Level of noise-induced hearing loss and associated factors among workers in Bishoftu central air base of Ethiopia

doi:10.21203/rs.3.rs-3010524/v1

Background

Excessive occupational exposure to noise results in occupational hearing loss was considered a major public health problem. Since the expansion of the aviation industry and its exposure to the intensity of noise level was simultaneously high.

Objective

This study aims to assess the level of noise-induced hearing loss and associated factors among workers in the Bishoftu central air base in Ethiopia.

Methods

Institutional-based cross-sectional study was conducted among 260 central air base workers through face-to-face interviews; an environment noise survey with an audiometric test had been conducted to collect data. Data were entered by Epidata version 3.1 and SPSS was used to analyze the data. Finally, a statistical analysis such as descriptive statistics and binary logistic regression analysis was applied. A P-value < 0.05 at 95%CI was considered statistically significant.

Result

The overall prevalence of noise-induced hearing loss and hearing impairments were 24.6 and 30.9%, respectively. From this, 16.9% bilateral and 7.7% unilateral noise-induced hearing loss. The highest prevalence noise level was recorded for workers who were exposed to noise levels greater than 90 dBA. Around 21.9% were males with an age category between 18 to 27 years were exposed to noise-induced hearing loss. Similarly, the lowest experience was between 1 to 10 years of exposure to noise-induced hearing loss by 16.9% with working department transport helicopter (4.7%), transport airplane (4.7%), and daily maintenance (3.5%). Those participants who were exposed to noise-induced levels at work place previously were five times (AOR = 5.0, 95% CI: 1.74–14.36) more likely exposed to the sound noise level at the workplace than those workers not exposed at work place previously. Those workers who were exposed to greater or equal to 90dBA noise level were 4.98 times (AOR = 4.98, 95% CI: 2.59–9.58) more likely exposed to noise-induced levels than those who were exposed to less than 90dBA noise level. Moreover, male air base workers were 3.5 times more likely exposed to hearing impairment than female workers (AOR = 3.5, 95%CI 1.01–12.0) and participants who have abnormal otologic examinations were 1.9 times more likely exposed to hearing impairment than those who have normal otologic examinations (AOR = 1.9, 95%CI 1.02–3.54).

Conclusion

Workers were exposed to high levels of occupational noise that could induce immediate hearing loss. The prevalence of hearing loss was significantly high.

Noise

Noise-induced hearing loss

Hearing conservation program

Noise level

An occupational environment is a workplace where risk factors such as biological, physical, chemical, and ergonomic factors exist that can predispose individuals to develop diseases. Noise is one of the physical hazards that cause hearing loss and other health problems¹. Noise is an excessive or unwanted sound that potentially results in annoyance and/or hearing loss whereas, the sound is a pressure variation (wave) of pure tone that travels through air and is detected by the human ear in a pleasant manner². Acoustic trauma that is related to exposure to explosive sounds can cause immediate permanent hearing loss^3,4.

For the past 10 years, the evidence showed that aircraft noise exposure leads to increased risk for poorer cardiovascular health has increased. A recent review suggested that the risk for cardiovascular outcomes such as hypertension, heart attack, and stroke, increases by 7 to 17% for a 10dB increase in aircraft or road traffic noise exposure⁵. And another review for children revealed that there were associations between aircraft noise and high blood pressure⁶, which may have similar implications for adult health⁷.

Different aircraft types have different noise levels and frequencies. Generally, noise from the aviation industry originates from three main sources: aerodynamic noise, engine/mechanical noise, and noise from aircraft systems. Noise can also be produced by aircraft equipment, transmission systems, propellers, rotors, hydraulic and electric actuators, air conditioning and cabin pressurization systems, alert systems, and communications equipment ^8,9.

Noise-induced hearing loss (NIHL) is a major problem among personnel serving in the armed forces as their profession exposes them routinely to extreme noise levels that affect their hearing. The soldiers, airmen, and sailors are exposed to high levels of noise produced by aircraft., ships, heavy mechanical transports, and the weaponry they use^10,11. High-level impulsive sounds from small caliber weapons and blasts specific to military environments are particularly damaging to hearing. Peak levels may be as high as 185 dBA¹².

Several studies have reported that the predictive factors related to NIHL were being male sex, higher age, duration of exposure or service year, noise intensity level, exposure to an explosion, vibration, smoking, ceruminous occlusion of the external auditory canal, and utilization of hearing protective devices as preventive factors^11,13,14.

