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What is Considered a Standard Threshold Shift?

Richard Danielson, PhD

February 6, 2006

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Question

Do we have documentation of what is considered a significant shift in hearing? They're using 15dB or more in at least two different frequencies, but not sure if that is a good guideline or not.

Answer

Noise-exposed workers receive an annual hearing test for comparison to baseline hearing threshold levels that were obtained when the worker was first assigned to a noise-hazardous job. When these periodic tests reveal a meaningful change from baseline data, this change should trigger certain steps (e.g., checking the fit of hearing protectors, reinstructing the worker on risks of hearing loss from noise, etc.) to prevent further progression of hearing loss. Ideally, monitoring audiometry programs can identify those noise-exposed workers with measurable hearing threshold shift at the most noise-sensitive audiometric frequencies and not tag too many false positives. Your question, "what is considered a significant shift in hearing?", can be answered in terms of compliance with federal hearing conservation standards or in terms of "best practices" that encourage a "pro-active" approach to preventing noise-related hearing loss.

The current federal hearing conservation standards of the Occupational Health and Safety Administration (OSHA) have not changed since 1983. Using OSHA criteria, a reviewer of audiograms will identify an STS (or "Standard Threshold Shift") if the hearing threshold has changed (relative to the baseline audiogram) an average of 10dB or more at 2000, 3000, or 4000 Hz in either ear. Age corrections (listed as an appendix in the OSHA standard) may be used when determining STS, although they are not required. Another federal agency, the Mine Safety and Health Administration (MSHA), has also adopted the OSHA STS criteria.

More recently, the National Institute of Occupational Safety and Health (NIOSH) has proposed new criteria to identify noise-induced hearing loss (NIHL) earlier, thereby going beyond attempts to conserve hearing by instead focusing on preventing NIHL. (Similarly, safety and health personnel are gradually changing the names of their programs from hearing conservation programs to hearing loss prevention programs.) NIOSH has recommended that OSHA and MSHA revise their STS criteria to a definition of an increase of 15 dB or more at any frequency (500, 1000, 2000, 3000, 4000, or 6000 Hz) in either ear that is confirmed for the same ear and frequency by a second test within 30 days of the first test. Because the STS must be confirmed in a second hearing test, these criteria are referred to as the "15dB TWICE" method. Interestingly, the criteria which you mentioned (15dB or more in at least two different frequencies) seems similar to the 15dB TWICE method, but is clearly NOT the same as the NIOSH criteria. Moreover, if a 15dB shift occurred in two of the frequencies of interest in the OSHA criteria (i.e., 2000, 3000, or 4000 Hz), it would be mathematically impossible to have anything EXCEPT an STS. Even if the third frequency "improved" by -10dB, it would yield an average shift of 10dB. By the way, NIOSH does not support the use of age corrections.

The "15dB in two frequencies" criteria that you mention are not a common method of determining STS. In 1992 and 1996, Julia Royster conducted studies for NIOSH, in which she compared the performance of eight known criterion (none of which was 15dB in two frequencies):

  1. OSHA STS: in either ear, a change of 10 dB or more in the average of hearing thresholds at 2000, 3000, and 4000 Hz.

  2. OSHA STS TWICE: in either ear, a change of 10 dB or more in the average of hearing thresholds at 2000, 3000, and 4000 Hz is present on one annual audiogram and is persistent in the same ear on the next audiogram.

  3. American Academy of Otolaryngology-Head and Neck Surgery (AAO-HNS) SHIFT: in either ear, a change of 10 dB or more in the average of hearing thresholds at 500, 1000, and 2000 Hz, or 15 dB or more at 3000, 4000, and 6000 Hz.

  4. NIOSH SHIFT (1972 version): in either ear, a change of 10 dB or more at 500, 1000, 2000, or 3000 Hz, or 15 dB or more at 4000 or 6000 Hz.

  5. 15-dB SHIFT: in either ear, a change of 15 dB or more at any test frequency from 500 through 6000 Hz.

  6. 15-dB TWICE (current NIOSH version): in either ear, a change of 15 dB or more at any test frequency from 500 through 6000 Hz is present on one annual audiogram and is persistent at the same frequency in the same ear on the next audiogram.

  7. 15-dB TWICE 1B4 kHz: in either ear, a change of 15 dB or more at any test frequency from 1000 through 4000 Hz is present on one annual audiogram and is persistent at the same frequency in the same ear on the next audiogram.

  8. 10-dB AVG 3B4 kHz: in either ear, a change of 10 dB or more in the average of hearing thresholds at 3000 and 4000 Hz.
None of the criterion evaluated were best in every respect (e.g., tagging workers early, giving high percentage of true positives, tagging a moderate percentile of workers so follow-up is practical, not considering low frequencies that are unlikely affected by noise exposure, etc.). However, NIOSH selected the 15dB TWICE approach because it identifies a high number of workers and provides a warning of noise-induced shifts at 6000 Hz, a noise-susceptible test frequency. While most occupational health programs use the OSHA STS criteria, NIOSH's STS criteria is being closely examined for possible application in industrial and military programs.

No matter what technique is used, our overall objective should be to prevent unnecessary hearing loss. As the American Academy of Audiology stated in its 2002 Position Statement, "No one needs to lose his or her hearing in order to earn a living. Noise-induced hearing loss is preventable." Good luck in your efforts.

Addendum:


A reader has asked for a clarification between the OSHA's basic definition of STS, described in OSHA's Hearing Conservation Amendment [29 CFR 1910.95 (1983)] and OSHA's guidelines for a recordable STS [29 CFR 1904.10 (2002)].

I'm happy to elaborate on how a basic STS is different from a recordable STS. OSHA's basic definition of STS (as I described in my initial response) is unchanged since 1983's description, although the requirements for reporting an STS to OSHA were updated, effective January 2003. As shown below, if an individual had an STS, but did not have a hearing loss (defined, in this case, as a pure tone average at 2,3,4 kHz of 25 dB or more), OSHA would not require you to report this shift. This figure may help clarify the OSHA reporting rule:





Reconfirmation of STS: If the annual audiogram shows a STS, a hearing retest may be performed within 30 days. If the retest does not confirm the STS, then the case need not be recorded. However, if the retest confirms the STS and the hearing loss is work-related, it must be recorded within 7 calendar days of retest. If a retest is not performed, then the case (again, if work-related) must be recorded within 37 days of test.

Results of subsequent testing: If later testing is performed as part of the hearing conservation program, and indicates that the STS is not persistent, then the employer may erase or line-out the recorded entry.

Determination of work-relatedness: Work-relatedness must be determined according to specifications of 29 CFR 1904.5 of OSHA's Recording and Reporting Occupational Injuries and Illness standard. If an event/exposure in the workplace caused, or contributed to, the shift in hearing or "significantly aggravated" a previously existing hearing loss, then the STS is recordable.

See links at http://www.caohc.org/current_events/oshafinal.php for more information


Dr. Richard Danielson is the Manager for Audiology and Hearing Conservation for NASA-Johnson Space Center, Houston, TX. He works for the National Space Biomedical Research Institute and Baylor College of Medicine. He is a retired U.S. Army audiologist, who served as a consultant to the Army Hearing Conservation program and director of the Army Audiology and Speech Center, Walter Reed Army Medical Center. He can be contacted at richard.danielson@jsc.nasa.gov.


Richard Danielson, PhD


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