The Exchange Rate and oise Induced Hearing Loss

Transcription

1 The Exchange Rate and oise Induced Hearing Loss lice H. Suter, Ph.D. uter & Associates, Portland OR resented at the NHCA Annual Conference t. Petersburg FL, Feb. 23, 2013

2 Exchange Rate Defined: Relationship between noise level and duration incorporated into a permissible exposure limit (PEL) Also called: Doubling rate Time-intensity tradeoff

3 Before 3 db or 5 db Early research measured TTS, PTS, and cell damage (mainly OHCs) Results: No single function fits all conditions Depends mainly on: Noise level - PEL Audiometric frequencies examined Type of noise (i.e. different types of

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5 temporal patterns made a difference This reflected in early DRC Glorig, 1961, precursor to ISO standard permitted # exposure cycles/day as a function of: noise level on-time

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7 CHABA 1966 Criteria for exposure to continuous, fluctuating, and intermittent noise Based mainly on TTS studies Issued curves for equally hazardous exposures Allowable exposure levels based on on-fractions

8 CHABA Criteria Postulates 1. TTS2 is a consistent measure of the ffects of a single day s exposure 2. All exposures that produce a given TS2 will be equally hazardous 3. NIPTS after many years of exposure ill be the same as TTS2 resulting from an -hour exposure.

9 Shifts Frequenc y Median 20% 10% 1000 Hz 10 db 20 db 30 db 2000 Hz 15 db 30 db 45 db 3000 Hz 20 db 40 db 60 db

10 CHABA Assumptions Faulty TTS2 relationship to PTS still murky Interruptions not evenly spaced and of equal duration in normal work environment Off-times not uniform Off-times usually not sufficiently quiet to recover from TTS (E.Q. about 65 dba) Allowable NIPTS excessive!

11 implifications leading to the 5 dba R and eventually to OSHA ore Octave assumptions: band curves converted to A weighting Interruptions of equal duration and spacing are distributed uniformly throughout the day Uniform off-times Continuous noise criteria generalized to other types of noise

12 3 db Equal Energy Rule Hypothesis: Equal amounts of sound energy will produce equal amounts of hearing loss, regardless of how the sound energy is distributed in time. Caveat - limit to an 8-hour day. Not the same as total energy

13 History US Air Force 1955 ISO 1971 (ISO 1999) EPA 1974 EU 1986 ISO 1990 ACGIH 1994 NIOSH 1998

14 Who uses which rule: Most nations have followed the ISO standard and use 85 dba and 3 db ER. 24 nations 85/3 8 nations 85/5 2 nations 90/5 In the U.S. 3 Fed. agencies 85/3 3 Fed. agencies 90/5

15 EPA s Levels Document Required by the Noise Control Act to identify safe levels of environmental noise for the entire population Data used: Baughn, 1973 (year published) Burns and Robinson, 1970 Passchier-Vermeer, 1968

16 EPA cont. Used 3-dBA ER in its Criteria and Levels Documents, data from occupational noise sources. Identified safe level - 65 dba for 24/hr. exposure or 70 dba for 8/hr. exposure to protect even the most sensitive frequency (4kHz) Because noise in general environment is highly intermittent, adjusted occupational data by adding 5 db to identified safe level.

17 Animal Studies Intermittent Noise Studied TTS, PTS, cell loss Some animal experiments supported 3 db Several showed benefits from intermittencies Relationship between cochlear damage, PTS, and TTS not consistent Laboratory conditions more conducive

18 Field Studies Most done 40 years ago - all have some weaknesses Continuous and varying noise: Burns & Robinson, 1970 Evans and Ming, 1982 Baughn, 1973 Intermittent noise: several from outdoor occupations found less hearing loss than predicted by ISO standard, e.g. forestry workers miners

19 Recent Developments Southampton Report, 1981, consensus that 3 db sufficient, but minority advocated for addition of 5 db to Leq when impulse noise present. Studies show importance of impulse noise and high-level transients combined with continuous and varying noise. Kurtosis metric. Zhao et al., 2010

20 Accelerated Presbycusis Kujawa and Liberman 2006, 2009 Lin et al., 2011 Campo et al., 2011 Seemingly innocuous exposures cause greater hearing loss later in life Shows need for more conservatism in assessing occupational hearing loss:

21 Conclusions Development of the 5-dB ER was based on faulty assumptions 3-dB ER has a long history of acceptance by agencies and professional groups in the U.S. 3-dB ER used by vast majority of nations P-V data set was not the only one used by ISO and EPA

22 Conclusions cont. Recent research shows noise with high-level transients more damaging Accelerated presbycusis All of which accentuates the need for conservatism and further supports the 3-dB ER over the 5- db ER.