# I. RECOMMENDATIONS FOR A NOISE STANDARD
The National Institute for Occupational Safety and Health (NIOSH) recommends that employee exposure to noise in the workplace be controlled by requiring compliance with the standard set forth in the following sections. Control of employee exposure to the occupational limits stated and adherence to the precautionary procedures prescribed will improve the protection of the working population from incurring noise induced hearing loss that could impair their abilities to understand everyday speech.
Such control and adherence at the workplace is believed sufficiently effective to reduce also the possibility of other forms of occupational Injury and illness related to noise. This standard is amenable to techniques that are valid, reproducible, and presently available. It will le reviewed and revised as additional information becomes available.

# Section 1 -Applicability
The provisions of this standard are applicable to occupational noise exposures at places of employment and are intended to apply for all noise even though additional controls may be necessary for certain specific types of noise, such as some impact and impulsive noise. For the purposes of this standard the noise exposure is determined for an 8-hour workday.

# Section 2 -Definitions
As used in this standard, the term: (a) "Administrative control" means any procedure that limits daily exposure to noise by control of the work schedule. (b) "Audiogram" means a graph or table obtained from an audiometric examination showing hearing level as a function of frequency.
1-1 (c) "Baseline audiogram" means an audiogram obtained from an audio metric examination that is preceded by a period of at least 14 hours of quiet.
(e) "Audiometer setting" means a setting on an audiometer corres ponding to a specific combination of hearing level and sound frequency.
(f) "Daily Noise Dose" means that value for D derived from the equation:
D - L + Sl + + 2.

# Ti T2 Tn
where Cl, C2 . . . , Cn are the actual durations of exposure for an employee at the various noise levels, Tl, T2, . . . , Tn are the respective duration limits obtained from Figure 1-1 and D is the Daily Noise Dose.
(g) "dBA -Slow" means the unit of measurement of sound level indicated by a sound level meter conforming as a minimum requirement to the American National Standard Specification for Sound Level Meters, ANSI SI.4 (1971) Type S2A, when used for A-weighted sound level, slow response.
(h) "Engineering control" means any procedure other than administrative control that reduces the sound level either at the source of the noise or in the hearing zone of the employees.
(i) "Hearing level" means the amount, in decibels, by which the threshold of audibility for an ear differs from a standard audiometric threshold.
1-2 (j) "Environmental noise level" means the noise level in dBA-Slow as measured in accord with Section 3(c).
(k) "Effective noise level" means (1) for employees not wearing ear protectors, the environmental noise level; (2) for employees wearing ear protectors, the result of subtracting the dBA reduction, R, for the ear protectors (determined as specified in Appendix A) from the measured environmental noise level. Effective noise level is expressed in units oi dBA-Slow.
(1) "Noise exposure" means a combination of effective noise level and exposure duration. (1) Occupational noise exposure shall be controlled so that no worker shall be exposed in excess of the limit described as line B in  The Indicated duration limits which exceed 8 hours are to be used only for purposes of computing Daily Noise Dose and are not to be regarded as defining noise exposure limits for work days which exceed 8 hours.
employment is believed necessary to permit the Department of Labor to conduct an extensive feasibility study.
(iii) At no time shall any worker be exposed to effective noise levels exceeding 115 dBA-Slow.
(c) Measurements (i) Compliance with the permitted daily noise exposures defined by Section 3(b) shall be determined on the basis of measurements made with a sound level meter conforming as a minimum to the requirement of the American National Standard Specification for Sound Level Meters, SI.4 (1971)
Type S2A, and set to use an A-weighted slow response.
(ii) All measurements shall be made with the sound level meter at a position which most closely approximates the noise levels at the head position of the employee during normal operations.
(iii) An acoustical calibrator accurate within plus or minus one decibel shall be used to calibrate the sound level meter on each day that noise measurements are taken. (i) A baseline audiogram for each employee who is initially assigned or reassigned to work subject to conditions stated in (a) of this section shall be taken within 30 days of assignment to such employment, in the sixth year of such employment, and once every sixth year thereafter. It is recognized that some delay in implementation of this requirement may be necessary for employers with a small work force.

# 1-5 (ii)
A baseline audiogram should be taken for each employee presently assigned to work subject to conditions stated in (a) of this section at the time of effective date of this regulation, in the sixth year, and once every sixth year thereafter.
(iii)
In addition an audiogram, not necessarily baseline, for all exposed employees should be taken every second year.
(c) Each audiogram shall contain (1) employee's name or identifying number,
(2) employee's job location, (3) significant aural medical history of the employee, (4) the examiner's name and signature, (5) the date and time of test, (6) serial number of the audiometer, and (7) last exposure to high level noise: number of hours since exposure; type of exposure; and noise level, if known.
(d) Each employee's audiogram shall be examined to determine whether it indicates for either ear any threshold shift (higher threshold), that equals or exceeds 10 dB at 500, 1000, 2000, or 3000 Hz, or 15 dB at 4000 or 6000 Hz as evidenced by a comparison of that audiogram with the employee's most recent baseline audiogram and with his initial baseline audiogram as corrected to his current age by the method described in Appendix B. If either comparison indicates a shift as described above:
(i) refer the employee for appropriate medical evaluation, (ii) if the employee needs personal protective equipment or devices, insure that he has the appropriate effective equipment and that he is instructed in the proper use and care of the equipment, and
(iii) if the audiogram was not a baseline audiogram, take a baseline audiogram within sixty days.
(e) Audiometric tests shall be pure tone, air-conduction, hearing threshold examinations, with test frequencies including 500, 1000, 2000, 3000, 4000, and 6000 Hz and shall be taken separately for the right and left ears.

# 1-6 (i)
The tests shall be conducted in a room whose ambient noise levels conform to all requirements except that part concerning octave bands whose center frequencies are less than 250 Hz of the American National Standard Criteria for Background Noise in Audiometer Rooms, ANSI S3.1 (1960 R-1971), when measured by equipment conforming to American National Standard Specification for Sound Level Meters, ANSI SI.4 (1971) Type 2 and American National Standard Specification for Octave, Half-Octave, and Third-Octave Band Filter Sets, ANSI SI.11 (1966, R-1971).
(ii)
The tests shall be administered using an audiometer which conforms to the requirements for limited range pure tone audiometers prescribed by the American National Standards Specifications for Audiometers, ANSI S3. 6 (1969), and which is of the discrete frequency type. If a pulsed tone audiometer is used, the on-time of the tone shall be at least 200 milliseconds. The instrument used in the testing shall be either a manual audiometer, or a self-recording audiometer which is subject to the following additional restrictions:
(1) The chart upon which the audiogram is traced shall have printed lines at positions corresponding to all multiples of 10 dB hearing level within the intensity range spanned by the audiometer. The lines shall be equally spaced and shall be separated by at least 1/4 inch.
Additional graduations are optional. The pen which traces the audiogram shall have a fine point so that the tracing shall not exceed 2 dB in width.
(2) It shall be possible to disable the stylus drive 1-7 mechanism so that the stylus can be manually set at the 10-dB graduation lines for calibration purposes.
The slewing rate for the audiometer attenuator shall be 6 dB/sec or less except that an initial slewing rate greater than 6 dli/sec is permitted at the beginning of each new test frequency, but only until the second subject response.
(A) The audiometer shall remain at each required test frequency for 30 seconds (+ 3 seconds). The audiogram shall be clearly marked at each change of frequency and the actual frequency change of the audiometer shall not deviate from the frequency boundaries marked on the audiogram by more than + 3 seconds.
(5) If an audiogram fails to pass the following criteria, the subject shall be retested:
At each test frequency it must be possible to place a horizontal line segment parallel to the time axis on the audiogram, such that the audiometric tracing crosses the line segment at least six times at that test frequency.

