fil (Age) + ,
where LnE= A-weighted 8-hour TWA sound level for noiseexposed workers in dB; L*)=parameter for nominal TWA sound level in control population in dB; shape parameter on dB effect; or-intercept parameter; 0i = slope coefficient for age effect; $ 2 , = the slope coefficient for die jth duration o f expo sure (years) interval, where 7 = 1,2,3 represent ex posure intervals o f 2 -4 years, 5 -1 0 years, and > 10 years o f exposure, respectively.

# m o d e l d e v e l o p m e n t
The first step in the analysis was to fit several hierarchi cal logistic regression models and compare nested models using likelihood ratio tests (LRTs) to identify which param eters significantly improved the fit to the data (Fienberg, 1987). The fit of the model to the data was evaluated using a likelihood ratio test and examining the log likelihood statis tic, G, which is defined by the expression where the summation is over all individuals in the sample (Breslow and Day, 1980b).
In general, die lower the value o f G , the better the fit between die model and die data. Differences in G statistics for nested models may be interpreted as chi-squares (Breslow and Day, 1980b).
To be consistent with the methodology used in the 1972 NIOSH Noise Criteria Document (NIOSH, 1972), the model was initially fit assuming that the sound level for die control population ( was 79 dB and the shape parameter (  ) was I. This was accomplished by first fitting models with main effects only and then adding interaction terms between (a) duration exposed and daily TWA sound level (L); (b) dura tion exposed and age; and (c) age and sound leveL These interaction terms tested whether there should be allowance for differing slopes by levels o f other variables. Models with linear main effect o f age, duration exposed, and sound levels were fit with an assumption that all control 8-hour TWA sound levels ( Lq) were 79 dB. This assumption was made because individual noise exposure data for controls were un available but were known to be less than 80 dB (Lempert and Henderson, 1973). Other models with categorical main ef fects o f age and duration were also examined. The final steps of the analysis involved further model refinements that in cluded (1) assuming there is a nondecreasing relationship o f prevalence with sound level and duration; (2) refitting func tional fbnns identified by the LRT strategy accordingly; (3) assuming more flexible models for incorporating the effects o f sound level by permitting the shape parameter (^) to vary; (4) permitting the control sound level (L^) to vary from 79 dB; and (5) conducting sensitivity analyses o f the impact o f critical assumptions.
A final fonn o f the model was fit such that all the pa rameters (including Lq and ^) were solved for simulta neously. This model form was fit with the following restric tion: the control level, L^, was bounded at 55 dB and 79 dB. For the final model, a two-sided 90 percent confidence inter val was calculated for several noise levels nsing the paramet ric percentile bootstrap method (Efron and Tibshirani, 1986;Efron, 1982). The same restrictions on w o e applied to 1000 bootstrap samples generated to obtain the confidence limits for excess risk. Graphical displays o f bootstrap-based confidence limits were smoothed nsing localized linear re gression smoothers in S-PLUS (Statistical Sciences, Inc., 1993).

# . E x c e s s r i s k e s t i m a t i o n
Excess risk for a particular age is defined as die differ ence between the risk o f hearing handicap for the noiseexposed population, given exposure duration, and the expo sure sound level, (where > L<j),  id the risk o f hearing handicap among controls. The excess risk associated with exposure to noise evaluated at a given age was esti mated from logistic models using the following relationship: Excess R isk=Pi -Pi.
Hence, excess risk is assumed to be equivalent to the in crease in risk o f hearing handicap associated with noise ex posure.

# . S e n s i t i v i t y a n a l y s e s
Sensitivity analyses were performed to examine how model assumptions may affect the results (ix*. excess risk estimates). Assumptions evaluated in this analysis included (1) die shape o f the dose-response relationship; (2) the sound level, Lq, for the control population; and (3) the effect o f using different definitions o f hearing handicap. The first two issues were addressed during model development, where each assumption was varied while die other remained fixed.
A comparison o f how excess risk estimates varied with different definition o f hearing handicap was also examined in this analysis. The new definition (1 -4 kHz A1 average) was compared to definitions previously used by NIOSH (1972)biaural hearing levels averaged over 1 -3 kHz and 0.5-2 kHz. The analyses o f different hearing handicap definitions were based on oar final model for each definition o f hearing handicap: the model in which the control sound level ( I q) and shape parameter (^) were simultaneously estimated.