A few countries have published prevalence studies of noise-induced hearing loss in military aviation industries. The two studies conducted in the Indian air force among 1000 personnel and analysis of audiometric notch revealed that an overall prevalence of NIHL was 22.9% with a higher prevalence among the technical trades at 26.18% and 57.3% and 34.3% at 6 and 4kHz respectively^10,13, while, in maintenance crew of institute of aviation medicine of Royal Malaysian air force, 41.2% had NIHL¹⁵.

A similar study conducted by the Israel air force showed that the prevalence of NIHL was 57.5% among those aged 50 years¹⁶. In other studies conducted in international airports like Malaysian Airport, the prevalence of NIHL was 33.5%¹⁷, Saudi Arabia King Kahilid international airport 33.5%¹⁸, and Jomo Kenyatta international airport 16%¹⁹.

In Ethiopia, there is no study conducted in aviation industries but there was a study conducted in the textile and metal industry with the prevalence rate for the NIHL was 34 and 22%, respectively^14,20. Although aviation workers in Ethiopia are exposed to noise every day the level of noise exposure and its health effects are not known so understanding the noise level, prevalence, and factors related to noise-induced hearing loss is essential for effective noise prevention and control strategy. Therefore, this study aims to assess the level of noise-induced hearing loss and associated factors among workers in the Bishoftu central air base in Ethiopia

2.1. Study setting

The institution-based cross-sectional study design was conducted at the Ethiopian central air force base which is found in Bishoftu town, Ada’a district, Oromia regional state between June to July 2018. The study area was 50km away from the capital city, Addis Ababa and it was established in 1966 E.C.

2.2. Study design

A cross-sectional study was conducted that included 260 central air force base workers from June to July 2018 in central Ethiopia. Following the verbal agreement, the air force workers were interviewed face-to-face using a standardized pretested questionnaire.

2.3 Sample size determination

The sample size was determined using single population proportion formula considering the prevalence of NIHL was 22% among Akaki basic metal industry workers¹⁴. A 95% confidence interval and a 5% margin of error were used to calculate the sample size. The sample size was determined using single population proportion formula:

\(\frac{{(Z\alpha /2)}^{2} \left(p\right)\left(q\right)}{{\left(SE\right)}^{2}}\) = \(\frac{{\left(1.96\right)}^{2}\left( 0.22\right)\left(0.78\right)}{{\left(0.05\right)}^{2}}\) = 264, by considering a 10% non-response rate, the total sample size was found to be 290.

Where:

n = Sample size

P = prevalence of NIHL in Ethiopia, Akaki Basic metal industry 22%¹⁴.

d² = marginal error = 5%

Zα/2: A standard Z score of 1.96 corresponds to a 95% confidence level.

The total number of workers found in the central air base was estimated to be around three thousand. Using the reduction formula; nf = n/(1 + n/N), the final sample size was calculated to be 264.

2.4. Sampling techniques

A total of 264 workers were needed. The 264 workers were allocated proportionally to the size of the nine departments. Then, workers were selected by a simple random sampling method, using the workers’ rosters as a sampling frame from each department. But finally, only 260 participated in the study because four with a history of ruptured tympanic membranes were excluded according to the exclusion criteria.

2.5. Data collection

Basic socio-demographic and work-related data were collected using a pretested structured questionnaire. Otologic examination and other physical examinations (body mass index, blood pressure, and other vital signs) were performed by the clinician. The hearing level was determined by pure tone audiometric measurement using a digital recording audiometer known as Digital Diagnostic Audiometer AC50-D in the audiometer room of the surgeon clinic. The testing frequencies were at 250, 500, 1000, 2000, 3000, 4000, and 8000Hz.

The threshold values were recorded in 5dB increments at every frequency by putting the red color phone on the right ear and the blue on the left ear. Early or moderately advanced NIHL usually results in the typical ‘boilermakers’ notch at 4 kHz, with spread to the neighboring frequencies of 3 kHz and 6 kHz ²¹

(20) and some hearing recovery at 8 kHz²². The fact that frequencies around 4 kHz are most affected by noise is most likely due to the resonance frequency of the outer ear/ear canal as well as the mechanical properties of the middle ear²³. But the cut-off point for reference we used was 4000 and 6000 Hz ^13,22. For hearing impairment, “at least 25 dB (A) in the better hearing ear (average over the frequencies 0.5, 1, 2 and 4 kHz)”. In grading of hearing impairment; those who hear less or equal to 25 dB A considered normal, 26–40 dB A slight impairment, 41–60 dB A moderate impairment, 61–80 dB A severe impairment, and above 80 dB A as profound impairment²².

