the role of intermittence in pts

The role ofintermittence in PTS

W. Dixon Ward

Hearing Research Laboratory, University of Minnesota, 2630 University•4venue, SE, Minneapolis, Minnesota 55414

(Received 1 September 1990; revised 1 January 1991; accepted 18 February 1991 )

Exposure of chinchillas to noise that is continuous results in auditory damage that is a function of the total energy of the exposure, provided that a critical exposure is not exceeded. Breaking a continuous exposure into 45 exposure periods given once a day Monday through Friday for 9 weeks (an interrupted exposure) is shown to result in a slight reduction in damage, but breaking each of the 45 daily exposures into short noise bursts presented at regular intervals (interrupted and intermittent exposures) reduces the damage more significantly. The shorter the noise bursts, the greater will be the reduction in damage. Too few data are available to establish a principle that will predict correctly the amount of reduction afforded by a particular temporal pattern; while the "equal energy" principle predicts no reduction at all, the "mean level" principle derived from studies of temporary threshold shifts (e.g., a noise at 80 dB half the time and at 100 dB half the time has a mean level of 90 dB and will have the same effect as

a continuous 90-dB noise) predicts too much reduction.

PACS numbers: 43.66.Ed, 43.50.Pn, 43.50.Qp [WAY]

INTRODUCTION

Only seldom is a measurable hearing loss produced by a single continuous exposure to a known constant noise. More typical of real life are exposures that fluctuate in level and are broken by periods of effective quiet, times during which the noise level is so low that, even if it were sustained, it would neither produce TTS nor interfere with recovery from TTS.

An exposure that involves long periods of effective quiet (several hours) will be defined as being interrupted, one with short break (a few seconds to an hour) will be characterized as intermittent, and one in which the level varies but remains above effective quiet is simply time varying.

It is possible to establish limits for exposures that are continuous; time-varying; interrupted; intermittent; interrupted and intermittent; interrupted and time-varying; or interrupted, intermittent and time-varying; and, of course, for the single short event. These limits will depend on the criterion tolerable damage, the spectrum of the noise, and the duration of noise bursts and pauses as well as the sound level, duration, and perhaps crest factor.

In actual practice, however, most official limits usually consider interrupted exposures--specifically, continuous 8- h exposures to a noise of fixed level, 5 days per weekmas being the most basic, because there does exist enough empirical evidence on the permanent effects of such exposures to permit establishment of a valid relation between exposure and effect. This evidence was summarized by Passchier-Ver- meer (1968) and showed that, for the median worker exposed to a steady noise for 8 h/day: ( 1 ) no loss developed if the level were below 80 dBA; (2) a level of 85 dBA produced an average hearing loss of 10 dB at 3, 4, and 6 kHz after 10 years of daily exposure; (3) at 90 dBA, the losses were 15-20 dB at these frequencies; and (4) at higher levels, losses at all frequencies increased roughly linearly with level. Selection of a specific limit as being a tolerable daily exposure, for example 90 dBA, therefore implies acceptance of some de-

gree of hearing loss, and the question at that point relevant to limits for other patterns of exposure becomes: "What intermittent and/or time-varying exposures are equivalent in effect to the 8-h exposure at 90 dBA?"

Three main answers to this question have been proposed: ( 1 ) exposures of equal energy, (2) exposures that produce the same TTS, and (3) exposures of the same time- weighted average (TWA). The equal-energy theory, for- mally adopted in AFR 160-3 (1956) and by the Interna- tional Standards Organization (1981), is, of course, the most attractive because of its simplicity. If it were con- firmed, then the hazard associated with any daily exposure-time-varying, intermittent, impulsive or combination thereof, no matter how complicated---could be evaluated in terms of the simple time integral of the intensity, or of this quantity divided by time, the equivalent level, and the trading relation between intensity and time would be simple: Halving the exposure time and doubling the intensity (in- creasing the level by 3 dB) would keep the hazard constant.