# L RESULTS
Figure 1 shows the hearing threshold level distributions (10th, 50th, 90th percentiles) for different frequencies by age and sound level categories for exposed and control workers. All hearing thresholds shown are averages over the left and right ears. Data are classified into five age groups and three noise exposure categories (80-87 dB, 88 -9 2 dB, 92-102 dB). The boundaries for the age and sound level categories w o e selected to provide adequate sample size (Le., at least 30) in each cell. Sample sizes for the noise-exposed  groups are provided for each graph with median exposure duration. The sample sizes for the controls  are die same, within age groups (shown in top panel o f each col umn). The graphs show similar exposure durations within each age cell and increasing trends for median hearing threshold levels with age and sound level. In all cases, con trol bearing threshold levels are lower than the noiseexposed population. The tendency o f median hearing thresh olds to increase with increasing age and sound level is also illustrated. The spread o f the distribution (given by the 10th and 90th percentiles) is most marked at 3 and 4 kHz.  Despite the limited amount o f data in the low exposure region, the M antel-Haenszel age-stratified analysis provided evidence o f positive excess risk associated with sound lew is ranging from 80 to 84 dB ip =0.02), as well as 85 to 89 dB (p= 0.02) and 90 to 102 dB (p < 0.001).
Age was found to be a highly significant predictor o f hearing handicap d u e to noise whether it was modeled using a continuous variable ( - 2=211, d f-l) or a set o f categori cal variables (jr2=213, df=4). The fitted categorical effects for age suggested a linear trend (data not shown). This trend was also apparent when models including sound level and duration w o e f it Therefore, the simpler models with linear effects for age (as a continuous variable) were subsequently considered in the final models. The addition o f either years of exposure or sound level ( L^) significantly improved the fit erf the model containing age. The addition o f both terms further increased the goodness o f f it A statistically signifi cant interaction (^2=29.6.  20 years combined to > 10 years) and the model with separate parameters for each duration category. This sug gested that risks remain essentially flat after 10 years of ex posure, and that these two categories could be combined. This initial model was further refined to describe predicted risks o f hearing handicap as a nondecreasing function o f ex posure duration and sound level. The models also assume that the effects o f sound level depend on durations greater than or equal to two years.
To test whether a linear sound level effect ( $ = I) ad equately described the relationship between noise exposure and risk o f bearing handicap, higher order terms for the sound level effect were tested in the analysis. Using a qua dratic sound level term for exposure (^= 2 ) appreciably im  IV. Also shown are the results from fitting a final model in which the control value and the shape parameter were found to be 73 dB and 3.4, respectively (model 4, Table IV). Model 4 is denoted as the "best fitting model,*' because it produced (be best fit to die data. These results indicate considerable vari ability in excess risk estimates depending on model form and is likely due to lade o f data at lower sound levels. H us was most marked at average daily sound levels less than or equal to 85 dB. Figure 3 presents excess risk estimates with smoothed 90 percent confidence limits for 65-year-old males with greater than 10 years o f exposure as a function o f sound level for the " best fitting model" (model 4).

# A. S en sitiv ity a n a ly se s f . A s s u m p tio n r e g a r d in g c o n tr o l B -h o u r TW A m o u n d le v e ls
To examine tbe sensitivity o f risk estimates to the as sumed sound level for the control group, the value o f L- was varied from 60 to 79 dB and optimum values o f tbe shape parameter, were estimated. As Lq is varied, there is very little variation in the log likelihood statistic, G , whereas the excess risk estimate for noise exposure at a level o f 80 dB varies between 0.06 and 2.9 (Table V). The results also show that the optimum value o f ^ decreases considerably as tbe assumed value o f I - increases. This analysis suggests that information regarding the distribution o f occupational sound levels within the control population is important in estimat ing the risk o f noise-induced handicap in noise-exposed populations. The variability of excess risk estimates below 85 dB seen in Fig. 3 may be attributed to the lack o f accurate Risk estim ai an be generated nsmg die following equation: L o g it \ T r iY > 25 dB HL)]  -5X 857 + 00612(Age) + (D urabon=l)K ( L*e-L*)/ (102-73) 1*. where. /$ = 2^653, 3.989, and 6.4206, respectively, fotfte/i dotation o f exposure for 2-4 yean, 5-10 yean, and > 1 0 years, tespectively and y ii die AI-*dgfcced faiaunl avenge over 1-4 kHz. For the l o t fitting no d rl, y was estimated to be 1 4 and L- = 73 dB. The term (1 0 2 -7 3 ) in f e denominator o f fte coefficient describing the effect o f datation and w a d level, standardize- the exposure t e n  e h t o t die au mom capoaire equals one. This was done for ease o f comparison to  id i differing wrimm- for 1 and sound level data among control subjects and the sparseness of the data for workers exposed at sound levels below 85 dB.

