Some research suggests that young children may be relatively more susceptible to noise induced hearing loss (NIHL) than adults, and that the unique noise footprint associated with military jet aircraft is particularly damaging to hearing. This pilot study looked for evidence of NIHL in adults who have been exposed to military jet noise in early childhood, while living in Married Quarters on active RAF fast jet stations. Many Married Quarters lie within 70 dB(A) Leq contours, fewer in 83 dB(A) Leq contours. A cross-sectional pilot study was undertaken to examine the hypothesis that military jet noise exposure early in life is associated with raised hearing thresholds.
Keywords: jet noise, military, hearing loss, childhood, environmental exposure
|How to cite this article:|
Ludlow B, Sixsmith K. Long term effects of military jet aircraft noise exposure during childhood on hearing threshold levels. Noise Health 1999;2:33-9
| Introduction|| |
It has been suggested that children, as a vulnerable group, are more susceptible to hearing damage in their childhood. In addition, military jet aircraft noise has been cited as a probable cause of hearing damage, in particular by its unique footprint, potentially a more damaging type of noise than other transport noise sources.
Clearly, these contentions beg the question:
'Is jet noise that different from other noise in its effects?'
Obviously there may be differences in effect linked to rise time, duration and total energy - all subjects of further research. However, if jet noise per-se is particularly harmful, one would expect to be able to find evidence of 'damage' - threshold shift - in identified adults who were known to be exposed to jet noise in childhood.
Thus the hypothesis that 'exposure to jet noise causes threshold shift' was considered. It was assumed that a significant proportion of RAF recruits would be the children of RAF staff, hence lived in RAF quarters near airfields during their childhood, hence been exposed to jet noise for some time whilst growing up. We accepted that the exact jet noise exposure dose for each subject would be difficult to quantify, and that the sound footprint would be more akin to 'airport' noise than 'overflight' noise. Some overflights may have been experienced by the subjects for from time to time military aircrew practice overshoots and display sequences at their parent airbase. Notwithstanding, it was likely that the children would have lived within a 70dB(A) Laeq contour, some may have lived within an 83dB(A) contour and we had access to archived noise footprints to confirm that some military quarters fell within these noise contours.
A pilot study was undertaken to compare a group of recruits who were the progeny of RAF staff and had probable childhood exposure to RAF jet noise with a larger control group of recruits who were not so exposed. This paper reviews the preliminary results.
Other field studies (Green et al, 1982; Wu et al, 1995) which do not suffer from the influence of extraneous variables find no significant threshold shifts in children living with airport noise. Fisch (1981) notes that although his study of children living around the UK's Heathrow airport found no evidence of NIHL, 5% of children display low levels of sensory-neural hearing loss at any one time, of which only a tiny proportion is likely to be noise induced. Classic NIHL is characterized by a dip at 4 kHz; any other pattern of hearing loss is likely to be congenital. However, we believe this to be the first study to look at the effects of MILITARY aircraft noise closest to air bases.
| Method|| |
The participants were 153 Royal Air Force noncommissioned air and ground crew aged between 16 and 25 years. All had less than 12 month's service and were posted to general service or specialist training wings at the time of the study. Forty-eight were the children of RAF personnel and had lived in married quarters or other residential areas on or very near to stations where fast jet squadrons were based. These participants formed the jet noise exposed group. The remaining 105 participants acted as controls.
A further 8 jet noise exposed and 18 control participants were identified, but did not complete the full procedure or were deemed unsuitable. [Table - 1] gives reasons for their exclusion.
The selection of participants was not randomized. Instead, all recruit intakes between mid April and late June 1997 were canvassed for suitable jet noise exposed participants.
Materials and Procedure
n enlistment into the RAF, all noncommissioned entrants undertake general service training. At the recruit training squadron, a full air-conduction audiogram is recorded in the frequency range 0.5 8 kHz and is noted on the recruit's medical file. Audiograms are recorded using a bank of ten Graystad Microlab 10 audiometers using TDH-39 type earphones, operated by RAF medical officers. These records formed the basis for the current study.