The equal-TTS principle, by contrast, is complicated to apply, because the relation between intensity and time for constant TTS is not a linear one, even for continuous noise, and, in the case of intermittent noise, the specific burst duration and duty cycle are important. So the 1966 CHABA recommendation of exposure limits based on the equal-TTS principle (Kryter et al., 1966) has not found wide acceptance, although the fact that intermittent exposures produce less TTS than continuous ones has influenced some other

proposed limits. In particular, this evidence led to the adop- tion of the time-weighted average (TWA) by OSHA, in which the trading relation between level and time is 5 dB per halving (OSHA, 1983).

What constitutes exposures that are equally hazardous in terms of permanent hearing loss cannot be answered by further empirical studies of humans because industrial exposures that might be hazardous cannot be rigorously con- trolled even when they are not now specifically interdicted.

164 J. Acoust. Soc. Am. 90 (1), July 1991 0001-4966/91/070164-06500.80 ¸ 1991 Acoustical Society of America 164

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The few data involving time-varying exposures before hear- 9 60 ing protectors began to be required in the noisier industries

5o (Passchier-Vermeer, 1974) indicated a tendency for actual losses to be less than would be expected on the basis of the

o•4o equivalent level, suggesting that Leq overestimates the hazard of intermittent and time-varying noises.

• 3o

Since humans cannot be used to study the effect of temporal pattern on hearing loss, then animals must be utilized. A program of research was therefore initiated about a dozen years ago in which the growth of hearing loss and cochlear damage as a function of exposure severity are studied in the monauralized chinchilla, beginning with continuous and in- • o- terrupted exposures, in order to establish a baseline for as- sessing the result of making an exposure intermittent. A single 2-oct-wide noise (700-2800 Hz) was used throughout, in order to avoid complications associated with spectrum. It was hoped that any general relation between damage and exposure pattern would not be unique to a particular spectrum. Auditory thresholds were measured by standard shock-avoidance techniques (Nelson et al., 1976), and final physiological damage was assessed in the form of the number of destroyed hair cells.

I. CONTINUOUS EXPOSURES

It soon became apparent that, to a first approximation, single exposures (uninterrupted except for an occasional 20- to 30- min period during which auditory testing was per- formed) conformed to the total-energy principle: Equal total amounts of energy produced equal damage (Ward et al., 1981 ). Figure 1 shows the results of these continuous exposures as a function of that energy in J/m 2, the top panel indicating damage measured in terms of the median permanent threshold shift at 2 kHz (the frequency most affected by the noise) in groups of four animals, the bottom panel the median number of destroyed outer hair cells (DOHC) in the entire cochlea (the normal complement of OHC is 7500 or so). Different symbols are used for different exposure durations; the two main durations used were 15 days and 0.15 days (squares and circles, respectively). The SPL of the noise associated with a given datum is indicated by the adja- cent number.

Thus, for example, the damage caused by 150 days of exposure to 82 dB, 15 days of 92 dB, 1.5 days of 102 dB, and 0.15 days of 112 dB is about the same, although 0.015 days (22 min) of 120 dB clearly exceeded a critical exposure. Furthermore, the growth of damage with exposure is an ex- ponential function with a slope of about 1 dB/dB for PTS, and 0.5 log unit/dB for DOHC. The latter relation implies that damage in the form of DOHC increases as the square root of the energy and so for a particular exposure duration the damage is proportional to the pressure. Zero damage, either 0-dB PTS or 70-100 DOHC (the median number of DOHC in a non-noise-exposed young chinchilla) corre- sponds to an exposure of around 25-30 J/m 2, which happens to be 8 h of 90 dB. Ryan and Bone (1978) found zero effect for an equivalent exposure ( 1 h of 100 dB of a similar noise).

o

EXPOSURE DURATION (CUM.) 0120

•) Z2. MIN

201--- 0 220 MIN " Z• 2200 MIN (I.5 DAYS) Oil4 10 • [] 15 DAYS 0 •? 150 DAYS • 85

I I I • I I I I I I I 20 40 I00 2• 400 103 2 4 I• 2 4 I0

TOTAL CUMULATIVE EXPOSURE (J/M

FIG. 1. Relation between auditory damage and exposure severity for single uninterrupted exposures to 700- to 2800-Hz noise in the chinchilla. The top panel shows the median permanent threshold shift at 2 kHz in groups of four animals. The bottom panel indicates the median number of destroyed outer hair cells in the cochleas of seven or eight animals; the number beside each point indicates the SPL of that noise exposure.