# . D e f i n i t i o n o f h e a r i n g h a n d i c a p
To examine whether excess risk estimates varied by the definition o f hearing handicap used, we compared the 1 -4 kHz AI average definition to two other definitions using (he same fence ( > 25 dB HL), die unweighted biaural frequency averages o f 0 5 -2 kHz and 1 -3 kHz. All three definitions were examined using a model that included age and a dose metric effect defined as ( L^-tim e s duration catego ries (e.g., 2 -4 , 5 -1 0 , and > 1 0 years). The resultant esti mated shape parameters for the 0 i -2 kHz and 1 -3 kHz biaural averages were 4.5 and 4.9, respectively, with Lq equal to 55 dB for both.
Under these models, excess risk estimates were affected by both die definition of hearing handicap and die age o f the worker. We also found that changing from the articulation index to a simple average o f 1-4 kHz did not substantially affect excess risk estimates (results not shown). For workers aged 65 years (with > 10 years of exposure), excess risks for the 1 -3 kHz definition are high' than excess risk for the new definition, particularly for sound levels above 85 dB (Fig. 4A). However, among workers aged 45 with similar years o f exposure, excess risk estimates are similar for all sound levels for the 1-3 kHz definition and the new defini tion (Fig. 4B). For younger workers (aged 30 years) with 5 to 10 years o f exposure, excess risk estimates for the definitions that included 3 kHz and/or 4 kHz, are similar for all sound levels (Fig. 4C).

# IV. DISCUSSION
The results o f these analyses indicate that there is an excess risk o f noise-induced hearing handicap (NIHH) at 8-hour time-weighted average (TWA) sound levels greater than o r equal to 85 dB. The excess risk below 85 dB was not well defined in our analysis. However, the M antel-Haenszel test result suggests that there is a positive and statistically significant excess risk at levels between 80 and 84 dB.
These findings also indicate two major areas o f uncer tainty for quantifying the risk o f noise-induced hearing handicap. The first concerns the sensitivity o f the analysis to the assumed sound level for the control group ( 1^). The second relates to the shape o f the dose-response relationship between the sound levels among the noise-exposed group ( me) , duration exposed, and the risk o f NIHH. Risk esti mates w o e found to vary considerably for values of below 85 dB, depending on the assumed control sound level (Lq ), and the shape parameter ( 0 ) for the sound level effect (e.g., linear, quadratic, or cubic) in the models.
Sound Lawel to <B HG. 3. f a  1 * risk (percent) of bearing (/-weighing. 1-4 kHz) and bootstrap-based 90% confidence limits from model 4 (Table IV) far 65-year-old males exposed far greater than 10 years to vatying levels o f noise ( L^) . The previous NIOSH (1972) estimate o f excess risk for a 40-year working lifetime o f exposure to noise was approxi mately IS percent at 85 dB. A linear regression model o f log bearing levels was used in the previous analysis (NIOSH, 1972) to estimate the risk o f hearing handicap. NIHH was defined as an average biaural hearing level greater than 25 dB based on unweighted averages o f 0 .5 -2 kHz o r 1 -3 kHz. The model described in the 1972 NIOSH criteria document (NIOSH, 1972) is mathematically equivalent to a probit model in which the risk o f a hearing level greater than 25 dB is o f interest The results from the previous NIOSH analysis (NIOSH, 1972) also appear to be consistent with the assump tion that the control group was exposed to sound levels near 79 dB.
It is clear that models which include a quadratic or cubic effect for die sound level effect fit significantly better than the linear effect model and produce lower excess risk esti mates for sound levels below 85 dB than similar models used in the 1972 NIOSH analysis (NIOSH, 1972). As shown in Table IV, the point estimates o f excess risk at 85 dB from die quadratic and cubic models are 8 percent and 3 percent, re spectively. The quadratic and cubic models fit better than the linear model, mainly due to the effect o f sound level in the low exposure region. For sound levels less than or equal to 90 dB, the excess risk estimates from fitting a linear model (Table IV) are slightly higher than those in die NIOSH (1972) analysis. Thus, the disparity in excess risk estimates presented in Table IV may be attributed primarily to the different functional forms (i.e., shape o f the sound level ef fect) o f die fitted models. The logistic model used in this analysis assumes the existence o f a plateau in risk after 10 years o f exposure duration.
The analysis comparing different indicators o f NIHH show that patterns o f excess risk as a function o f average daily sound level depend on age. Differences in excess risk were nominal for die 1-4 kHz average, irrespective o f whether HTLs were weighted by the frequency-specific ar ticulation indexes. These differing results by age may be attributable to the fact that the effect o f aging on risk of hearing handicap may overshadow any incremental increases in excess risk due to noise exposure. In the upper range o f duration and sound level, the dose-response curve shows signs of a plateau effect The analysis also suggests hat the effect o f sound intensity and duration o f exposure is depen dent on frequency. Hearing damage at 3 and 4 kHz is ex pected to occur sooner than loss at lower frequencies (0 .5 ,1 , or 2 kHz). Definitions that exclude the higher frequencies tend to. be less sensitive to noise damage and may require longer durations o f exposure to a given sound level to see significant excess risks in die population.
Figure 4A and B suggests that the most suitable defini tion o f hearing handicap may depend on the population char acteristics, such as age, exposure duration, and degree o f hearing handicap already accrued, as well as whether one chooses to identify preclinical or later stages o f hearing handicap. The addition of the most sensitive frequencies to a hearing handicap definition is a valid option if the goal is to have a measure that addresses both prevention and identifi cation o f hearing handicap A. Data lim itations
The cross-sectional design o f this study presented limi tations for estimating die risk o f noise-induced hearing handicap. For example, the 8-hour TWA sound levels,  NE^ were determined at one point in time and are assumed to be representative o f exposure over the entire length o f an em ployee's jo b experience. This may have introduced a sub stantial source o f error in the estimation o f . As a means o f reducing this error, die screened ONHS population in cluded only workers who remained in the same job for the entire time that they worked at the study facility. These workers were then assigned an 8-hour TWA sound level based on noise measurements and job activities at the time of the survey. It is possible that larger errors in estimating 8-hour TWA sound levels over a long period o f time may have occurred for workers with longer durations o f exposure. It is also possible that die workers with long durations in cluded in this study represented a population which may have been less sensitive to the adverse effects o f noise on hearing. This may have contributed to the observed decrease in risk with increasing sound level, for durations greater than 20 years. Hence, the cross-sectional design o f the survey introduces areas o f concern for predicting NIHH risks over a working lifetime.