# (iv)
The audiometer shall be maintained in calibration in accordance with the provisions of Appendix C. If noise exposures to which employees could be exposed exceed the limits specified, personal protective equipment (i.e., ear protectors) shall be provided by the employer to be used in conjunction with an audiometric testing program, as specified in Section 4, subject to the f oJlowing requirements:
(i) The use of personal protective equipment to prevent occupational noise exposure of the employer in excess of the prescribed limits is authorized only until engineering and administrative controls and procedures can be implemented to maintain the occupational noise exposures within prescribed limits.
(ii) Any ear protector used by an employee shall reduce the effective noise level to which he is exposed so that his noise exposure is within the limits prescribed in Section 3(b).
(iii) Insert -type ear protectors shall be fitted by a person trained In this procedure.
(iv) Inspection procedures to assure proper Issuance, main tenance, and use of personal protective equipment shall be established by the employer.

# 1-9 (b)
The employer shall provide training In the proper care and use of all personal protective equipment.
Section 8 -Apprisal of Employees of Hazards from Noise Each worker exposed to noise shall be apprised of all hazards, relevant symptoms, and proper conditions and precautions for working in noisy areas.
The information shall be kept on file and readily accessible to the worker at all places of employment where the noise levels equal or exceed the limits prescribed in Section 3 (b).

# Section 9 -Monitoring & Recordkeeping Requirements
(a) Employers will be required to maintain records of:
(i) environmental exposure monitoring for a period of 10 years, (ii) all audiograms for a period of 20 years.
(iii) all audiometric calibration data for a period of 20 years.
(b) When exposure times of less than 8 hours/day are required in a specific work area or ear protection is used to meet the exposure limits, records of the method of control shall be maintained.
Appendix A -Determination of dBA Reduction R for Ear Protectors 1. The pure tone attenuation vs. frequency characteristics of the ear protector (normally supplied by the manufacturer) shall have been determined
In accordance with the American National Standard for Measurement of the Real-Ear Attenuation of Ear Protectors at Threshold, ANSI Z24. 22 (1957). The "-10.0" correction term is to account for possible noise spectrum irregularities and noise leakage which might be caused by long hair, safety glasses, head movement, or various other factors.

# Let
3. If the octave band levels of the noise are not known, then the dBA reduction R may be computed simply as R --10 log S -3.0 where S  antilog (-0.1 X Qj) + antilog (-0.1 X Q2)
- antilog (-0.1 X Q3) + antilog (-0.1 X Q4) + antilog (-0.1 X Q5) + antilog (-0.1 X Q6) + antilog (-0.1 X Q7)
This calculation is approximate, and is based upon the assumption that the octave band levels are equal. For most types of noise it will give results close to those obtained by the more accurate method of (2) above.     (1) Set audiometer to 70 dB hearing threshold level and measure sound pressure levels of test tones using an NBS-9A-type coupler, for both earphones and at all test frequencies.
(2) At 1000 Hz, for both earphones measure the earphone decibel levels of the audiometer for 10 dB settings in the range 70 to 10 dB hearing threshold level. This measurement may be made acoustically with a 9A coupler, or electrically at the earphone terminals.
(3) Measure the test tone frequencies with the audiometer set at 70 dB hearing threshold level, for one earphone only.
(4) In making the measurements in (1) -( 3) above the accuracy of the calibrating equipment shall be sufficient to prove that the audiometer is within the tolerances permitted by ANSI S3.6-1969.
(5) A careful listening test, more extensive than that required in the biological calibration, shall be made in order to ensure that the audiometer displays no evidence of distortion, unwanted sound, or other technical problems.
(6) General function of the audiometer shall be checked, particularly in the case of a self-recording audiometer.
(7) All observed deviations from required performance shall be corrected.
C. An exhaustive calibration shall be performed at least every five years. This shall include testing at all settings for both earphones.
The test results must prove unequivocally that the audiometer meets for the following parameters the specific requirements stated in the applicable sections of ANSI-S.  as noted in parenthesis.
(1) Accuracy of decibel level settings of test tones (Sections 4.1 .4. 1 and 4.1 .4.3).
(2) Accuracy of test tone frequencies (Section 4.1.2).
(3) Harmonic distortion of test tones (Section 4.1.3).
(4) Tone-envelope characteristics, i.e., rise and decay times, overshoot, "off" level (Section 4.5).
(5) Sound from second earphone (Section 4.4.2).
(6) Sound from test earphone (Section 4.4.1).  ation. In the case of periodic sound pressures, the interval must be an integral number of periods or an interval that is long compared to a period.

# III-l
Frequency -The frequency of a function periodic in time is the reciprocal of the primitive period. The unit is the cycle per unit time and must be specified. The unit cycle per second is commonly called Hertz (Hz).
Level -In acoustics, the level of a quantity is the logarithm of the ratio of that quantity to a reference quantity of the same kind. The base of the logarithm, the reference quantity, and the kind of level must be specified.
Noise -(1) Noise is any undesired sound; and, by extension, noise is any unwanted disturbance within a useful frequency band, such as undesired electric waves in a transmission channel or device. (2) Noise is an erratic, intermittent, or statistically random oscillation. Since the definitions of noise are not mutually exclusive, it is usually necessary to depend upon context for the distinction.
Noise Level -(1) Noise level is the level of noise, the type of which must be indicated by further modifier or context. The physical quantity measured (e.g., voltage), the reference quantity, the instrument used, and the bandwidth or other weighting characteristic must be indicated.
(2) For airborne sound, unless specified to the contrary, noise level is the weighted sound pressure level called sound level; the weighting must be indicated.
Oscillation -Oscillation is the variation, usually with time, of the magnitude of a quantity with respect to a specified reference when the magnitude is alternately greater and smaller than the reference.
Period -The period of a periodic quantity is the smallest increment of the independent variable for which the function repeats Itself.

# III-2
Periodic Quantity -A periodic quantity is an oscillating quantity whose values recur for certain increments of the independent variable.

# Sound -(1)
Sound is an oscillation in pressure, stress, particle displacement, particle velocity, etc., in a medium with internal forces (e.g., elastic, viscous), or the superimposition of such propagated oscilla tions. (2) Sound is an auditory sensation evoked by the oscillation described above.
Sound Absorption -Sound absorption is the change of sound energy into some other form, usually heat, in passing through a medium or on striking a surface.
Sound Level (SL) -Sound level is a weighted sound pressure level, obtained by the use of metering characteristics and the weightings A,B, or C as specified in the American National Standard Specification for Sound Level Meters, ANSI-S1.4-1971. The weighting employed must be stated.
Sound Pressure -The sound pressure at a point is the total instanta neous pressure at that point in the presence of a sound wave minus the static pressure at that point.
Sound Pressure Level (SPL) -The sound pressure level, in decibels, of a sound is 20 times the logarithm to the base 10 of the ratio of the pressure of this sound to the reference pressure. The reference pressure must be stated. The following reference pressure is in common use for measurements concerned with hearing and with sound in air and liquids: 2 X 10 N/M^. Unless otherwise explicitly stated, it is to be understood that the sound pressure is the effective (rms) sound pressure.