II. INTERRUPTED EXPOSURES

With a baseline established, the next step was to examine the effect of breaking up a 15-day exposure at 92 dB and a 1.5-day exposure at 102 dB to 45 equal exposures (8 h for 92 dB, 0.8 h for 102 dB) given Monday through Friday for 9 weeks. These schedules gave the results shown as closed points at 2100 J/m 2 in Fig. 2: The median PTS and DOHC counts for both groups were reduced by about the same amount from the continuous-exposure values, implying that the two daily exposures were equally hazardous, as the equal-energy principle would predict. However, the fact that there was a reduction from the continuous-exposure values implied that interruptions of this magnitude did reduce the hazard somewhat; the reduction in damage could be inter- preted as being equivalent to a reduction in exposure from 2100 to about 600 J/m 2, implying a correction of around 5 dB. The reduction was statistically significantly different from zero at the 5% level of confidence, but just barely so. Certainly the reduction was not as great as expected; apparently the 16-h quiet periods provide very little recovery of processes associated with the development of permanent damage. Indeed, two subsequent interrupted exposures failed to support the proposition that any amelioration is provided by interruption. Although breaking a 1.5-day exposure at 110 dB into forty-five 48-min daily noise bursts (closed triangles at 13 000 J/m 2) reduced the PTS at 2 kHz

165 J. Acoust. Soc. Am., Vol. 90, No. 1, July 1991 W. Dixon Ward: Intermittence and noise 165


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FIG. 2. Effect of interruptions and intermittence on auditory damage from a given cumulative exposure to noise. The open symbols indicate the results for single uninterrupted exposures (as in Fig. 1 ). Closed symbols represent median results for interrupted exposures in which a given total exposure is broken into 45 daily continuous exposures given Monday through Friday over a period of 9 weeks. The half-closed symbols indicate the median damage when each of those 45 daily exposures is further broken into 40 noise bursts presented once every 12 min or into 10 noise bursts given every 48 min.

from 27 to 19 dB, the DOHC count was doubled. And an

exposure of forty-five 8-h periods of 105 dB (closed square at 41 700 J/m 2) gave the same PTS as the continuous 15-day exposure. At best, then, reduction of hazard of permanent damage provided by 16-h recovery periods (or even longer) is minimal, at least for the chinchilla.

III. INTERMITTENT INTERRUPTED EXPOSURES

However, the same is not true when each of the 45 daily exposures is broken into periods of alternating noise and quiet. A 48-min exposure was changed to 40 noise bursts of 1.2 min presented every 12 min. This exposure, involving an on- fraction R of 0.1, was given at levels of 102, 110, and 94 dB to three successive groups of animals, and in all cases the PTS was considerably reduced, compared to the average of the continuous and the interrupted results (small half-closed triangles in Fig. 2). However, the variability of the data does not permit a clear conclusion as to how the reduction can best be expressed quantitatively. Does the function relating damage to exposure energy simply shift to the right by a constant amount (the long-dashed line in Fig. 2)--i.e., is it parallel to the nonintermittent growth curve?--or is it simply shifted down by a constant fraction (the short-dashed

line), in which case it would be a function with a lower slope but the same intercept?

The PTS data imply that the former is the case, because the intermittent 94-dB exposure gave no PTS instead of the 10 dB or so predicted for a nonintermittent exposure of 330 J/m 2. However, the DOHC data do not support this alternative, as the 94-dB exposure apparently produced the same 300 DOHC as a nonintermittent exposure. One could therefore defend either of the following propositions: that this particular pattern of intermittence reduced the damage to about half to one-third of what it would have been for a

nonintermittent exposure, or that it reduced the capability of the exposure to produce damage by a factor of 10, in which case the effect could be characterized as a 10-dB correction.