The noise level was measured in workstations corresponding to each sampled worker's position by locating the microphone in the worker’s hearing zone (2 feet wide sphere) using a digital sound level meter model 840029 Taiwan²⁴. The equipment meets OSHA requirements which meet IEC 61672 class 2 and ANSIS 1.4 type 2 frequency and time weighting specifications, DIN 45633, and JIS 1502 whose frequency weighting selectors A and C which were conformity to standards. It has also time weighting (SLOW and FAST) which are dynamic characteristic modes. In this study, the FAST was selected to capture the peak noise level which may be occurred intermittently. The sound level meter was calibrated at 94 dBA before and after each measurement was conducted. Since the characteristic A weighting is simulated at the Human ear response, it is recommended to be used for environmental noise level measurements²⁵. During measurement maximum effort was made to keep the microphone dry, avoid vibrations during measurement, and carry out measurements in steady temperature and humidity as much as possible as recommended²⁶.

2.6. Data quality

The audiometer was calibrated at the outset of the study and recalibrated regularly using biological standards. Biological standards are healthy individuals on which the instrument is calibrated under the same environmental conditions or obtained by testing the hearing sensitivity of young, healthy adults and averaging the sound level at specific frequencies at which the tones were barely perceptible²⁰. All audiometric tests were carried out in a quiet room within the surgeon clinic designed for audiometric tests before the interviewees entered their work shift to avoid the effects of temporary threshold shifts

2.7. Data analysis

Data was entered into Epi-Data version 3.1 and analyzed using SPSS statistical package, version 21. Descriptive statistics were computed to display mean frequency and percentage. Bivariate and Multivariate logistic regression analyses were performed to identify factors associated with NIHL. The associations were described using an odds ratio with a 95% confidence interval. The logistic regression model was run to examine whether there was an association between the potential explanatory variables and outcome variables. The cutoff for statistical significance was p-value ≤ 0. 05. Moreover, crude and adjusted odds ratio with their corresponding 95% CI was used to measure the strength of associations between the independent and the outcome variables. For the Environmental measurement, the logarithmic results were analyzed and compared with the standards²⁵.

3.1. Socio-demographic characteristics of respondents

Out of A total of 264 samples needed 260 were included for the assessment of NIHL and 262 for general assessment of hearing impairment in the study. Two of the workers with tympanic rupture or perforated eardrums differentiated with otoscopic physical examination were excluded from the study of NIHL but included in the assessment of general hearing impairment with a 98.5% of response rate. The mean age of workers was 30 ± 7 years. The majority of age categories were between 18–27 (47.7%) years with service of 1–10 (73.8%) years. Most of the participants of the workers were males (82.0%) and single 161 (61.9%). Most of the educational level study participants were Technical/ college diploma 147 (56.5%) followed by degree and above 80 (30.8%). Of those participants, 254 (97.7%) were military personnel with the job category of maintenance technicians115 (44.2%), flight and ground technicians 73(28.1%)(Table 1).

Table 1: Socio-demographic characteristics of respondents of Central Air base workers of Bishoftu, Ethiopia

Characteristics		F	P (%)
Sex	Male	226	86.9
Sex	Female	34	13.1
Age in years	18-27	124	47.7
	28-37	91	35
	38-47	36	13.8
	> 47	9	3.5
Service in years	1-10	192	73.8
	11-20	50	19.2
	>20	18	7
Religion	Orthodox	184	70.8
	Muslim	14	5.4
	Protestant	58	22.2
	Catholic	3	1.2
	Other	1	0.4
Marital status	Single	161	61.9
Marital status	Married	99	38.1
Educational status	Write and read	1	0.4
	Primary school 1-8	7	2.7
	Secondary 9-12	25	9.6
	Technical/ diploma	147	56.5
	Degree and above	80	30.8
Category of the participants	Civil	6	2.3
Category of the participants	Military	254	97.7
Working Department	Security	23	8.9
	Cisinia	12	4.6
	Su-27 jet	35	13.5
	Transport helicopter	29	11.2
	Transport airplane	18	6.9
	L-39 jet	19	7.3
	Logistics	10	3.8
	Garage	25	9.6
	Daily maintenance (DM)	89	34.2
	Security	26	10
Job category	Pilot	8	3.1
	Mechanic technicians	115	44.2
	Electronics and communication maintenance	17	6.5
	Flight and ground technician	73	28.2
	Navigation crew	7	2.7
	Armament	5	1.9
	Driver	5	1.9
	Welder	4	1.5