# III-4

# Response of the Ear
Upon inspection of the definition of sound pressure level, it is evident that there is no reference to frequency. In actuality, the ear does not show equal response to all frequencies, and in fact, it is more sensitive to the middle frequencies than to the low or high ones. Studies have been made which determine the sound pressure levels of simple tones at various frequencies which sound just as loud to an observer as a 1000 Hz tone of a particular SPL. The results of such comparisons are given as equal loudness curves in Figure 1 . The number of each curve, loudness level in phons. is the SPL of the 1000 Hz tone used for comparison in 3 determining the curve.

# Measurement Scales
These equal loudness contours have been taken into account in the standardization of several frequency weighting networks which are included on most sound measuring equipment. The frequency characteristics of these networks are given in Figure 2. The A scale corresponds approximately to the 40-phon equal loudness contour, the B-scale corresponds to 70-phons, and the C-scale corresponds to the 100-phon contour. With these weighting networks, which modify sound pressure level to approximate the ear's response, the term to be used is sound level, and the weighting used must always be stated. (?'The A-sound level is 36 dB" or "86 dBA" are appropriate ex--5 2 pressions.) The reference pressure is 2 X 10 N/M ).
The A scale is commonly used in measuring noise to evaluate its effect on people, and the A-weighted sound level is considered an adequate number to indicate or rate the hazard of a certain noise. Explanation of these measurements is given in Part IV.

# III-5

# Correction of Original Baseline Audiograms for Age
To determine whether there has been a significant change in an employee's hearing due to noise exposure by comparing an audiogram taken since the original baseline audiogram with that audiogram it la necessary to make a correction for difference in age.
The best way to make this correction is to use data from a non-noise exposed group from the same area tested in the same manner as the group under consideration. Quite often this is not possible; therefore, It Is necessary to establish an age correction that can be used universally. The data from these four studies with respect to differences in hearing level from age 20 are quite similar. However, the greater changes apparent
In the National Health Survey data at the upper frequencies could be expected because this population was not screened for significant noise exposure.

# III-6
Thus, to provide a uniform correction, tables B-l and B-2 (Appendix B) derived from the NIOSH data, will be used as specified in the standard.

# Conversion of Octave Band Levels to dBA Levels
When the octave-band sound pressure levels of a noise have been measured it is often desirable to compute the A-weighted sound pressure level from them. This can be done as follows:
1. From each octave band level, subtract (or add) the A-weighting correction value shown in Table III. corresponding to the frequency of the octave band. This method is quite accurate although it does involve some approximation.

# Calculation of dBA Reduction R for an Ear Protector
Calculation of dBA reduction R for ear protectors can be done as follows:
1. When the octave band levels of the noise are known: If the dB attenuation levels of the ear protector were known for each octave band, then the dBA reduction of the ear protector could be determined by subtracting these attenuation levels from the original octave band noise levels, and then calculating the dBA level of the resulting attenuated octave band levels using the method described in the previous section. One would then subtract this dBA level from the original dBA level to obtain the dBA reduction.

# III-7
However, the American National Standard for Measurement of Real-Ear Attenuation of Ear Protectors at Threshold, ANSI Z24. 22 (1957), prescribes pure-tone tests at 125, 250, 500, 1000, 2000, 3000, 4000, 6000, and 8000 Hz.
We shall also assume that the 4000 Hz octave band attenuation level can be obtained by averaging the pure tone attenuation levels at 3000 and 4000 Hz, and that the 8000 Hz octave band attenuation level can be obtained by averaging the pure tone attenuation levels at 6000 and 8000 Hz.
This method has been formulated concisely in Appendix A of the Recommended Noise Standard, also including a factor of 10 which is to be subtracted to account for possible noise spectrum irregularities and noise leakage which might be caused by long hair, safety glasses, head movement, or various other factors. It ignores noise in the 31.5 Hz, and 16,000 Hz bands, but these rarely contribute substantially to the dBA level.
When the octave band levels are not known it is assumed that the noise has a uniform "pink" spectrum, i.e., equal levels in each octave band.
This type of noise is representative of "average" occupational noise, and the error introduced by making this approximation is usually small. The assumption results in a simplified formula for calculation, as presented in Appendix A of the recommended Noise Standard. It is recommended, however, that the more exact method described in (1 ) above be used whenever octave band noise levels are available.

# IV. REVIEW OF EFFECTS OF NOISE ON MAN
Exposures to noise levels found at the workplace, particularly in mechanized Industries, are likely to be the most Intense and sustained of any experienced In dally living. As such, they represent the severest form of acoustic insult to man and therein pose the greatest harm to human function. Real or alleged effects of occupational noise exposures include the following:
-Temporary and permanent losses in hearing sensitivity.
-Physical and psychological disorders.
-Interference with speech communications or the reception of other wanted sounds.
-Disruption of job performance.
These different listed effects of noise can be classified in various ways. For example, the first two effects can be treated in the context of health or medical problems owing to their underlying biological basis.
That is, noise-induced hearing loss, as will be described later, involves damage to the cell structures of the hearing organ, and physical or psychological disorders due to noise presume alterations in normal physiologic or nervous system responses. In contrast, the remaining two effects, interference with sound reception and performance loss, are deemed annoyance or economic problems since they involve no pathology or physical dysfunction to the organism.
The aforementioned noise effects can also be classified as "auditory"
and "extra-auditory" in nature. In this regard, hearing loss and speech interference caused by noise are deemed auditory effects since they both involve disturbance to the hearing organ and/or its functional processes.

# IV-1
Noise effects on physical and psychological health states md/or performance represent extra-auditory effects in that they extend beyond or are apart from auditory experience per se.
The intent of this part is to summarize current knowledge of these various adverse effects of noise as it relates to occupational noise conditions, and to establish their importance relative to needs for noise control in industry.

# Hearing Loss
The ear is the organ structure of the body especially adapted and most responsive to the pressure changes underlying airborne sounds or noise. Anatomically, it is divided into three subdivisions -the outer, middle, and inner ear. Some key structures within these subdivisions are shown in Figures 3 and 4 structures by high level sounds subjects them to shearing forces or other mechanical stresses that may jar them loose from their supporting cells or otherwise damage them.1 Another is that constant intense sound stimula tion forces the hair cell receptors to high metabolic levels that cannot be maintained. As a result, the metabolic processes essential for cellular life become exhausted or poisoned, leading to the death of the cells
12 involved.

