Exchange Rate: Ear vs. Dosimeter. William W. Clark Ph.D. NHCA Feb. 24, 2013

Transcription

1 Exchange Rate: Ear vs. Dosimeter William W. Clark Ph.D. NHCA Feb. 24, 2013

2 Introduction Predicting the hazard posed by exposure to excessive noise (occupational or otherwise) is a tricky business. Modern sound level meters more advanced But only can determine the characteristics of the sound Cannot measure the ear s response Response is the tricky part: Hazard refers to the biological processes that cause permanent hearing loss, and this is very complicated-- From a sound level perspective: some sounds are harmless and most likely will never cause a hearing loss (<75 dba) some can cause temporary changes in hearing (75-80 dba) some can produce permanent hearing loss via metabolic fatigue (above 80 for prolonged periods) some can damage the inner mechanically --above peak levels of 140 db (humans) or 120 db (chinchillas)-- even for very brief exposures

3 Hearing effects tricky to assess As a first pass to assessing hearing risk, we can teach the sound level meter to respond like the ear for these cases: Don t count anything measured below 80 dba; Adopt a rule based on what we know about hearing for integrating sounds between 80 and 140 dba; Include a separate assessment of sounds with peak levels above 140 db; Incorporate current knowledge to create a rule that is simple to employ and at the same time scientifically valid.

4 Exchange Rate Goal is to assess noise hazard when the daily exposure is not continuous- When noise levels fluctuate during an 8-hr work shift, or When daily exposures are for periods that differ from the traditional 8- hr workday Requisite is that noise induced hearing loss (NIHL) is invariant for a given exposure, regardless of the level or temporal distribution of the source(s)

5 Equal Energy Hypothesis (3 db) Asserts that NIHL accumulates on the integral of the energy in the exposure, i.e., the pressure-squared distribution x time- (independent of spectral or temporal properties) First proposed by Eldred, Gannon, and von Gierke (1955) on the basis of human TTS studies Stated simply: H= f(e) H= hearing loss (NIHL) E= acoustic energy (sum pressure squared distribution x time expressed as A-weighted equivalent level) f= arbitrary function In other words, the biological effect of excessive exposure to noise covaries with the acoustic energy delivered to the ear.

6 Applications of EEH The EEH has been invoked in predicting: The amount of NIHL that will accumulate with continued exposure over a working lifetime; For daily exposures to intermittent or fluctuating noise, exposure level that would produce the same effect as continuous noise over a work day (usually 8 hrs); From an equal energy perspective, an increase of 3 dba in the SPL of an exposure must be offset by a halving of the exposure duration to maintain equivalent exposure. Therefore, the EEH implies a time-intensity exchange rate (ER) of 3 db per doubling or halving of exposure time to produce the same effect.

7 Support for 3 db Exchange Rate Most experts recognize the 3 db rule as more logical. They argue that it is logical that if the sound level is doubled, then the allowable exposure time should be cut in half. It follows, then, that the allowable time should be halved for every 3 db(a) increase in sound level. This is precisely the case if the 3 db(a) exchange rate is used Source: Canadian Centre for Occupational Health

8 5 db exchange rate Adopted in the US as a regulation in 1969 under the Walsh-Healy Public Contracts Act of 1936; Promulgated into OSHA s Occupational Noise Standard in 1971; OSHA considered testimony from experts, empirical data published in peer-reviewed journals, and concluded that the 5 db exchange rate was the most appropriate choice for characterizing occupational noise exposures; Part of the rationale was that most daily exposures included breaks that allowed the ear to recover somewhat.

9 Mammalian hearing is not linear Physiological phenomena incompatible with the EEH Acoustic reflex Stapes axis of rotation at high levels Efferent mediated protection Cochlear toughening Protective effects of intermittence Nonlinear normative function (MET) Other sensory systems (skin)

10 Pascals 20 db SPL Dynamic Range of IHC

11 Pascals 20 db SPL Dynamic Range of IHC Gain from OHC Active Process

12 Pascals 20 db SPL -120 Acoustic Reflex, Contralateral Efferent Suppression Dynamic Range of IHC Gain from OHC Active Process

13 Chaba DRC report Kryter, Ward, Miller, Eldredge Botsford Chaba Working Group

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15 Kraak Kraak and colleagues have done much work, much published in German Methodologies include retrospective studies of PTS in workers, TTS (ITTS) studies, and animal studies Excellent review in English published in 1981 General conclusion of all the work well expressed in his introduction:

16 sound influences may be expressed in terms of dose quantities with respect to their hearing-damage effects. The dose, however, is not comprehended as an energy quantity as has been the practice up to now. Except for sound with very high peak levels, L>=135 db, the time integral over the magnitude of sound pressure is the appropriate dose quantity adequate to create the damaging effect. If we consider this weighting of the sound pressure with the exponent one in the equivalent continuous sound level, we have to choose a level exchange rate of q=6. (p.191)

17 Kraak s retrospective studies 78 groups of people, with exposures ranging from dba Exposure is considered as the sum of the noise effect (Bn) and the age effect (Ba) Both calculated using the 6-dB ER Results well described by linear fit:

18 Summary Graph from Kraak Axis is pressure

19 Kurtosis Recent studies have suggested another metric for evaluating noises that are complex (steady noise with impulsive peaks) and not Gaussian Kurtosis metric has been proposed as an additional value to be added to the Leq Original suggestion by Erdreich (NIOSH), and several papers published Most in animal models, all originate from NIOSH or Hamernik group

20 chinchilla Mechanical Metabolic man Mechanical Metabolic

21 Summary Predicting hazard from measures of occupational exposure is complicated and has been the subject of debate for 6 decades, and it s still debated, although most accept the 3 db ER. Jury is still out on kurtosis- makes sense, but may be better as an additional penalty for impulsive noise with peaks that cause mechanical damage (Aranda de Toro et al., 2011). And even though it seems the jury has already voted on the 3 db ER, Dr. Dobie and I decided to review the evidence, and he will discuss our findings.