Abbreviations: F- frequency; P- percentage

3.2. Personal and health-related history of the workers

The study revealed that 19 (6.2%) workers were previously exposed to noisy work. Additionally, in the central air base, those who were involved in grinding or welding metal and military service were exposed to noise levels by 14. 2, and 94.6%, respectively. Regarding audiometric hearing tests only 57(21.9%) had hearing tests before the present job and 109 (41.9%) had pre-employment hearing tests when they were hired by the institution. In this assessment, the previous experience of disease conditions of workers was assessed which showed as 22 (8.5%) ever had ear discharge or infection, 37 (14.2%) had a cold in the last fortnight before a test, 14 (5.4%) ever had a head injury. Whereas 42 (16.2%) do have ear problems, 37 (14.2%) do feel hearing problems, and 45 (17.3%) have hearing differences between the left and right ears during a test. Regarding tinnitus, 39 (15%) have ringing or trouble noise in their ear or head, and hypertension 10 (3.8%)( Table 2).

Table 2: Personal and health-related history of the workers of central air force workers of Bishoftu, Central Ethiopia

Characteristics of the variables		F	%
Previous work in another noisy area	Yes	19	7.3
Previous work in another noisy area	No	241	92.7
Ever been grinding or welding metal	Yes	37	14.2
Ever been grinding or welding metal	No	223	85.8
Ever been in military service	Yes	246	94.6
Ever been in military service	No	14	5.4
If the military is ever exposed to a gunshot or explosion in battle war	Yes	46	18
	No	200	82
Ever been trained /given service as a pilot	Yes	10	3.8
Ever been trained /given service as a pilot	No	250	96.2
If yes type of plane	L – 39	1	10
	Transport helicopter	6	60
	Su – 27	3	30
Had a pre-employment hearing test	Yes	109	41.9
Had a pre-employment hearing test	No	151	58.1
history of hearing loss before this job	Yes	6	2.3
history of hearing loss before this job	No	254	97.7
Ever had ear discharge or trauma or infection	Yes	22	8.5
Ever had ear discharge or trauma or infection	No	238	91.5
Ever had a head injury	Yes	14	5.4
Ever had a head injury	No	246	94.6
Had a broken ear drum identified by a health professional (self-report)	Yes	5	1.9
	No	255	98.1
Had a cold in the last fortnight (last two weeks)	Yes	37	14.2
Had a cold in the last fortnight (last two weeks)	No	223	85.8
Have an ear problem now	Yes	42	16.2
Have an ear problem now	No	218	83.8
Hearing problem now	Yes	37	14.2
Hearing problem now	No	223	85.8
Otitis	Yes	8	3.1
Otitis	No	252	96.9
Filling of hearing difference b/n two ears	Yes	45	17.3
Filling of hearing difference b/n two ears	No	215	82.7
Filling of tinnitus in the ear or head	Yes	39	15
Filling of tinnitus in the ear or head	No	221	85
A family lost hearing before the age of 50 years	Yes	6	2.3
A family lost hearing before the age of 50 years	No	254	97.7
Hypertension identified by a health professional	Yes	10	3.8
Hypertension identified by a health professional	No	250	96.2
Diabetic identified by a clinician	Yes	2	0.8
Diabetic identified by a clinician	No	258	99.2
Vibration in your section	Yes	139	53.5
Vibration in your section	No	121	46.5
Body mass index	Underweight (below 18.5)	13	5
	Normal weight (18.5 – 24.9)	186	71.8
	Overweight (25 – 29.9)	59	22.8
	Class I obesity (30 – 34.9)	1	0.4