# IV-3
Since direct observation of the cell structures of the intact ear on a live human is impossible, injury to these parts are inferred from audio grams which show losses in threshold hearing levels for certain pure tone frequencies relative to some reference value. Such losses, when due to noise, may be of a temporary or permanent nature. Temporary hearing loss, more usually called temporary threshold shift or TTS, can be produced by brief exposures to high level sound and shows recovery following a period of time in quiet. Figure 6 describes an example of TTS caused by a twohour exposure in the laboratory to a broad-band noise of fairly high level (103 dBA). In this instance an audiogram was taken on the listener just before and at various times after the cessation of the noise exposure.
Differences between pre-and post-exposure threshold levels for the specified test frequencies display the amount of TTS induced by the noise.
TTS is greatest immediately after exposure and progressively diminishes with increasing time in the quiet, reflecting ear recovery from the apparent noise overstimulation .
As a general rule, a noise capable of causing significant TTS with brief exposures is probably capable of causing significant permanent losses in hearing, given prolonged or recurrent exposures. In fact, some limited evidence from animal studies suggests the presence of minor hair cell damage even in those ears showing complete recovery from noise-induced temporary threshold shift. 13 In any case, daily exposures to TTS-producing noises for several hours per day for months or years would pose a risk of permanent hearing loss. That is, the ear is not likely to recover completely with recurrent exposures of this type. Rather, only partial recovery may occur IV-4 in due course with new resting thresholds established, poorer than those found at the beginning of the overall noise experience. This residual loss is indicative of permanent hearing damage. Figure 7 describes these permanent losses in one group of employees as a function of their years of exposure to workplace noise levels appoximating the level used 14 in the TTS example above.
The TTS component in the losses evident in these hearing data was eliminated by allowing sufficient time after the workshift ended before taking audiograms on the worker group. Deductions have also been made in the hearing levels to remove those hearing changes related to aging (i.e., presbycusis).
Figure 7 indicates that the most significant noise-induced hearing losses occur first in the high frequency range, most prominently at 4,000 cycles per second or Hertz (Hz). Decreased sensitivity to these high frequency sounds may go unnoticed by the listener since they are relatively unimportant to speech reception. With increasing exposure years, however, the losses grow and also broaden to involve other frequencies which are more critical to speech reception, namely, those in the range 500 to 3,000
Hz. In actuality, in the United States, generally accepted procedures for rating hearing handicap for speech consider losses only at frequencies 500, 1000, and 2000 Hz on the audiogram.^ Controversy centers around the need to include 3000 Hz in these judgments since consonant discrimination may depend on hearing sensitivity for sounds higher than 2000
The pattern of permanent hearing loss shown in Figure 8 seems typical of noise-induced hearing damage as revealed in noise and hearing surveys in assorted industries (see Table IV). Why high frequency hearing, parti cularly around 4000 Hz, shows most vulnerability to noise is not altogether IV-5 clear. One possible explanation is that the resonant frequency of the ear canal is in region 2000 -5000 Hz which, in effect, adds strength to corres-19 20 ponding frequencies in an incoming noise signal. - Complicating evaluations of hearing loss due to industrial noise are a number of factors. First, hearing sensitivity normally decreases with age, and these losses (presbycusis) are quite similar to those caused by excessive noise, i.e., differentially greater losses at the higher fre- Other causes of hearing loss besides noise and age include use of 23 drugs, illness and disease processes, blows on the head. Special audio metric procedures are sometimes necessary to diagnose a given case of hearing loss in order to determine the likelihood that it may have been caused by excessive noise rather than other agents.
Even when there is clear audiometric evidence of noise-induced hearing loss, questions may be raised as to whether such damage was produced entirely by workplace noise. It is apparent that off-job noise conditions, particularly in recreation, can pose some risk of hearing change by themselves or can 24 exacerbate the acoustic insult associated with the job situation.

# IV-6
While indicating the need for close appraisal of audiometric results, the aforementioned complicating factors should not be construed as minimizing the seriousness of noise and hearing loss problems in industry. As will be noted in subsequent section, noise surveys in assorted manufacturing, construction, mining, transportation and farm operations indicate exposure conditions potentially harmful to millions of workers. Indeed, the popula tion at risk with regards to noise-induced hearing loss may be greater than any other hazard in the work environment. Audiometric data already col lected on samples of employees in many of the jobs surveyed above for excessive noise show them to have poorer hearing relative to other groups of workers not so exposed. Composite reviews of published occupational  Obviously, the rate and number of such impact sounds constituting an exposure period are also factors in making hazard judgments for these types of sounds.
Noise and hearing surveys in industry dealing with these types of exposure conditions are just beginning. Most permanent hearing loss data reflooting impulse noise hazards have been based on military studies involving gunfire.

# IV-9
Physical and Psychological Disturbances Aside from hearing loss, noise may trigger changes in cardiovascular, 35 37-39 endocrine, neurologic, and other physiologic functions, - all of which are suggestive of a general stress reaction. These physiologic changes are produced typically by intense sounds of sudden onset, but can also occur under sustained high level or even moderately strong noise conditions.
Whether repeated noise induced reactions of this type can ultimately degrade one's physical and mental health is still uncertain. For example, the aforementioned physiologic changes tend to subside with recurrent exposures to the same sounds, suggesting adaptation and presumably no health difficulty. These observations, however, may not have been conducted over a sufficiently long time period to judge the possible long-term cost 40 of this adaptation to the health of the organism.
There are some reports which show that prolonged exposure to high level noise may lead to physiologic disorders in animals. For example, Anthony and Ackerman reported that guinea pigs presented with intense siren-type noises for fairly long periods of time eventually revealed endocrine and metabolic deficits which reduced their ability to cope with 41 the noise stress.
Additional siren exposure here resulted in gastro intestinal ailments, cardiovascular disease and even tissue damage in the kidneys and liver. Reproductive dysfunction and reduced resistance to infectious disease have also been reported in animals subjected to recurrent 42 43 or prolonged high level noise conditions. '
The results of these studies have not been without criticism.in some instances, they have lacked for certain controls, e.g., handling of test animals in noise but not non-noise groups, or not differentiating the groups. Also, rodents have often been used as subjects, and these animals are known to have special IV-10 susceptibility to the effects of certain sounds. Furthermore, the sound levels used in many of these experiments have usually been well above those normally encountered by man even in the noisiest environmental situations.
With regard to human exposures, there are Indications in the foreign literature which suggest that routine exposures to intense industrial noise might lead to chronic physical disturbances. A German study, for example revealed a higher incidence of circulatory and neurologic irregularities among steel workers in noisy jobs as compared with other worker groups in 44 less noisy plant areas.
Neurological examinations of Italian weavers exposed daily to intense noise showed their reflexes to be hyperactive, and in a few cases, electroencephalography traced a pattern of desynchron-25 ization similar to that seen in personality disorders.
A study in the Russian literature showed workers in noisy ball-bearing and steel plants to have a relatively greater prevalence of cardiovascular irregularities 45 such as bradycardia.
Complaints of fatigue, irritability, and social conflicts in many noise exposed workers have also been reported in connection 25 44 with several of the investigations just noted. '
The fact that those who work in high noise levels show greater medical difficulties than those who work under quieter conditions is not conclusive evidence that noise is the crucial causal factor. In each case, it is possible that the differences in the specified health parameters may be explained by other factors such as age, other environmental contaminants, work load and job habits. In any case, replication of these findings seems indicated with attempts being made to statistically isolate and better control factors that could confound possible noise effects on a variety of health indicators.