Participants read and signed an information sheet explaining the rationale and procedure of this study, as required by the RAF Clinical Research Committee (1995). They then completed a questionnaire asking for details of the participant's age on RAF entry, gender, level of education, parental occupation, places of residence before age 16 years, auditory health and regular exposure to leisure or industrial noise.
| Results|| |
The exposed and control groups were equivalent in age, participation in noisy activities, levels of education's, duration of service with the RAF and reported hearing disorders see [Table - 2].
The age of participants in both groups had a mean of 20 years. The exposed group was comprised of proportionally more female recruits (29.2%) than the control groups (19.0%), although this difference was not statistically significant as measured by a Chi squared (Chi 2 ) test. There was little variation in educational standards, as the majority of participants in both groups were educated to a least GCSE or A-level standard (exposed group 93.8%, control group 91.4%). The proportions of participants within this educational range did vary between groups, with relatively more control group trainees declaring A-level standard. All but two participants held the entry rank of Aircraftman/woman (AC) at the time they were surveyed.
On all five measures of participation in noisy activities, the control group were marginally more likely to have reported regular exposure to leisure or industrial noise. This may reflect the relatively higher proportion of male recruits in the control group over the exposed group, especially with reference to industrial noise. All differences were statistically non-significant as measured by Chi  tests.
[Table - 3] shows the medians, means and standard deviations of the left-right average hearing threshold levels for each group. At all frequencies the medians show that the exposed group had lower, or equivalent hearing thresholds to the control group. This may reflect the slightly higher proportion of males and more participation in noisy activities in the control group over the exposed group. When the mean is taken as the measure of central tendency, the pattern is maintained except at 1 kHz where the exposed group mean is slightly lower.
Wilcoxon rank sum tests revealed no significant differences in audiometric test results between the two groups at frequencies 2-8 kHz. The medians for test frequencies implicated in noiseinduced hearing loss (generally, 3-6 kHz) showed that the two groups were very similar, the only difference being at 4 kHz where the control group displayed a threshold 5 dB higher than the exposed group.
[Table - 4] presents audiogram data by group and gender. From this it can be seen that the general patterns of hearing threshold shifts described above are preserved. Both male and female control group participants tend towards slightly raised mean hearing threshold levels at higher frequencies compared to exposed group participants.
Spearman's rank-order correlation coefficient revealed no significant positive relationship between military jet noise exposure and raised hearing threshold levels at 4 kHz. The number of years each person spent living on or near flying stations up to the ages of 5 and 16 years respectively, and the number of years spent in schools on or near stations, are not significantly correlated with raised hearing threshold levels see [Table - 5].
We were aware that the RAF recruits may represent a survivor population hence not be representative. Therefore we reviewed the records of all recent applicants who had been unsuccessful in their attempt to enlist due to an unacceptable audiogram at the Careers Office examination. (Screening audiograms are conducted as a matter of routine) When compared to a sample of recent intakes into the RAF, children of RAF/Services personnel (who may have been exposed to jet noise as a result of a parent's posting) were represented equally in both groups. This indicates that the main analysis has not been biased by the "healthy worker" effect.
| Discussion|| |
When the data from the Service personnel groups are compared, there is no indication that increased childhood exposure to military jet noise is associated with raised hearing thresholds. The control group displayed slightly raised hearing threshold levels in comparison to the exposed group at all frequencies, possibly due to the larger proportion of male participants, and greater involvement in noisy activities. They hypothesized noise-induced hearing loss in jet noise exposed children has not been demonstrated.
There is a long-running debate on the accuracy of the standard zeroes for audiometric testing determined by ISO 389. Robinson et al (1981) produced evidence that the change from the use of the NBS 9A coupler to IEC 318 coupler using TDH-39 earphones caused systematic errors in the standard to appear. When this equipment is used on otologically normal samples, the standard zeroes defined by ISO 389 may be in error by +2.5 dB at 0.5 kHz and + 4 dB at 6 kHz. Similar conclusions have been drawn by other researchers (Fearn and Hanson, 1983). The audiogram data used in this project was recorded at No 1 Recruit Training School at RAF Halton. At the school a bank of Graystad Microlab 10 audiometers test 10 recruits simultaneously. Calibration of the machines is carried out using an NBS type 9A coupler. Due to difficulties in testing at 0.5 and 6 kHz using the equipment at No 2 Recruit Training Squadron, it can be argued that raised thresholds at these frequencies are unlikely to indicate noise-induced hearing loss. This is supported by the high prevalence of a 6 kHz notch in audiometric data from young otologically normal people in the MRC's epidemiological work (Davis, 1995).