3.3. Use of personal protective devices, hearing loss perception, and training

About 33.2% of the participants were using personal hearing protective devices, of those 62.79% used ear plugs, and 37.21% ear muffs. In terms of frequency of using hearing protective devices, only 15.9% used them always, and 84.9% used them sometimes. Out of 260 participants, 86.2% mentioned hearing protective devices were not available, 6.9% didn`t know about HPD`s purpose and 3.5% of them perceive as HPD not beneficial. In the case of safety training or education, 49.8% of the participants didn’t get safety training with different duration. Regarding noise`s health effect, 96.2% of the workers didn`t get training on the effect of noise on their health at their workplace. Additionally, 17.3% of the participants used personal protective equipment and 3.5 % didn’t know the use of PPEs. Furthermore, about the law or rule that obligates the workers as they used personal protective equipment only 17.3% told as a rule was available at their institution but 3.5% of the participants didn`t know and 79.2% of them believe that some rules and regulations enforce the workers.

3.4. Environmental noise measurements

The noise level of different planes and departments at personal exposure was measured at the maximum idle running of motors and maximum running of different jets, and plane motors at the place of pilot exposure, and workers exposure. The study revealed that the sound level of transport airplane L-100 emitted at idle was 123 dBA and at maximum power emitted was 128 dBA at the ground technician's site of exposure but at the site of pilot exposure was 76 dBA at ground running of four engines.

The other transport helicopter M-17 sound level measurement revealed a maximum level of 109 dBA externally but 102 dBA passenger`s seat and at idle engine run the maxim sound level was 99 dBA, at the position of a pilot the maximum sound level captured was 96 dBA. Moreover, the Su-27 jet with two engines emitted 110 dBA at idle state and 128 dBA at maximum engine run-up but it was difficult to measure the internal part at the pilot site due to high production of high pressure and sucks the surrounding air once it started to run up the engine. Additionally, it was forbidden to reach nearby the jet, and the other detail sound level of the plane was measured (Table 3).

Table 3: The maximum sound level measured by dBA at idle and maximum engine run-up of the planes and jets of the central air force of Bishoftu, Ethiopia

Type of plane	Max. sound level at idle engine run-up	Max sound level at max engine run-up	Pilot`s exposure at max engine
Transport Airplane	123	128	76
L-39 Jet	105	115	98
Cesena	81	110	90
Transport Helicopter	99	109	96
Su-27 Jet	110	128	-

NB: The measurement was taken at the position of the exposure of the workers

3.5. Prevalence of Noise-induced hearing loss (NIHL) and hearing impairments

This institutional-based study attempted to assess the prevalence, the magnitude of noise emitted, and determinant factors of noise-induced hearing loss among central air base workers. The overall prevalence of noise-induced hearing loss was 24.6%. About 16.9% bilateral and 7.7% unilateral noise-induced hearing loss. The highest prevalence noise level was recorded for workers who were exposed to noise levels greater than 90 dBA. Around 21.9% were male with an age category between 18 to 27 (10%) years were exposed to noise-induced hearing loss. Similarly, the lowest experience was between 1 to 10 years of exposure to noise-induced hearing loss by 16.9 % with working department transport helicopter (4.7%), transport airplane (4.7%), and daily maintenance (3.5%) (Table 4). Furthermore, the overall prevalence of hearing impairment among the workers was 81 (30.9%) with 45 (17.2%) bilateral and 36 (13.7%) unilateral hearing impairments. Among the bilateral hearing impairment, slight impairment 23 (63.9%), moderate impairment 10 (27.8 %), severe impairment 1 (2.8%), profound impairment 2 (5.6%), and from those unilateral right ear, slight impairment 36(78.3%) moderately impairment 8(17.4%) and profound hearing loss 2 (4.3%), left ear, slight impairment 35 (57.4%) moderate impairment 20 (32.8%), profound impairment 6 (9.8%).

Table 4: Prevalence of NIHL among central air force workers of Bishoftu, Ethiopia