# IV-11
Concern may be raised as to whether excessive noise poses any added hazards to persons with preexisting health problems. That is, losses in performance may occur when the noise appears and also when it disappears.
Apparently, change in noise levels is the degrading factor. Of interest here is that sound levels required to cause notable performance changes may exceed hearing conservation limits for routine workday exposure. Thus, noise standards for safeguarding hearing could also offset possible noise 38 effects on job efficiency.
More moderate levels of noise may actually benefit task performance relative to quiet conditions. The presence of such noise may provide for a more uniform acoustic background, masking stray sounds which could be otherwise audible in a quiet work area and cause distraction. Also on the positive side, pulsating-type sounds may pace or drive performance and, in effect, reduce fatigue on tasks that are trying. The rhythm component in music may serve this purpose too.^"- Not all performance capacities suffer equally from the disruptive effects of noise. In fact, noise may aid performance on simple, wellpracticed or repetitive tasks through causing increased arousal in an otherwise boring job. There are indications in fact, that only those tasks which require unremitting attention (e.g., vigilance in machine monitoring, product inspection) or which place extreme mental demands on IV-14 the employee (short-term high-memory loads) may be most vulnerable to the degrading effects of n o i s e .^ In short, tasks which by themselves tax the total capacity of the Individual do not permit any accommodation to noise and consequently show loss. In some Instances, noise seems more Inclined to disturb the quality rather than the quantity of work. That Is, noise might not change work output, but may cause more errors. Along these lines, performance under noise may be subject to worker fluctuations, with periods of poor performance being Interwoven with periods of heightened 56 effort.
These performance swings, when averaged across the total work session, may yield little or no overall performance decrement.
Individual differences are quite commonly found In investigations of noise effects on performance. Attitudes toward noise are a basic factor in this variance. A recent laboratory study found subjects sensing that they had no control over randomly occurring noise intrusions to perform 57 poorer than those who could terminate such sounds.
Personality factors also seem to underlie performance variations in noise. Tense, anxious persons, as defined by personality inventories and certain physiologic indicators, seemed less able to cope with certain laboratory tasks as 58 compared with those who were more relaxed.
The importance of attitude factors in noise-performance studies is even more evident in field investigations. For example morale and egoinvolvement in one's job can override stresses imposed by noise.Other employees dissatisfied with their job situations can use noise as a "scape goat" for poor performance. It should be mentioned, too, that through a process of self-selection, only the more noise tolerant employees probably stay at a noisy job. The more noise sensitive persons would remove themselves from these situations. Measures of absenteeism and labor turnover could IV-15 reflect the latter occurrences, but could be difficult to relate causally to noise.
The numerous factors that can influence noise effects on performance make for highly qualified conclusions and conservative predictions. Generally speaking, intense sounds preferably impulsive and a taxing task performed by a tense person offer the combination of conditions most likely to display a noise-induced decrement in performance.

# Cumulative Effect
This discussion has separately treated different adverse effects of noise that may result from occupational exposures. It is unrealistic, however, to conceive of employees In their everyday job routines experiencing one form of noise disturbance to the exclusion of the others. Indeed, one's daily encounters with workplace noise may degrade hearing, mask reception of desired sounds, heighten emotions and physiologic activity, disrupt con centration , or otherwise hinder job efficiency or safety. The collective impact of these noise effects clearly poses a significant challenge to the employee's health, productivity, and well-being.
The establishment of noise exposure limits for safeguarding the employee's hearing will in the main prevent the occurrence of the worst physical defect that noise can have in humans. Adherence to these limits may also have other benefits. Specifically, physical and psychological disturbances from occupational noise exposures will be less likely to arise under more controlled exposure conditions. Of course, the question remains as to whether excessive noise by itself can cause physical and mental disorders.
Nevertheless, noise levels which meet hearing conservation requirements may also be within limits that do not cause losses in job performance.

# IV-16

# V. PROCEDURES FOR REDUCING EXPOSURES
The information in this part (1)  is not an attempt to present exact figures as to how many employees are exposed to hazardous noise levels; rather it substantiates that noise is Indeed a common occupational hazard which could affect a large number of employees. The companies were asked to answer the following question: "How many of your employeees are located In areas where noise levels are 90 dBA or above?" When this question was asked in August 1971, the recommended level for an 8-hour exposure limit was still 90 dBA, however, it can be assumed that a greater population is at risk. In interpreting the answers to the question stated in the preceding paragraph and the results which appear in Table VI the following points must also be considered:
V-l 1. The answers from this question cannot be used to determine how many employees are incurring hazardous noise exposures because information concerned with length of exposure time and the exact level of the exposure is not available.
2. The 341 plants volunteered this information, and the information is based upon their own noise level evaluations.

# Many other plants involved in the questionnaire survey could not
-r did not answer the question.
4. The table does not contain information concerned with the 4,511,000 transportation workers, the 3,502,000 construction workers, the 626,000 miners, or the 4,746,000 agricultural w orkers.
The projected numbers of employees located in areas of noise 90 dBA F. Use sharp and properly shaped cutting and drilling tools.

# Substitution of Machines
A. Substitute belt drives for gears.

# Vibration Dampening
A. Increase mass.
B. Increase stiffness.
C. Use rubber or plastic linings to dampen noise.
D. Improve supports.

# Reduction of Solid-Borne Transmission
A. Flexible mounts for motors and other types of machinery.
B. Flexible hose in pipes or electrical conduits.
C. Flexible coupling on shafts.

# . Reduction of Noise Caused by Fluid Flow
A. Install or replace intake and exhaust mufflers on internal combustion engines and compressors.

# Isolate Noise Source
A. Construct sound reduction enclosures around equipment or parts of equipment.

# Isolate Operator
A. Provide a relatively sound-proof enclosure for the operator or attendant of one or more machines.

# V-4
Of the items listed above, the preferred procedures for reducing environmental noise are those which are directed at reducing the noise at its source (Items 1-7). Generally, these procedures have proven to be far more efficient in terms of actual noise reduction than the procedure listed as Item 8 . Furthermore, source noise controls provide protection for both the operators of the equipment as well as workers in the immediate exposure environment.

# Administrative Controls
Another effective approach to reducing the hazard of excessive noise exposure is to limit the dally amount of exposure which each employee receives, by means of strict control of the work schedule.
The following are several methods suggested by the Department of Labor "1. Arrange work schedules so that employees working the major portion of a day at or very close to the criteria limit are not exposed to higher noise levels.
2. Ensure that employees who have reached the upper limit of duration for a high noise level, work the remainder of the day in an en vironment with a noise level well below criteria limit.
3. Where the man-hours required for a job exceed the permissible time for one man in one day for the existing sound level, divide the work among two, three, or as many men as are needed, either successively or together, to keep individual noise exposure within permissible time limits.
4. If less than full-time operation of a noisy machine is needed, arrange to run it a portion of each day, rather than all day for part of the week.
V-5
Perform occasional high level noise producing operations at night or at other times when a minimum number of employees will be exposed."
When personnel are rotated, extreme care must be taken to insure that no single employee is exposed to a high level noise for a period longer than is allowed by the noise exposure limits.

