Hearing loss can significantly affect the quality of life of workers and their families, leading to a social handicap   . Noise-induced hearing loss is a preventable health problem worldwide   . However, it continues to harm workers in several sectors of activity, especially in industrial or artisanal environment  . The prevalence of noise-induced hearing loss in industrial settings was 37% to 59.7% in America in 2015  ; 7% in China in the automotive industry in 2015  ; 58.5% in Tanzania in a textile industry in 2015  ; 26% in a steel processing plant in Benin  . These frequencies, although high, are those of the formal sector where the application of regulations tends to reduce the effects of noise on human health. Workers in the informal sector are exposed uncontrollably to high noise levels. In Benin, epidemiological data relating to hearing loss in grain millers is almost non-existent and yet it is a very useful activity for feeding in the community. In fact, the working conditions in the flour mills expose to several occupational nuisances at the same time: noise, flour dust, chemicals, stress at work, prolonged sitting posture. The objectives of the present study were to: i) identify auditory and extra-auditory symptoms related to noise exposure; ii) measure noise level at the different workstations; iii) determine the prevalence of hearing loss; iv) implement prevention strategies among grain millers in the Dantokpa market.
2. Materials and Methods
2.1. Study Design
This was a descriptive cross-sectional study that took place from August 1 to September 31, 2020.
2.2. Study Framework
The study took place at Dantokpa market, the largest market in Benin, and precisely at the workstations of the grain millers. The millers work in rooms of 8 m2, most of which have only one door for ventilation. Each room has two to four machines with independent operation. Many of the workers are day laborers for the owners of the millstones.
2.3. Study Population and Sampling
The study population was made up of millers from the Dantopka market with at least one year of seniority in the activity. An exhaustive recruitment has been carried out. The millers with a hearing history before taking up the post as well as those who did not perform an audiometry were excluded from the study.
2.4. Collection of Data
Data collection was carried out through a “face-to-face” interview. The data were collected using a standardized form. The information provided related to socio-demographic, professional and clinical characteristics.
The noise level was measured using a sound level meter at all stations. The sound level meter was placed at each workstation at the actual time of a grain grinding activity, with the mill running. Instantaneous measurements were made over a period of 5 mm. The reading was made directly on the screen of the device.
Audiometry was performed using an audiometer calibrated to millers outside their workplace at the National Hospital Center for Pneumo-phtisiology after an otoscopic examination and after an auditory rest of 48 hours and before taking up the post. The ENT examination was performed by a physician using an otoscope and specula. Both ears were examined. The clinical features sought were: earwax plug in the ear canal, tympanic perforation, ear discharge.
The millers with a plug of earwax were invited to a washing of the auditory canal before the audiometry.
When the ENT examination is normal, audiometry can be performed. The audiometry was not performed in a soundproof room but in a room isolated from other hospital activities. A mechanical audiometer associated with a headset was used. The evaluation of the hearing loss was performed at frequencies of 500 Hz, 1000 Hz and 4000 Hz. The hearing loss is mentioned with the red prn for the right ear and in blue for the left ear. The hearing loss was calculated by [(lost 500 Hz) + (lost 1000 Hz) + (lost 2000 Hz) + (lost 4000 Hz)]/4. Audiometry results were interpreted by an occupational physician and an ENT specialist.
2.5. Variables of Interest
Hearing loss has been defined according to the classification of the International Bureau of Audio phonology (BIAP). We distinguish according to the level of the average tonal hearing loss calculated by the arithmetic mean of the deficits at the frequencies of 500 Hz, 1000 hz, 2000 hz and 4000 hz, normal hearing: ≤20 dB; mild deafness: 21 to 40 dB; moderate deafness: 41 to 70 dB; severe deafness: 71 to 90dB; profound deafness: 91 - 119 dB and total deafness or cophosis: ≥120 dB. Occupational deafness was defined according to Decree 2013-50 of February 11, 2013 establishing the list of occupational diseases in the Republic of Benin by a bilateral hearing loss greater than or equal to 35 dB in the better ear in a worker exposed to noise with a seniority of at least one year. The noise level was classified according to WHO standards due to the absence of any normative document on the subject in the Republic of Benin. The first exposure threshold for preventive action is 80 dB (A) for 8 hours (alert threshold). The noise exposure threshold that must trigger corrective measures by the employer (noise reduction at the source or provision of hearing protection) is 85 dB (danger threshold).