Intermittence, at least regular intermittence, has two major parameters, burst duration and pause duration, with on-fraction R determined by the ratio of burst duration to the sum of burst and pause durations. In order to determine the relative importance of these parameters, two more groups of animals were run in which the daily exposure con- sisted often 4.8-min bursts at 110 dB, given in one case every 48 min for 8 h (R = 0.1 ) or in the other case every 12 min for 2 h (R = 0.4). When compared with the animals given forty 1.2-min exposures at this level, the results would show the relative effect of quadrupling the burst duration from 1.2 to 4.8 min while holding the on-fraction constant at 0.1 versus quadrupling the on-fraction from 0.1 to 0.4 but keeping the burst duration constant at 4.8 min.

The results in this case were straightforward although surprising: The 4.8-min burst conditions (4.8 min, R = 0.1; 4.8 min, R = 0.4) produced identical values of PTS and DOHC (the large half-closed triangles in Fig. 2) that were greater than those produced in the earlier 1.2-min, R = 0.1 group. Thus, in this case, the noise burst duration is the more important parameter, with pause duration playing a minor role, at least for pause durations of 7.2 min or more. These results were surprising because a principle that had been firmly established about intermittence was that on-fraction was indeed important in determining TTS and even PTS. An early study of the effects of intermittence on PTS (Miller et al., 1963) showed that, where a 2-h exposure to a fairly pink noise at 115 dB SPL produced a 61-dB PTS at 2 kHz in the cat, sixteen 7.5 min bursts produced only 22 dB of PTS when the bursts came once an hour (R = 1/8), 16 dB when the interval was 6 h (R = 1/48), and no PTS at all when a single burst was given each day (R = 0.005). A similar result was shown in the chinchilla for the 114-dB 220-min exposure that developed a median PTS of 28 dB at 2 kHz; when the exposure was expanded to 440 min of 30 s on, 30 s off (R = 0.5), the PTS dropped to 24 dB (not a significant change), but when the schedule was 30 s on, 4.5 min off (R = 0.1 ), the PTS fell to 12 dB. (A series of twenty-two 10- min exposures, given twice a week for 11 weeks, generated no PTS.) Apparently the relative role of burst and pause duration is not simple, but must be dependent on the specific absolute values of each.

One more exposure of 13 000 J/m 2 was also given to confirm the importance of burst duration. In this case the burst energy was the same as for the forty 1.2-min bursts at



dB, but came in 0.12-min bursts (7.2 s) at 120 dB so that the on-fraction was 0.01. No PTS resulted from this exposure (half-closed circle at 13 000 j/m2), despite the fact that one- tenth as much energy in a steady exposure at this level ( 120 dB for 22 min) had essentially wiped out the OHC and produced a PTS of 50 dB.

Figure 3 shows the PTS produced by all the 13 000-J/m 2 exposures as a function of burst duration, from 7.2 s to 1.5 days. At least there is finally something that looks lawful, even though the point at 7.2 s may not be completely appro- priate (the level in that case was 120 dB instead of 110 dB). The slope is about 6.5 dB of PTS per tenfold change in burst duration, although no doubt this figure will depend on the total energy.

In summary, then, a decade of exploratory studies has not yet given any unequivocal solution to the question of the role of intermittence in reducing hazard from noise. In the chinchilla, 16-to 24-h breaks in exposure have little effect on the eventual damage from a given cumulative energy, yet shorter pauses that occur during a day's exposure produce a marked reduction, with both burst duration and pause duration important in some complicated way. Nevertheless, it seems safe to say that the equal-energy theory of damage is far from the truth; whether the equal-TTS or equal-TWA theories are closer remains to be seen when more informa-

tion about the role of burst duration and pause duration has been obtained.

That the situation is complicated is not particularly surprising. It has been assumed all along that the effect of intermittence would depend on the interaction of at least two factors: the recovery from the fatigued state of the cochlea that occurs during effective quiet, and restoration of the con- tractile strength of the middle-ear muscles. Now time-dependent changes in the efficacy of the efferent protective system may have to be considered. In the present case, one must also take into consideration a unique feature of the chinchilla, namely, the fact that little recovery from even moderate values of TTS occurs during the first 4-8 h of quiet following a day's exposure (delayed recovery); if the TTS is over 20 dB, then the chinchilla will not recover normal sensi-

tivity by the time of the next day's exposure, and if an expo-

N 3:

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• I0

O. I I I0 I00 I000

BURST DURATION (MIN)

FIG. 3. Relation between median PTS at 2 kHz and noise burst duration for

all cumulative exposures of 13 000 J/m 2.

sure becomes hazardous when the TTS produced by one exposure has not recovered by the time the next begins, then the failure of long interruptions to reduce damage will be explained.