# V-6
VI.

# DEVELOPMENT OF THE STANDARD
Attempts at limiting human exposure to noise have been based on damage risk criteria. The purpose of such criteria is to define maximum permissible levels of noise for stated durations which, if not exceeded, would result in an acceptably small effect on hearing levels over a working lifetime of exposure.
Previous  1. Measurement of spectral distribution (Noise Spectrum).
2. Determination of the temporal characteristics of exposure (Noise Duration).

# Identification of a protection goal (Biologic Response).
In the discussion which follows, selected damage risk criteria listed in  As may be seen from this figure, although the DRC differ in estimates of safe sound pressure level per octave, they all weight the spectrum similarly.
The second major Impetus for measurement of noise based on octave band analysis came from research which indicated that, at least for most audio metric frequencies, the amount of threshold shift observed (either temporary or permanent) was closely related to the frequency or spectrum of the stimulus. Results of "stimulation deafness" (temporary threshold shift) studies indicated that for pure tone stimuli the maximum shift in hearing appears to be about one-half octave above the frequency of total Prior to 1956 damage risk criteria set as a goal for protection (see Protection Goal), the prevention of hearing loss at all frequencies. This necessitated assessment of the noise at each octave band. After this time, VI-3 however, much more qualified protection goals were established (usually pro tection of loss in the so-called "speech frequencies") such that only knowledge of the sound pressure in certain critical octave bands (not to" be confused with aural critical bands) was required In order to assess the risk of noise exposure to hearing. This approach characterized the damage risk criteria developed by the Air Force in 1956, The American Academy of Ophthalmology and Otolaryngology in 1957, the International Standards Organ ization in 1961, and the American Academy of Ophthalmology and Otolaryngology in its revision of the 1957 criteria in 1964 (see Table IX).
The procedure for rating noise hazards by this mehtod consists of measuring the octave band levels in the critical octaves, and then comparing the measured levels with damage risk contours. This is best exemplified by the use of the "Noise Rating" curves developed by the International 80 Standards Organization.
The octave band levels of the noise are measured and then compared with the noise rating curve (Figure 9). The highest curve which is exceeded by the level of these bands yields the noise rating number (N). For this particular scheme, a noise rating of 85 was suggested as the protection criterion.

# The Use of A-weighted Sound Level
Since the publishing of the first Intersociety "Guidelines for Noise 81 Exposure Control," a relatively new approach, A-weighted sound level measurement, has become a popular measure for assessing overall noise hazard. As stated in Part III, the weighting on the A-scale approximates the 40-phon equal loudness contour (Figure 8). Use of the A-weighting is thought, therefore, to insure the rating of noises in a reasonably similar manner as would the human ear.

# VI-4
Several studies have been conducted In order to evaluate the efficacy of using A-weighted sound levels In rating hazardous exposures to noise.
82 In a study of 580 Industrial noises, Botsford showed that the A-weighted sound level Indicated the hazard to hearing as accurately as did limits expressed as octave band sound pressure levels in 80% of the cases and was slightly more conservative than octave band measures in 16% of the noises. Passchier-Vermeer2^ found that, except In one noise condition, sound level In d BA was as accurate as Nose Rating (NR) in estimating noise 83 induced hearing loss. In a study of hearing loss in 759 subjects, Robinson concluded that the error incurred from using dBA in predicting hearing level was within + 2 dB, even for noises ranging in slope from + 4 dB/ 84 octave to -5 dB/octave. A recent study found that even though dBA perhaps discounted too much low frequency energy, in all cases but one it predicted TTS2 resulting from exposures to noises of different spectra (slopes of -6 dB/octave, 0 dB/octave, and 6 dB/octave) as well as or better than other noise rating schemes which employed spectral measurements in octave-bands.
As a result of its simplicity and accuracy in rating hazard to hearing, the A-weighted sound level was adopted as the measure for assessing noise exposure by the American Conference of Governmental Industrial Hygienlsts

# VI-5
In keeping with the several precedents which have been established for its use in rating the hazard resulting from industrial noise exposure, and because it has been shown to be a reasonably accurate measure of such hazard, the A-welghted sound level measurement has been recommended for use in rating noise hazard in the Recommended Standard.

# Protection Goal
The limit of noise exposure that is established ultimately depends upon the degree of hearing which is to be protected and the number of persons in an exposed population to be protected. If a very strict protection criterion is contemplated such that no person exposed to noise will develop hearing loss at any frequency, the maximum permissible noise level governing a daily, or near daily, exposure would be quite low. Conversely, if the protection goal were to permit a certain amount of hearing loss in a small percentage of workers over a working lifetime, then the permissible exposure level would be raised accordingly. For example, Figure 10  formula whereby the ability to hear everyday speech was estimated by multi plying the hearing levels at 500, 1000, and 2000 Hz by 0.8 and then computing the average over these three frequencies. The major contribution of this formula was the introduction of the concept that hearing loss for speech could be estimated by the average hearing levels at what has now become known as the "speech frequencies"-500, 1000, and 2000 Hz.
The American Medical Association in 1947 recommended that hearing loss for speech be determined by the pure tone hearing losses at 500, 1000, 2000, and 4000 Hz. The four frequencies were given a weighting in accordance with what was presumed to be the importance of each frequency in hearing for speech (i.e., 15% at 500 Hz, 30% at 1000 Hz, 40% at 2000 Hz, and 15%
at 4000 H z ) . This guideline further suggested that hearing loss for speech does not begin until the weighted average hearing loss equaled 10 dB, and total loss for speech hearing occurred when the loss at 500 Hz reached 90 dB or the losses at the other 3 frequencies reached 95 dB.
In a later article which reviewed the assumptions In computing hearing 94 loss for speech, the AM A made the following observations and recommendations:
The 1947 formula was inadequate for calculating hearing loss for speech in sensorineural hearing loss. (This is particularly interesting in that the method used today for computing hearing loss for speech, developed by the AAOO in 1959 and accepted by the AMA in 1961, eliminated the most VI-9 sensitive indicator of sensorineural hearing loss (i.e., losses at 4000 Hz.))
(2) Everyday communication should be the basis for evaluation of hearing disability. "Ideally, hearing impairment should be evaluated in terms of ability to hear everyday speech under everyday conditions. The ability to hear sentences and to repeat them correctly in a quiet environment is taken as satisfactory evidence for correct hearing of everyday speech. Because of present limitation of speech audiometry, the hearing loss for speech is estimated from measurements made with a pure tone audiometer. For this estimate, the simple average of the hearing levels at the 3 frequencies 500, 1000, and 2000 cps is recommended.
V I -10 "In order to evaluate the hearing impairment, it must be recognized that the range of impairment is not nearly so wide as the audiometric range of human hearing. Audiometric zero, which is presumably the average normal threshold level is not the point at which Impairment 1. The basis of hearing impairment should be not only the ability to hear speech, but also the ability to understand speech.

# The ability to hear sentences and repeat them correctly in quiet
is not satisfactory evidence of adequate hearing for speech communication under everyday conditions. 2) above, the ability to understand speech under everyday conditions is best predicted on the basis of the hearing levels at VI-11 1000, 2000, and 3000 He.