Characteristics		Noise-induced hearing loss
Characteristics		Yes(%)	No(%)	Total (%)
Sex	Male	57 (21.9)	169 (65)	226 (86.9)
Sex	Female	7 (2.7)	27 (10.4)	34 (13.1)
Age category in years	18 - 27	26 (10)	98 (37.7)	124 (47.7)
	28 - 37	22 (8.5)	69 (26.5)	91 (35)
	38 - 47	14 (5.4)	22 (8.5)	36 (13.8)
	>47	2 (0.8)	7 (2.7)	9 (3.5)
Service in Years	1-10	44 (16.9)	148 (56.9)	192 (73.8)
	11-20	14 (5.4)	36 (13.8)	50 (19.2)
	> 20	6 (2.3)	12 (4.6)	18 (6.9)
Working Department	Security	0 (0.0)	21 (8.2)	21 (8.2)
	Cesena	3 (1.2)	9 (3.5)	12 (4.7)
	Su - 27	10 (3.9)	25 (9.7)	35 (13.6)
	Transport helicopter	12 (4.7)	17 (6.6)	29 (11.3)
	Transport airplane	12 (4.7)	6 (2.3)	18 (7.0)
	L-39	7 (2.7)	12 (4.7)	19 (7.4)
	Garage	2 (0.8)	8 (3.1)	10 (3.9)
	Logistic	8 (3.1)	16 (6.2)	24 (9.3)
	Daily maintenance (DM)	9 (3.5)	80 (31.1)	89 (34.6)
Noise level( dBA)	≥ 90	43(16.5)	65(25)	108(41.5)
Noise level( dBA)	< 90	21(8.1)	131(50.4)	152(58.5)

3.6. Factors associated with noise-induced hearing loss(NIHL)

Factors like level of work in noise before the air force, having a hearing difference between two ears, feeling hearing problem, and noise level (dBA) have a p-value less than 0.05 in bivariate analysis and were a candidate for multivariate logistic regression. However, due to potential con-founders, some variables were significant associations in the bivariate analysis but their significance disappeared in the multivariate analysis. Thus, the level of work in noise before the air force, and noise level (dBA) were significant associations with noise-induced hearing loss in multivariate logistic regression analysis.

Those participants who were exposed to noise-induced levels at work place previously were five times (AOR = 5.0, 95% CI: 1.74 - 14.36) more likely exposed to the sound noise level at the workplace than those workers not exposed at work place previously. Those workers who were exposed to greater or equal to 90dBA noise level were 4.98 times (AOR = 4.98, 95% CI: 2.59 - 9.58) more likely exposed to noise-induced levels than those who were exposed to less than 90dBA noise level (Table 5).

Table 5: Results of logistic regression model for risk factors associated with NIHL among central air force workers of Bishoftu, central Ethiopia

Characteristics		NIHS		COR (95% CI )	AOR (95% CI)
Characteristics		Yes	No	COR (95% CI )	AOR (95% CI)
Worked in noise before an air force	Yes	9	10	3.04 (1.18 -7.87)**	4.99 (1.74- 14.36)***
Worked in noise before an air force	No	55	186	1	1
Have hearing differences b/n the two ears?	Yes	18	27	2.45 (1.24 - 4.83)**	2.2 (0.9- 5.47)
Have hearing differences b/n the two ears?	No	46	169	1	1
Feel hearing problem	Yes	14	23	2.1 (1.01 - 4.39)**	1.1 (0.41- 2.98)
Feel hearing problem	No	50	173	1	1
Noise level (dBA)	≥90	43	65	4 (2.26 - 7.52)**	4.98 (2.59- 9.58)***
Noise level (dBA)	<90	21	131	1	1

Abbreviations: ** Significant at P < 0.05 bivariate analysis, *** Significant at P < 0.05 multivariate analysis, COR- Crude odd ratio, AOR- Adjusted odd ratio, CI- Confidence intervals

3.7. Factors associated with hearing impairment (HI)

Factors like level of sex, hearing the difference between two ears, feeling hearing problems, tinnitus/ ringing in the ear, and normal otologic examination have a p-value of less than 0.05 in bivariate analysis and were a candidate for multivariate logistic regression. However, due to potential con-founders, some variables were significant associations in the bivariate analysis but their significance disappeared in the multivariate analysis. Thus, sex and normal otologic examination were significant associations with hearing impairment in multivariate logistic regression analysis. The result revealed that male air base workers were 3.5 times more likely exposed to hearing impairment than female workers (AOR = 3.5, 95%CI 1.01 - 12.0). Moreover, those participants who have abnormal otologic examinations (having waxy, otitis, or tympanic rupture) were 1.9 times more likely exposed to hearing impairment than those who have normal otologic examinations (AOR = 1.9, 95%CI 1.02 - 3.54)(Table 6)

Table 6: Results of logistic regression model for risk factors associated with HI among central air force workers of Bishoftu, central Ethiopia.