But if there is no rapid long-term recovery in the chinchilla, then why do the short interruptions have such a great ameliorative effect? Can it all be attributed to recovery of the protective reflex and the efferent system? The chinchilla's reflex operates for all noise levels employed in the present studies (Woodford et al., 1976), and no doubt is revived by periods of quiet just as in the human, although nobody has yet studied this in detail. But, if this is the case, then the ameliorative effect of the reflex renewal will be reflected in a

lower TTS at the end of a day's exposure. It seems worthwhile, therefore, to see if there is a consis-

tent relation between the TTS produced by the exposures in the present series of studies and the final PTS. In the case of all 1.5-day and longer single exposures and of course for all the 9-week interrupted and intermittent exposures, this TTS is an ATS (asymptotic threshold shift), as an asymptote is reached within the first few days of exposure and then either remains the same or sometimes actually decreases (the middle-ear-muscles get stronger?). Figure 4 shows the relation between this ATS and the final PTS for all exposures. The 9- week (interrupted, or interrupted and intermittent) data and the results of the 150-day exposure at 82 dB all fall along a line defined by PTS = ATS/2, except for the exposure to 7.2-s bursts at 120 dB, where no PTS occurred despite an ATS of 31 dB. Thus the lower ATSs of the intermittent exposures are reflected in lower PTS. However, the 15-day continuous exposures all lie below this line, and the single 220- min exposures are even lower (ATS was probably not reached in that case, though). It would appear that these results support Kraak's (1982) proposal that PTS is a function of the integral of TTS over time rather than being dependent on TTS or ATS per se.

At this point it would be desirable to compare the fore- going results with those of others, but nobody seems to have conducted relevant related research on either the chinchilla

N

• 20

n

IO

rio

0 I0 20 30 40 50 60 70 80 90

CTS (ATS) AT 2 KHZ (dB)

FIG. 4. Dependence of median permanent threshold shift on asymptotic combined threshold shift for single uninterrupted exposures (open symbols), interrupted exposures (closed symbols), and interrupted plus intermittent exposures (half-closed symbols).

1(•7 J. Acoust. Soc. Am., Vol. 90, No. 1, July 1991 W. Dixon Ward: Intermittence and noise 167


or any other animal. There have been numerous studies com- paring TTSs from various interrupted and even intermittent exposures, but few attempts have been made to determine the ameliorative effect on PTS of intermittence (or of inter-

ruption in animals who do not display the delayed recovery characteristic for all exposures). Dolan et al. (1976) compared 8-h continuous exposure to 8 X 1 h/day, but the exposures were at or above 114 dB SPL, and so were supracritical even when interrupted. Moody et al. ( 1976, 1978) used the workday pattern of interrupted exposure in monkeys, but made no comparison with continuous or intermittent exposures. Saunders et al. (1977) exposed chinchillas for 6 h/day for 9 days but at a level that produced no PTS, so the results are indeterminate. The most relevant study comes from the CID group, who have been conducting extensive experi- ments using a 500-Hz octave band of noise. They compared the effects of 36 days of exposure for 6 h/day at 95 dB with the result of 9 days of continuous exposure. Their results (Clark et al., 1987; Bohne et al., 1985 ) are in agreement with the present ones in that the 18-h interruptions produced only a small reduction, albeit a reliable one, in the PTS and hair cell destruction. However, the PTS from the continuous exposure was so small to begin with that one could not determine the effective reduction afforded by the interruptions. They also confirm the fact that delayed recovery is characteristic of all TTS in the chinchilla, and report a gradual (sometimes precipitous) drop in ATS after the first few days of exposure, one that is much greater than in the present chinchillas (which would be consistent with the development of increased protective reflex activity, as the reflex is much more effective in reducing transmission of 500-Hz energy than of the present 700- to 2800-Hz noise). A subsequent study using a 4-kHz octave band (Bohne et al., 1987) compared several forms of interruption involving 6-h exposures, but in all cases the interruption reduced damage to zero (as the level was only 86 dB, even continuous exposure gave little damage) so the reduction could not be quantified.