A potential confounder in the difference in hearing thresholds at 0.5 and 1 kHz between the groups has not been controlled for. Although the questionnaire asked for details of parental occupation, the probing of the question led personnel to indicate either that parents were serving with HM Forces, or were civilian. This has meant that comparison of socio-economic status based on the main breadwinner of the household has not been possible, so has been omitted from the results section. It may have been possible to argue that the difference was due to relatively lower socio-economic status and so standards of health care. It could also be argued that the RAF provides a better standard of health care than the National Health Service, meaning that fewer conductive hearing disorders go untreated.
Calculation of participants actual noise dose has not been attempted. During the planning stages of this work it was felt that calculation of a figure would lend a spurious accuracy to measures of noise exposure reported by participants. In place of personal dose calculation, the questionnaire asked about general interests and activities which may have contributed to significant leisure or industrial noise exposure. This may be seen as a potential shortcoming of this study; it remains that the exposed group described her has experienced military jet noise associated with activity around fast jet stations, in excess of that likely to have been experienced by the control group (see [Table - 1] for a summary of rejected participants).
This study cannot refute all of the claims of alleged noise-induced hearing loss in children. The RAF does not routinely carry out air-conduction audiometry at frequencies higher than 8 kHz, for we believe that there is no clinical significance to audiometric thresholds in this range and considerable evidence of the spread of the "normal" range of high frequency thresholds.
There is currently very little data on how much military jet noise that any given population are likely to experience during childhood. Similarly, there is scant agreement on how much exposure to leisure noise the average young adult has, and how much PTS is likely to have resulted (Davis et al, 1985). There are problems in assessing noise exposure retrospectively, especially in early childhood exposure which may not be adequately remembered or reported. Any future work would need to consider these issues to be of further value in answering the question this study had addressed.
| Conclusions|| |
Firstly, this study has found no evidence that RAF personnel who have lived on or near fast jet stations while very young display raised hearing threshold levels associated with noise-induced hearing loss.
Secondly, there is no indication that applicants to the RAF who may have lived on active flying stations during childhood are less likely to be successful than applicants without any jet noise exposure.
| References|| |
|1.||Davis, A. (1995) Hearing in adults: the prevalence and distribution of hearing impairment and reported hearing disability in the MRC Institute of Hearing Research's National Study of Hearing. London: Whurr |
|2.||A. C. Davis, H. M. Fortnum, R. R. A. Coles, M.P. Haggard and M.E.Lutman (1985) Damage to hearing arising from leisure noise: a review of the literature. Prepared for the Health and Safety Executive, by MRC Institute of Hearing Research. From "A review of the literature pertaining to socioacusis, with bibliography", R.E.Walford, 126 May 1984. An unpublished review commissioned by the MRC Institute of Hearing Research. |
|3.||Fearn, RW and Hanson, DR. (1983) Audiometric zero for air conduction using manual audiometry. Br. J. Audiol; 17: p 87-89. |
|4.||Fisch, L. (1981) Aircraft noise and hearing impairment in children. Br. J. Audiol.; 15: 231-240. |
|5.||Green, KB, Pasternack, BS and Shore, RE. (1982) Effects of aircraft noise on hearing ability of school-age children. Arch. Environ. Health; 37, 5: 284-289. |
|6.||RAF Clinical Research Committee. (1995) Medical, Dental and Nursing Research in the Royal Air Force: Guidance Notes for Investigators. File reference IHMT/1645/3/CRC, revised April 1995. |
|7.||Robinson, DW, Shipton, MS and Hinchcliffe, R. (1981) Audiometric zero for air conduction. Audiology; 20: 409431. |
|8.||Wu, T-N, Lai, JS, Shen, C-Y, Yu, T-s and Chang, P-Y. (1995) Aircraft noise, hearing ability, and annoyance. Arch. Environ. Health, 50, 6: 452-456. |
RAF Institute of Health, Buckinghamshire HP22 5PG
Source of Support: None, Conflict of Interest: None
[Table - 1], [Table - 2], [Table - 3], [Table - 4], [Table - 5]