# B. M odeling c a v e a ts
The data limitations described above also placed limita tions on the modeling approach and interpretations presented in this paper. One data limitation with implications for mod eling the risk of noise-induced hearing handicap, was the lack o f in fo rm a tio n on the distribution o f 8-hour TWA sound levels among the control population. This is a crucial omis sion because all excess risk estimates depend on die risk of handicap among workers with low levels o f occupational noise exposure (in this study, defined as exposure to sound levels less than 80 dB).
Due to this lack o f data, a very simplistic assumption was made: sound levels in the control population could be represented by a single number. This is problematic in terms o f model interpretation. First, it ignores the possibility that there may be a distribution o f sound levels below 80 dB for this population. Second, this assumption results in a model that implies that the estimated value (Lq) is a threshold sound level at which no excess risk o f noise-induced hearing handicap is predicted regardless o f the duration o f exposure. Hence, the statistical criteria used in model development are valid only if all of the controls were below a defined thresh old.
These modeling issues underscore the fact that all mod els are likely to be dependent on assumptions used to ac count for uncertainty in the available data. This analysis did not model hearing threshold levels as a continuous variable. Therefore, calculation o f NIPTS using these models are not possible. The analysis also did not extensively explore other possible shapes for die sound level function other than (Z*he-Furthermore, modeling exposure duration as a categorical variable limits finer examination o f the relation ship o f duration o f exposure on risk o f hearing handicap. Given this updated analysis o f the NIOSH (1972) data, it is o f interest to compare these results to estimates generated using methodology developed by the International Standards Organization (ISO 1971(ISO , 1990. which was adopted in the ANSI S3.44 standard (ANSI, 1996). This standard was is sued to provide a more accurate and more generalized model of the relationship between NUTS and occupational noise exposure for people at different ages and duration o f expo sure. ANSI S3.44 (ANSI, 1996) provides mathematical pro cedures for estimating bearing handicap due to noise expo sure for populations free from auditory impairment (other than that due to noise).
The data from studies by Passchier-Venneer (1968)  id by Bums and Robinson (1970) are the basis o f die ANSI S3.44 (ANSI, 1996) standard for estimating NIKI'S. As with the NIOSH (1972) study, most o f die noise-exposed workers were exposed to daily noise levels ranging from 85 to 95 dB.
The Passchier-Venneer (1968) and Robinson (1970) models are represented by different mathematical equations which include an aging (non-noise) component in dB and a N1PTS component in dB. For each model, the equation for NIPTS was determined by age correcting the noise-exposed workers' hearing threshold levels to get the NIFFS compo nent. An empirical equation was developed for NlFTS in terms o f noise level and exposure time. For each model, the aging and NIPTS components were combined to compute total hearing threshold level in dB (ANSI, 1996) 53.44 (ANSI, 1996). Johnson (1978) provides the methodol ogy used to develop risk percent calculations using the per centage o f die population expected to exceed a specific hear ing threshold level (e.g., 25 dB) for a given population.
The excess risks generated from our analysis o f the 1-4 kHz AI definition are compared to excess risk estimates gen erated using the ANSI S3.44 (ANSI, 1996) methodology and Annex A as die unexposed population. Annex A was chosen over Annex B since the NIOSH study population was highly screened. Hence, the Annex A highly screened control popu lation is the most appropriate comparison to our study popu lation. As shown in Fig. 5, excess risk estimates from our best fitting model are similar to those estimated by ANSI 53.44 (ANSI, 1996) for workers aged 65 years with 45 years o f exposure. However, among workers aged 45 years with 25 years of exposure, excess risk estimates at sound levels great' than 90 dB are higher for this analysis as compared to ANSI S3.44 (ANSI, 1996). These results particularly in the range o f 80-90 dB are not surprising given the similarities in study design, data collection and time period for all o f these studies. Although these are qualitative comparisons, the dif ferences in estimates o f lifetime excess risk between ANSI S3.44 (ANSI, 1996) and this analysis do not appear to be substantial. This is illustrated in Fig. 6, which shows that excess risk estimates generated from ANSI S3.44 are located between the bootstrap-based 90% upper and lower confi dence limits from the best fitting logistic model. At age 45 years and 25 years o f exposure, excess risk estimates below 89 dB are within die lowo- bound o f the confidence limits from the logistic model. Thereafter, point estimates from ANSI S3.44 are found to be lower, particularly at sound levels greater than 92 dB.
For other definitions o f hearing handicap (0 .5 -2 kHz and 1 -3 kHz), ANSI S3.44 estimates o f excess risk are con siderably lower at 85 dB for workers aged 65 years with 25 years o f exposure. For the 0 .5 -2 kHz definition, excess risks at 85 dB from our logistic model and ANSI S3.44 (ANSI, 1996) are 12% and 1%, respectively. For die 1-3 kHz defi nition, the values are 16% for our model and 4% using ANSI S3.44 (ANSI, 1996) methods. At 80 dB, ANSI S3.44 gener ates excess risks o f 0% for both definitions, while rim atps from this analysis axe 5% and 6% for die 0 3 -2 kHz and 1-3 kHz definitions, respectively. Some o f the divergen t results may be due to differences in population characteristics o f the studies used to generate excess risks. The NIOSH data set represented a heterogeneous population o f workers from a variety o f geographic regions and worksites within the United States. The study populations used to develop the ANSI S3.44 (ANSI, 1996) models were likely to be more homogeneous with respect to industry, demographic and so cioeconomic (e.g., access to medical care) characteristics.