IV. VERY SHORT NOISE BURSTS: IMPACT NOISE

The most intermittent of exposures would be those involving impact noise, with bursts of a fraction of a second to a few seconds. A considerable amount of research has been

devoted to this topic, some even producing and measuring PTS instead of only TTS. Exposure to impact noise on a workday pattern was studied by Lim et al. (1982); they made no comparisons with steady noise, but did show that the AER loss was the same after one week as after 4 weeks of

exposure. Is it possible that in the chinchilla all of the damage from intermittent noise occurs in the first few days?

By and large, the evidence indicates that the hazard associated with industrial impact noise is predicted with reasonable accuracy by its equivalent levelmi.e., the intermittence has no particular ameliorative effect. Martin (1976) first proposed this equivalence, for drop-forge employees, and subsequent studies of these workers are in reasonable agreement (Kershaw et al., 1976; Kuzniarz et al., 1976; Tay- lor et al., 1984), although nobody seems to have actually shown that the predictive ability of Leq is superior to that of TWA.

Indeed, a few studies suggest that there should be some penalty for very short intense bursts of noise. Buck et al. (1980) showed that 1800 1 O-ms 146-dB noise bursts at 1 per 4 s produced considerably more damage in the guinea pig than a single 18-s exposure. Less convincing is the claim by Erlandsson et al. (1980) that impact noise produced more damage than pure tones of the same Leq, and the latest Swe- dish study on impact noise (Grenner et al., 1988) presents data that support the equal-energy principle. It is interesting that these investigators developed the following equation relating PTS from impact noise in the guinea pig to exposure energy E: PTS = -- 14.8 + 1.07 log E. According to this formula, PTS would be zero when log E is 13.8, i.e., when E is 24 J/m 2, just as for a single exposure for the chinchilla.

V. TIME-VARYING NOISES

The regularity of the pattern of intermittence may play some role in its ameliorative effect. For example, Erlandsson et al. (1987) found much less damage in the guinea pig from exposure to recorded shipyard noise than from exposure to a regularly repeated laboratory impact noise of the same Leq. And in general, real-life exposures to impact noise usually occur in a rather high continuous noise level, so that the exposure really should be considered time-varying instead of intermittent. There have been no attempts as yet, however, to determine the effect on PTS of systematically varying the level among several different values, in order to examine the possibility that the hazard of a time-varying noise is better predicted from its mean level than from its Le•, a relation known to be true for TTS. If an exposure alternates among 85, 95, and 105 dB, with equal time at each level, it will produce a TTS closer to that produced by a 95-dB steady noise (the mean level) than to that produced by a 100-dB noise (its equivalent level). Obviously, the reduction of damage observed in the present studies is in the direction implied by the mean-level principle if periods of quiet are assigned a value corresponding to the highest value of"effective quiet," say 80 dB. In such case, the exposure involving 4.8-min noise bursts every 12 min to 110 dB would have a mean level of 92 dB (60% of the time at 80 dB, 40% at 110)

although the Leq•2 h• is around 106 dB. The mean-level principle would therefore predict an effective reduction in effec- tiveness of 18 dB, while the equivalent level would predict only about 4 dB. The actual estimated reduction from Fig. 2 is about 10 dB, or about halfway between the two alternative possibilities.

If it is possible to venture any general conclusion about the state of affairs regarding the effect of intermittence on noise-induced hearing loss, it is typified by the previous in- conclusive comparison: Intermittence does reduce hazard, contrary to the equal-energy principle, but never as much as predicted by the equal-TTS principle.

ACKNOWLEDGMENT

This research was supported by NIH Grant NS 12125.

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the role of intermittence in pts

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