Characteristics		HI		COR (95% CI )	AOR (95% CI)
Characteristics		Yes	No	COR (95% CI )	AOR (95% CI)
Sex	Male	78	150	5.4 (1.59 - 18.13)**	3.5 (1.01- 12.0)***
Sex	Female	3	31	1	1
Hearing difference b/n two ears?	Yes	25	22	3.2 (1.69 - 6.17)**	1.6 (0.7 - 3.93)
Hearing difference b/n two ears?	No	56	159	1	1
Feel hearing problem	Yes	20	17	3.2 (1.56 - 6.44)**	2 (0.83 - 4.9)
Feel hearing problem	No	61	164	1	1
Having tinnitus/ ringing in the ear	Yes	23	18	3.6 (1.81 - 7.13)**	2.2 (0.93 - 5.25)
Having tinnitus/ ringing in the ear	No	58	163	1	1
Normal otologic examination	No	28	36	2 (1.19 - 3.82)**	1.9 (1.02 -3.54)***
Normal otologic examination	Yes	53	145	1	1

Abbreviations: ** Significant at P < 0.05 bivariate analysis, *** Significant at P < 0.05 multivariate analysis, COR- Crude odd ratio, AOR- Adjusted odd ratio, CI- Confidence intervals

This study revealed that noise-induced hearing loss (NIHL) among central air base workers was high. The overall prevalence of noise-induced hearing loss was 24.6%. The current noise-induced hearing loss was higher than the studies done in Saud Arabia King Khalid International Airport (12%)¹⁸, Jomo Kenyatta International Airport( 15.3%) ¹⁹, and Hong Kong metal industry( 18.6%) ²⁷. Similarly, the prevalence of noise-induced hearing loss was higher than the study conducted in India(22.9%)¹¹, and lower than the study conducted in Malaysian 41.2%¹⁵, and Israel air force ¹⁶.

These differences may be due to the different levels of noise emitted, different hearing conservation programs, different exposure levels of the workers, and the awareness level of the countries on the impact of noise. Additionally, the low level of awareness of the workers towards safety, lack of supply of personal protective devices, and enforcement of rules and regulations of safety in this country are not as applicable.

According to this study, the overall prevalence of hearing impairment among the workers was 30.9% (95% CI 26.0 - 36.6) with 17.2% bilateral and 13.7% unilateral hearing impairments. which was lower than the study conducted by the India Air Force ²⁸ but higher than the study conducted among workers at Malaysia Airport ²⁹ and Saud Arabia King Kahild International Airport workers ³⁰. this is due to noise-induced hearing factors like treating otitis infections and irrigating the compacted waxy of the workers` ear on regular follow up in different countries

Additionally, the study revealed that 41.5% of workers were exposed above 90 dBA with no specified period because of the nature of the work. Even some planes and jets like the Transport airplane (Antonovo), Su-27, Transport helicopter, and daily maintenance Garage measured at the workers` positions of different places revealed that high exposure to noise level due to the exposure of workers was not bounded with the time of exposure standards rather it depended on the nature of work.

This fails the Ethiopia occupational safety and health directives regulation on the level of noise and respective time exposure³¹. Another study conducted by Malaysian and Indian airport workers revealed that the highest noise level was recorded at aircraft starting its engine at different distances as well as during day and night time ^13,31. The main difference in noise levels might be due to the difference in planes, jets, and machines' sound emissions levels. In other non-air force industries like in Hong Kong metal industry 36% of metal industry workers were exposed for 8 hrs for 90 dBA and above ²⁷, UAE industries workers' permissible exposure level was 85 dBA but 89% of the workers were exposed above permissible exposure level. This might be related to sound level with a time of exposure being high.

Moreover, the predictive factors that were related to NIHL were previous noise exposure and level of noise. This finding showed high-level noise exposure to noisy work environments is significantly associated with NIHL. This finding aligns with many studies conducted in different countries^20,32. The main reason behind this one might be as the intensity of noise increases the hair cells found on the cochlea become eroded and damaged to the auditory nerve so this causing as the noise level increases the prevalence of noise-induced hearing loss becomes high.