# From (1) and (
The point at which the average of hearing losses in the stated three frequency range of 1000 -3000 Hz begins to have a detrimental effect oil the ability to understand speech is 25 dB re ANSI (1969).
With reference to the determination of hearing impairment (1. above), the dbility to "hear" speech, measured in terms of the lowest intensity at which a listener can barely identify speech materials, provides little information concerning communication difficulties under everyday con-QQ ditions. As Sataloff states, "It  implies the presence of obvious difficulties in hearing speech. Actually, the difficulty mofe often lies not so much in 'hearing' speech as in 'understanding' it." Furthermore, Davis and S i l v e r m a n^ observed that "... a man with severe high-tone nerve deafness (as is seen in occupational noise induced hearing loss), will always fall to hear certain sounds and will never make a perfect articulation score. On the other hand, the same man may hear some words, the easy low-frequency words, as well as anyone else does. He may have a normal threshold for speech."
This issue is further clarified if one compares the "typical" clinical picture of a person having a conductive hearing loss versus a person having A sensorineural hearing loss resulting from noise exposure. Both cases would be expected to have elevated speech reception thresholds (a measure of hearing for speech); however, in the case of the conductive loss, speech discrimination (measure of understanding) would be approximately the same as that iot a person having normal hearing, provided that the presentation level ift sufficiently above the speech reception threshold level. The person with occupational hearing loss (sensorineural), on the other hand, would have VI-12 relatively poor discrimination scores, and the effect of raising the pre sentation level to higher levels often serves to reduce the articulation 100 score (see example in Figure 11). In applying the AAOO-AMA formula to the cases shown in Figure 11 it is possible that both would be rated identically in terms of hearing impairment, yet the sensorineural case has much more difficulty in understanding speech than does the conductive case.
It is apparent, therefore, that the formula applied to compute hearing impairment should consider discrimination ability and that the pure tone frequencies used in the formula should be highly correlated with this latter function.
With reference to speech communication under everyday conditions (see
100Z. Furthermore, everyday speech rarely takes the form of complete sentence communications; thus, the number of speech cues available for 88 accurate speech perception under everyday conditions is greatly reduced.
From this discussion, it may be concluded that an appropriate predicting scheme for determination of hearing impairment must include some consideration VI-13 for an actual daily communication environment rather than some optimum condition as suggested by the AAOO -AMA.
With reference to predicting ability to understand speech on the basis of heavy levels at the pure tone average at 1000. 2000. and 3000 Hz (point 3 above), results of several studies indicate that hearing levels at these three frequencies predict hearing loss for speech under mild conditions of distortion better than the three frequency average at 500, 1000, and 2000. In chinchillas. However, they cautioned that their findings were only applicable to continous exposures and not to intermittent exposures.
Variables that are germane to Interrupted exposures but do not play a significant role in limiting hazard from short-term continuous exposures further complicate the problem of how the ear responds and integrates noise over time. One such variable is the "acoustic" or "middle ear" reflex.
When the ear is exposed to loud noise, the middle ear muscles contract, thus altering the impedance of the middle ear. This reflex, which serves to attenuate the noise reaching the inner ear, adapts out or disappears quickly if the noise is continuous and relatively unchanging over time.
However, if the noise level varies considerably or is interrupted on a regular or irregular basis, then the reflex is sustained.
A second variable which plays an important role in reducing the hazard of interrupted noises relative to short-term continuous noises concerns the off-time of the exposure cycles. Depending upon the over-all level of the noise and the nature of the relationship between on-time and off-time, a considerable reduction in the degree of temporary hearing threshold shift may be observed.
To date, the only empirical data available on permanent hearing losses resulting from intermittent exposures comes from a study of iron ore miners conducted by Sataloff et a l .^^ Their findings indicated that intermittent noises had to be some 15 dB more intense than continuous noises to cause the same additional hearing impairment in men ages 30 to 50 years. Although this 113 VI-18 evidence confirms Che general notion that intermittent exposures are less hazardous than continuous steady-state exposures of the same duration and noise level, the applicability of this rule to other schedules of intermittency must await further investigation.
Since 1960, several damage risk criteria have been proposed to limit 82,107,115 exposure to intermittent noise.
For the most part, these criteria, like the rules for assessing Intermittent noise exposure discussed below, have been based predominantly upon evidence collected from studies of temporary threshold shift.
At least three different rules have been proposed in order to assess the hazard of exposures to intermittent noise. The first of these rules, 28 developed by Ward et al.
was called the "on-fraction" rule. This rule states that the amount of temporary threshold shift resulting from a given intermittent exposure can be determined on the basis of noise level and average on-fractlon (the time the noise is on divided by the total duration of exposure). This procedure assumes that levels below 75 dB SPL are not hazardous to hearing; thus, the amount of on-time is taken as the total time the noise is above 75 dB SPL. In a critical test of the on-fraction rule, Selters and Ward^1 found that this rule was invalid when the regular on-off times exceeded two minutes.
For burst durations longer than two minutes, a second rule has been The essential feature of this approach is that residual TTS is translated into exposure time.
One of the crucial assumptions of the "exposure-equivalent" rule is that the course of recovery from TTS is Independent of the type of noise 33 that produce the TTS. In a recent article, Ward has presented data that question the validity of this assumption. It appears that intermittent exposure to high level, high frequency noise causes a considerable delay in the recovery of TTS relative to intermittent low frequency exposures.
A third approach in determining hazard from interrupted noise has been to determine the total on-time of the noise, regardless of how the noise bursts are distributed in time, and to consider the intermittent exposure in terms of an equivalent continuous exposure. This approach attempts to take into consideration the reduced hazard of interrupted noise by adjusting the rule which relates noise level and exposure duration.
Although possibly not as scientifically rigorous as the previously mentioned procedures, the "equivalent continuous" rule is not constrained by the assumption concerning the regularity of exposure cycles which is basic to the other rules.