# D. F u tu re cfirectiorts a n d d a ta ne e d s
This analysis indicates a need to collect and m alyze data from populations exposed to noise at sound levels below 85 dB to leani more about the shape o f the dose-response rela tionship below 85 dB. Like nm ilar studies conducted in the late 1960 and early 1970's, the screened ONHS data set had few subjects with exposures at levels below 85 dB. This contributed to a high degree o f instability in the risk esti mates as the modeling assumptions were varied. Although logistic modeling techniques were used in this analysis, other methods for evaluating excess risks can reasonably be ap plied to these data. Nonetheless, it seems plausible that the observed instability below 85 dB would persist using ocher modeling methods. Risk estimates in the range o f 88-95 dB are probably more reliable than the estimates for the lower ranges o f sound level. More recent longitudinal data sets may be useful in examining risk below 85 dB. To examine whether noisc-induced bearing handicap remains a problem for workers enrolled in OSHA-mandated bearing conserva tion programs (Department o f Labor, 1981a,1981b), we are currently examining appropriate longitudinal audxm etric da tabases. The present analysis indicates that new studies should be implemented to (1) characterize noise exposure for presumably " non-noise" o r low noise populations (includ ing populations exposed to nonoccupational sources o f noise); and (2) examine dose-response relationships for noise and hearing handicap among workers exposed to noise levels below 90 dB.