The other factor which was significantly associated with NIHL in this finding was previous exposure of the workers to noisy work environments after adjusting for other factors that mean especially workers before this institution in the metal industries and woodwork industries. This finding was consistent with the study conducted among metal industry workers in Ethiopia's Akaki Basic metal industry¹⁴. This one is due to the nature of NIHL because once the hair cells become damaged it becomes permanent. NIHL is sensor neural hearing loss. So one individual affected by the noise becomes permanent that was the case and those individuals exposed previously become positive for NIHL³².

Finally, the study revealed that low utilization of hearing protective device (HPDs) among air base workers was due to a lack of supplies and safety training, budget scarcity, negligence, and less awareness of the purpose of the protective devices. The study conducted in Saud Arabia King Kahild and India International Airport revealed that those workers who never used HPDs regularly developed NIHL^11,18,20,33. The result revealed that male air base workers were 3.5 times more likely exposed to hearing impairment than female workers (AOR = 3.5, 95%CI 1.01 - 12.0). This result aligned with other studies in different countries with different setups ^19,34,35. The reason may be due to the high exposure of males to noise than females especially in developing countries due to the nature of the work environments. Especially in the military setup and industries males are highly dominant in number than that females.

Moreover, those participants who have abnormal otologic examinations (having waxy, otitis, or tympanic rupture) were 1.9 times more likely exposed to hearing impairment than those who have normal otologic examinations (AOR = 1.9, 95%CI 1.02 - 3.54). The result was in contrast with the study conducted in the Dire Dawa textile industry²⁰. This significance or a risk factor may be occurred due to the occlusion or blockage of the ear canal.

The study has several drawbacks. Lack of generalizability with the Ethiopian air force since it was done only on the central base of the air force. Power interruption to do audiometric measurements, weight balance, and height measurements since the materials were digital and needed electric power. Furthermore, running the engine at the maximum was so difficult because of the nature of the plane and its fuel cost was so expensive, and senior staffs were reluctant to participate in the study

Workers of the Central Air base especially the flight line were exposed to high levels of occupational noise that could induce immediate hearing loss. Specifically, the magnitude of noise emitted by jets like Su-27, L-39, transport helicopters, transport airplanes (Antonovo), and Cesena was high when compared with MOLSA and OSHA standards of noise level exposure against permissible time exposure even if the workers were not continuously exposed to the noise. The prevalence of hearing loss was significantly high, especially for those workers exposed to noise levels above 90 dBA. The supply of HPDs and safety training related to noise was low. So Implementation of a hearing conservation program, giving noise education, and supplying adequate HPDs are essentials.

Ethical approval

An ethical approval letter was obtained from the Defense University College of health science ethics committee Review Board (IRB) on 03/05/2018 (No. መጠኮ/አጠ15/ተ/03/05/2018).The authors confirm that all experiments were performed under relevant guidelines and regulations. Written informed consent was obtained from each study subject.

Acknowledgments

This project would not have been possible without the support of the Defense University, college of health science giving us the chance to project work and funding budgets to enhance the college staff`s academic capacity and to solve the communities` problems and also would like to thank all the participants of our study, the management staff, and Air Force Clinic surgeon staff for their great cooperation to do this project. Finally, we would like to thank Professor Melakeberhan Dagnew for reviewing the manuscript.

Authors’ contributions

All authors contributed to the study design. AH principally did the survey, performed the data analysis, and drafted the manuscript. BZ and ZE did collect the data and interpreted the data. FK, SD, MD, SW, SM, GTT, and TNA interpreted the data and participated in manuscript revision. All authors critically reviewed draft versions and provided important intellectual content during revisions. All authors read and approved the final manuscript.

Funding

The study was funded by Defense University, College of health science. Funder played no direct role in designing the study, writing the manuscript, and submitting it for publication.

Consent to participate

All the participants voluntarily participated

Data Availability statement

The datasets analyzed during the current study were available from the corresponding author upon reasonable request

Conflicts of interests

The authors declare that they have no competing interests.

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No competing interests reported.

Level of noise-induced hearing loss and associated factors among workers in Bishoftu central air base of Ethiopia

Status:

Journal Publication

Version 1

Abstract

1. Introduction

2. Materials and Methods

2.1. Study setting

2.2. Study design

2.3 Sample size determination

2.4. Sampling techniques

2.5. Data collection

2.6. Data quality

2.7. Data analysis

3. Results

3.1. Socio-demographic characteristics of respondents

Discussion

Conclusion

Declarations

References

Additional Declarations

Status:

Journal Publication

Version 1