Hearing Health Risk in a Population of Aircraft Carrier Flight Deck Personnel

Military Medicine, Jun 2004 by Rovig, Glen W, Bohnker, Bruce K, Page, John C

Permanent Threshold Shift (PTS) Study

The length of service-stratified FD group was modeled and departmental rosters were reviewed to select subjects for the ENG and ADMIN comparison groups, whose medical records were then reviewed for evidence of noise-induced PTS and instances where hearing thresholds at 1,000, 2,000, 3,000, or 4,000 Hz exceeded 20 dB on the most recent monitoring audiogram. Per DoD instruction, PTS was defined as a persistent deterioration of 15 dB in either ear at one or more of the frequencies 1,000, 2,000, 3,000, or 4,000 Hz or a 10-dB average deterioration at 2,000, 3,000, and 4,000 Hz in one or both ears. For purposes of the study, thresholds of 25 or greater among this relatively young population were considered abnormal. Table I provides a chart showing pertinent demographics for the three groups. Although 77 subjects were initially identified for each group, only 52 of the 77 ADMIN personnel had suitable comparison audiograms because they are not typically enrolled in the Hearing Conservation Program and therefore do not receive annual monitoring audiograms. Within-group data are reported for those with less than 4 years of service and those with 4 or more years of service, enabling a reasonable cohort of more than 30 subjects each.

TTS Study

The purpose of TTS testing was to qualitatively demonstrate overexposure (and underprotection) as opposed to quantitatively predicting future hearing loss. all testing was manually administered by the first author, a licensed and certified audiologist, using a calibrated portable audiometer. Earphone enclosures were added to improve sound isolation, because the one-man, double-walled audiometric test booth was marginally certifiable at 500 Hz per DoD criteria. Pre-exposure audiograms were administered concurrent with the fitting of dosimeters on the morning of each volunteer's participation day. Postexposure audiograms were administered upon the subject returning to Medical spaces immediately after being released from his/her watch station at the end of the duty day. For purposes of the study, TTS was identified whenever thresholds at two or more of the six test frequencies in each ear deteriorated by at least 10 dB or when at least three thresholds deteriorated by 5 dB (after subtracting the number of thresholds that improved). It is emphasized that this was not a controlled experiment where the postexposure interval could be carefully dictated. An approximation of the interval between the last protected exposure exceeding 85 dBA and post-testing was extracted from the dosimeter's data log.

Results

Dosimetry data

Table II summarizes dosimetry data for 72 FD subjects and 4 ENG, using an 85-dBA damage risk criteria with a 3-dB exchange rate. Figure 1 illustrates 1,087 minutes of dosimetry data for subject 10, an aircraft handler with a TWA of 116.3 dBA. The lower tracing reflects 1-minute equivalent sound levels in dBA. The upper tracing captures unweighted peak sound pressure levels, also in 1-minute sections. Note the recurrent peaks at the instrument ceiling of 150 dB. The data are compressed into 10-minute sections to capture the entire monitoring day.