2.6. Data Analysis
Data analysis was performed with Epi-info 7.2.6 software. Proportions were calculated for qualitative variables and means with standard deviations for quantitative variables. A univariate analysis was used to search for factors associated with occupational deafness using the Chi 2 test at a significance level of p < 0.05, so a crude odds ratio was performed.
2.7. Ethical Considerations
Permission was obtained from Dantopka’s market managers and informed consent was obtained from participants. Data were collected with respect to confidentiality and human rights. Travel of participants was at the expense of the research team. Data management and use was done anonymously.
3.1. Socio-Demographic and Professional Characteristics
A total of 57 millers, all men, participated in the study out of the 64 identified. The reasons for non-participation were: 3 did not present in the hospital for the completion of the ENT consultation and audiometry, 2 had a history of hearing since childhood and 1 had a profound hearing loss in one ear prior to entry into the sector. Their median age was 25 years with extremes of 18 and 50 years, distributed as follows: 64.91% [18 - 28[ years, 21.05% [28 - 38[; 12.28% [38 - 48[ and 1.75% age ≥ 48 years. Among the millers 42.11% were in a couple, 52.63% were single and 5.26% were widowed/divorced. At the professional level, 57.89% had a seniority in the job of more than 2 years, there were 11 bosses versus 46 apprentices. The average daily working time was more than 12 hours for 45.61% and 42.11% had a daily income of less than 4 US dollars. Only 5.26% were engaged in extra-occupational activities involving noise exposure at the same time. The instantaneous measurement of noise at the different workstations allowed us to classify the millers by level of exposure as follows: 5.26% exposed to less than 80 dB (A); 26.32% exposed to between 90 and 100 dB (A) and 68.42% exposed to more than 100 dB (A). Table 1 exposes socio-demographic and professional characteristics of the informal cereal millers. None of the 57 millers had hearing personal protection equipment (PPE) available at the time of work. Figure 1 shows the working conditions at the grain crushing site. All the workers recognized permanent exposure to noise and 45.61% had a good knowledge of the auditory effects of noise.
3.2. Clinical Symptomatology
Conversation disturbance affected a total of 10 out of 57 millers and concerned the raising of the voice by the miller himself or his entourage and the increase in volume of the devices. The most frequent hearing symptoms were the sensation of auditory fatigue (89.47%); tinnitus (73.68%) such as ringing and whistling in the ears. On the extra-auditory level, the most represented symptoms were: headaches (71.93%); irritability (64.91%); palpitations (57.89%) and dizziness (56.14%). An increase in blood pressure was noted, greater than or equal to 140/90 mmHg in 43.86%. Table 2 describes the clinical symptomatology related to hearing loss in cereal millers.
3.3. Prevalence of Hearing Loss and Associated Factors
The prevalence of deafness was 87.72% 95% CI [76.32%; 94.92%] with 24.56%
Table 1. Socio-demographic characteristics of cereal millers, Dantokpa Cotonou (N = 57).
Figure 1. Working conditions of the grain millers at Dantokpa Market, Cotonou, Benin, 2020eeWW.
Table 2. Distribution of cereal millers according to auditory and extra-auditory clinical symptoms, Dantokpa Cotonou; (N = 57).
moderate deafness and that of occupational deafness was 29.82% 95% CI [18.43% - 43.40%]. Factors associated with occupational deafness were age greater than 28 years (ORb = 9.6; 95% CI [2.62-35.21], p = 0.0002) and high blood pressure (ORb = 4.98; 95% CI [1.45-17.14], p = 0.008). Table 3 and Table 4 present prevalence and factors associated with hearing loss.
Table 3. Distribution of cereal millers according to the degree of hearing loss, Dantokpa, Cotonou; (N = 57).