# VI-20
Intermittent noise exposure criteria based upon the first and/or second rules include those developed by Glorig, Ward, and N i x o n , 113 CHABA 107 82 Working Group 46, and Botsford. Botsford's i n t e m i t t e n c y criteria reflect a simplification and consolidation of the CHABA continuous exposure, long-burst intermittent, and short-burst intermittent contours into one general figure relating dBA level, total on-time (noise level above 89 d B A ) , and number of exposure cycles (see Figure 13). The limits of intermittent exposure expressed in these contours (shown in Table X) 27 have recently been adopted by the Second Intersociety Committee. Similar limits have been adopted as part of a revision of the German document con-116 cerned with assessment of industrial noise in working areas.
Recent research designed to investigate the efficacy of the limits proposed In Table X have generally shown that the limits do not acburately predict risk to hearing, at least so far as temporary threshold shift is concerned. In a laboratory study11^ designed to evaluate selected exposure conditions from Table X it was found that (1) the table shows concen trations of noise exposure within an eight-hour workday than can cause excessive amounts of temporary threshold shift and (2) the conditions did not yield equal effects on hearing, thus not affording equal protection.
Conversely in a study of forestry employees It was found that although the noise exposures were rated as hazardous according to Table X, the audiometric results indicated that the exposures did not pose a risk to hearing.
Considerably more data must be collected to evaluate present criteria which attempt to designate safe levels of exposure to Intermittent noise. on the other hand, suggested that the "off" level was frequently dependent. For example, the safe level of exposure for the octave band 300 -600 Hz was seen to be 89 dB SPL, whereas It was approximately 85 dB SPL for octave band 1200 -2400 Hz.
Recently, Botsford^ computed a dBA equivalent from the octave band damage risk criteria developed by CHABA. The results of this computation suggested that the "off level" based upon one-third octave or octave band sound pressure level will, in many cases, be below the level designated by Botsford (particularly in the case of strong narrow band components in the noise). Both the CHABA and Botsford criteria do not appear to be in accord with the intended meaning of a safe intermittent level in that present data suggest that there is a significant increase in the proportion of the popu lation having hearing impairment in those groups exposed to continuous noise VI-22 levels at and slightly below 85 dBA as compared with a non-noise exposed population.
Two lines of evidence suggest that the lower limit of Interrupted exposure is considerably below the levels mentioned above. In a review of  XI.
The audiometric van used for the hearing tests was capable of testing six individuals at one time. All employees were tested before the beginning of their work shift, and, due to scheduling problems, the number of employees in a test session ranged from one to six. When less than six employees were present at a testing session, an attempt was made to randomize the assign ment of audiometers. It was also necessary to use headphones with otocups to properly shield the employees from the possible effects of interference caused from hearing the other test tones in the van. However, it was found from the results of two independent studies in the NIOSH laboratory that there was no significant difference in measured thresholds between headphones fitted with otocups and those fitted with standard MX-41/Ar type ear cushions.
Before data analysis could be done, it was necessary to check the calibration data accumulated during the respective survey. Calibration of the audiometeres used to take the audiograms was usually performed before and after each survey. The data were corrected where necessary to the appropriate values given in the American National Standard Specifications for Audiometers, ANSI S 3 .6 (1969).
Used for purposes of data analysis were the three-frequency averages mentioned above In the definitions of hearing impairment. HLI (0.5, 1, 2)
and HLI (1,2,3) are used to denote these averages performed over both ears. (HLI stands for "hearing level index.")

# VI-25
The samples were grouped Into age and experience ranges to assure equal numbers per cell and a consistent spread of the data across the various dBA levels.
The following lists the steps made in the data analysis:
1. Hearing level indices for 87 and 94 dBA noise exposed individuals were grouped into 31 samples for three-way cross-classification with respect to dBA level, age group, and experience group. The data were transformed by taking natural logarithms, and the resulting variances of log HLI (0.5, 1, 2)
and log HLI (1, 2, 3) were computed for each sample. For each of the two dBA levels, Bartlett's tests for homogeneity of variances were performed over all age and experience combinations. Separate tests were performed for HLI (0. 5, 1, 2) and HLI (1, 2, 3) average noise Indices. Of the four Bartlett's tests, three showed no suggestion of nonhomogeneity of variance, but the fourth was significant at the 0.05 prbbability level. However, only one atypical variance was found within the "nonhomogeneous" group, and this was believed to be caused by an improbable combination of purely random variations and not indicative of a real elevation of variability for the cell in question. Thus, the conclusions were that variability of log HLI (0.5, 1, 2) and log HLI (1, 2, 3) for replicate subjects was stable over all cells defined by the crosa-clasalfication.
2. Fifth-degrfee orthogonal polynomial regression curves were fitted to log HLI v s . dBA for each age and experience cell using data for all dBA levels. Significance tests for nullity of regression coefficients were performed. For most of the curves which exhibited any significant trend, a straight line fitted the data within the limits of unexplained variability.

# VI-26
In several cases, fourth or fifth degree coefficients showed significance, but examination of the plotted points revealed these to be artifacts due to clustering of the dBA levels for those plots, i.e., too few levels of the independent variable so that the polynomial tended to "fit the random errors."
3. Histograms of pooled deviations of log HLI values from the respective regression lines for H L I (0.5, 1, 2) and HLI (1,2,3) were constructed by fitting normal distribution curves. Chi-square goodness-of-fit tests were performed. The tests revealed that the log HLI deviations from the means were normally distributed over the full range of variability to a very significant degree of approximation as shown in Figures 14 and 15 Means were found to be zero, and pooled variances were calculated for use in later stages of the analysis.
4. Regression lines for different age groups within an experiencelevel were tested for parallelism, and in every case, the lines were found to be parallel within the limits of error in the slope estimates. Pooled slopes were calculated, and the intercepts were revised to reflect the small differences between the separate and pooled slopes. Families of parallel lines were plotted. Tests for coincidence of sets of parallel lines were then made by the method of covariance analysis. This revealed significant difference at the 0 .0 1 probability level in all cases.
5. Regression lines for different experience levels within an age group were not found to be parallel, and, for each age group, the intercepts were compared by means of Student's t-tests. The "intercepts" were defined as ordinates of the regression lines at a dBA of 79, which represented the control group exposed to less than 80 dBA. These regression lines were found to be significantly different families of nonparallel lines from common intercepts.
VI-27 6.
For each age and experience combination, the normal distribution of pooled variation in replicate subjects was distributed about the re gression line with its zero mean centered at the ordinate of the line.
This model was then used to calculate a predicted percentage of subjects whose hearing levels exceeded a "fence". Thus, such percentages could be tabulated as a function of dBA for each age and experience category.
Furthermore, risk values were then derived as the percentage difference between employees exposed to noise levels 80 dBA or greater and those exposed to less than 80 dBA (Table XII and XIII).
This analysis indicates that the 85 dBA-Slow noise limit for an eighthour day, in conjunction with the medical program prescribed in the standard, will improve the protection of the working population from hearing loss that could impair their abilities to understand everyday speech. The reliability of the analysis is evidenced by homogeneity of the variance and normality of the population distributions. In other words, the evaluation is repeatable and is representative of a random sample.  Thus, the Intersociety analysis differs from that of NIOSH in several characteristics: use of one ear only, nonseparation of experience groups, use of SIL in noise levels, and use of a dissimilar composite population.
Some of these characteristics tend to produce lower risk values and con siderably more uncertainty than the NIOSH analysis, as evidenced in Table VII-3.
Another study whose analysis determined risk is published In ISO 90 Reconmendation R1999 (1971).
This analysis differs from the NIOSH analysis VI-30 in three ways. The first is that only the right ear was used. The second is that no separation of age groups into work experience groups was done. The third is that no screening for otological abnormalities was done in the ISO study. On the other hand, the entire sample of data used in this analysis is homogeneous in that all members of the sample 126 were taken from one comprehensive examination.
The lack of otological screening has some effect on incidence of hearing impairment for both the noise exposed and the non-noise exposed groups, but, when risk is calculated by subtracting the two incidences, the effect is essentially cancelled. Thus

# OCTAVE BAN D SPL R E 2x10 N/M
75 150 300 600 1200 2400 4800 9600          The hearing levels of the heavy earth equipment operators were found to be significantly higher than the non noise exposed groups. The paper bag workers had higher hearing levels but not as high as the earth equipment operators. The airport ramp personnel, however, had the lowest hearing levels, probably due to the intermittency of their ex posures .

# APPROX. SCALE 8PIRAL SANGLiON

# INNER

# CONT I N U A T I O N (6) O F T A B L E IX