Table 4. Associated factors of hearing loss in cereal millers Dantokpa, Cotonou; (N = 57).
Following the results, several corrective actions were carried out: an awareness campaign on the hearing risk linked to noise, a punctual distribution of anti-noise wadding, a recommendation for regular breaks and a limitation of the daily working time. Millers suffering from occupational deafness were referred to the ENT specialist for better care. An invitation to the millers to organize themselves in cooperatives to facilitate their medical follow-up was proposed.
Prevention of deafness in the informal work environment is poorly covered, unlike in the formal sector. To our knowledge, this is the first study in the country to assess deafness in informal millers. The strengths of the study lie in its comprehensiveness and the use of appropriate tools to assess noise and hearing loss. An important limitation of the study is that in the results, the effect of age on the hearing loss of individuals is not controlled for and is confounded by the effect of noise. Also, the realization of the audiometry did not respect the minimum 72 hours of cessation of exposure to noise required by the Beninese regulation before affirming the diagnosis of occupational deafness. This is due to the daily status of the workers and the loss of income for the millers. However, the millers with suspected occupational deafness were sent to the ENT specialist for additional voluntary exploration. Unfortunately, due to low income, only one miller was able to attend the ENT consultation and the diagnosis was confirmed. It should be noted, however, that several authors in the literature recommend a duration of 48 hours for the cessation of exposure to noise.
Comparison of the results with those of other authors is sometimes difficult because of occupational variability, the existence of few data in the informal sector and the use of different measurement tools.
Our study showed high noise levels at all workstations compared to the standards. These results are similar to those observed in Tunisia in workers exposed daily to noise throughout the production line of flour and semolina from reception, cleaning, milling of wheat, sieving of crushed wheat and its grading in different dimensions to the storage of finished products  . In Benin, in another informal sector among tinsmiths, an average noise level of 90.6 ± 4.8 dB (A)was found  .
The prevalence of hearing loss and occupational deafness in particular is very high among millers who are young workers with less than 5 years of service for the vast majority but who did not use hearing protection equipment. The prevalence of hearing loss is higher than that observed in several studies conducted in the formal sector: 24% in oil and gas extraction in New England, USA; 58.5% in Tanzania in 2014 among workers in a textile industry  ; in a steel production plant in Benin  ; the same is true for occupational deafness with only 5.8% in a sawmill and carpentry in Benin  .
On the other hand, in the informal sector, our results corroborate those of other authors such as Ayélo et al. who found a prevalence of 79.3% of hearing loss among tinsmiths with an average duration of exposure to noise of 13 years ± 10 months and not wearing hearing PPE  .
The prevalence of hearing loss therefore varies from country to country, from industry to industry and with the use of hearing protection equipment   . Indeed, the absence of hearing protection equipment exposes millers to very high noise levels with the risk of hearing accidents.
The association found between age and occupational deafness corroborates several findings in the literature   . Indeed, the older the age, the greater the risk of hearing loss if the subject is still exposed to noise. What is particular in the present study is the young miller who is, however, associated with occupational hearing loss. The association found between high blood pressure and occupational deafness is justified. Indeed, several authors have shown the risk of occurrence of arterial hypertension or a cardiovascular event in people usually exposed to noise    and therefore showed an association between noise-related hearing loss and cardiovascular disorders   .
Noise-induced hearing loss can be prevented if the right preventive measures are taken. Several prevention strategies are described in the literature depending on the intensity of the exposure, the exposing equipment, the duration of the exposure and the sector of activity   . The preventive measures implemented in this study, such as awareness of hearing risks and distribution of hearing individual equipment, remain ad hoc and must be reinforced by a formal organization of the sector and the implementation of an environmental and clinical monitoring program for millers.
The prevalence of bilateral hearing loss among millers in the Dantokpa market is very high, as is the prevalence of occupational deafness. The implementation of a permanent support program for the improvement of working conditions as well as the medical follow-up of these informal sector workers is necessary to limit the consequences of permanent and prolonged exposure to noise due to an activity that is vital for the community
Our thanks go to the managers of the Dantokpa Market and to the millers for giving us the opportunity to carry out this work.
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