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  Table of Contents 
Year : 2022  |  Volume : 26  |  Issue : 3  |  Page : 183-188

Assessment of occupational noise generation and associated noise-induced hearing loss among employees of a black tea processing factory in Darjeeling District, India

1 Department of Community Medicine, North Bengal Medical College and Hospital, Sushrutanagar, Darjeeling, India
2 Department of Community Medicine, Nil Ratan Sircar Medical College and Hospital, Kolkata, West Bengal, India

Date of Submission26-Dec-2021
Date of Decision21-Jan-2022
Date of Acceptance12-Mar-2022
Date of Web Publication26-Sep-2022

Correspondence Address:
Dr. Abhijit Mukherjee
34, SN Banerjee Road, New Barrackpore, Kolkata - 131, West Bengal
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/ijoem.ijoem_356_21

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Introduction: Exposure to high levels of noise is a problem among tea factory workers worldwide, but it is poorly studied in India. Aims: This study aimed to assess noise generation in various parts of a black tea factory and find out prevalence of occupational noise-induced hearing loss (ONIHL) and its determinants among employees. Materials and Methods: An observational cross-sectional study was performed in a black tea processing factory of Darjeeling District. Noise levels and exposure data at different parts of the factory were measured using a sound level meter. Participant characteristics were obtained using a questionnaire and noise-induced hearing loss data obtained by audiometry. A sound map was generated based on noise exposure data and a multivariable logistic regression performed to assess determinants of ONIHL. Results: Sound pressure levels ranged from 58.7 to 90.3 dBA, with the highest levels in the crushing–tearing–curling (CTC) room. Noise exposure of workers was the highest during curling process and the lowest during packaging in the packing room. The prevalence of ONIHL was found to be 28.3%, most of which were of moderate degree, and the highest prevalence was among CTC room workers. Multivariable analysis showed significant association only between daily noise exposure and the presence of ONIHL (AOR 1.68, P value = 0.018). None of the study participants used any hearing protection equipment during work. Conclusions: Generation of high levels of noise is a pertinent problem in the black tea factory, which, coupled with non-use of personal protective equipment, led to a high risk and prevalence of ONIHL.

Keywords: Black tea factory, Darjeeling, noise-induced hearing loss, occupational health, occupational noise

How to cite this article:
Bandyopadhyay A, Ghosal R, Dasgupta P, Mukherjee A. Assessment of occupational noise generation and associated noise-induced hearing loss among employees of a black tea processing factory in Darjeeling District, India. Indian J Occup Environ Med 2022;26:183-8

How to cite this URL:
Bandyopadhyay A, Ghosal R, Dasgupta P, Mukherjee A. Assessment of occupational noise generation and associated noise-induced hearing loss among employees of a black tea processing factory in Darjeeling District, India. Indian J Occup Environ Med [serial online] 2022 [cited 2022 Dec 3];26:183-8. Available from:

  Introduction Top

Noise, defined as any unwarranted disturbance within a useful frequency band, has long been associated with hearing loss.[1],[2] Noise-induced hearing loss (NIHL) is permanent, bi-lateral sensorineural hearing loss resulting from both momentary and prolonged exposure to noise.[3] Before the industrial age, the major cause of NIHL was environmental noise exposure.[2] However, since the past few decades, occupational noise-induced hearing loss (ONIHL), one that results from exposure to high levels of noise during employment, has cemented itself as not only the most common form of NIHL but also the most common occupational disease worldwide.[4],[5] ONIHL affects employees across industries, ranging from construction and aviation to music, manufacturing, and agriculture.[6]

India is one of the largest producers of tea globally, with most of its high-yield tea gardens being concentrated in the Eastern Himalayan foothills.[7] Withering, curling, and processing of tea leaves require massive machinery that expose employees to large amounts of noise. A study performed by Çiçek et al.[8] in Turkey found that most factory workers were exposed to noise of >85 dBA. It can therefore be surmised that employees of tea factories of India, which generally have poorer infrastructure and lack modern quality control systems, are also at a high risk of occupational noise exposure and therefore developing ONIHL.[9] However, a thorough search of the existing literature failed to yield research focusing on this subject. The current study aimed to address this gap by assessing noise levels generated at various sections of a black tea processing factory of Darjeeling District of West Bengal, India, and to find out the prevalence of ONIHL and its determinants among workers employed there.

  Material and Methods Top

Study design and area

An observational cross-sectional study was conducted at a black tea processing factory of Darjeeling District of West Bengal, India, from May to September of 2021.

Study population, inclusion criteria, and sampling

Sound pressure levels (SPLs) generated at various parts of the selected factory were measured to obtain noise-related data. ONIHL data were obtained from employees working at the factory. The complete enumeration sampling technique was employed to approach all 54 workers of the factory. Employees working in the factory for <1 year, those with already diagnosed permanent hearing loss before employment in the factory, and those who refused to provide written informed consent to participate in the study were excluded. Furthermore, each consenting participant underwent an otorhinolaryngological check-up by a specialist so as to rule out any other causes of hearing loss such as otitis media, a perforated tympanic membrane, impacted cerumen, etc.

Study tools and technique

The study utilized the following tools:

  1. A portable sound level meter (Digital InstrumentsTM SL-4012) to measure SPL in decibels (dB) at A-weighting.[10]
  2. A pre-designed, pre-tested, structured, researcher-administered questionnaire to interview study participants, consisting of two sections – sociodemographic and work-related characteristics of the workers.
  3. A portable audiometer (Arphi ElectronicsTM Proton SX-5) to conduct pure tone audiometry to identify NIHL.

Data collection

At first, the sound level meter was placed at different parts of the factory. Each black tea processing factory is divided into six distinct locations where a particular task is performed, namely, the unloading area (unloading of tea leaves), withering area (withering), crushing–tearing–curling (CTC) room (curling and fermentation), drying area (dying), sorting area (sorting of dried leaves), and packing room (packaging). Initially, the sound level meter was used to identify the highest sound pressure level (dBA) at the ear level in each area. Thereafter, multiple observations were made at incremental distances from this location to generate a 'noise map', which was further divided into individual 'noise zones'. Each zone until 75 dBA was calculated at (-2) dBA intervals, starting at the highest SPL. From the threshold of 75 dBA onward, each noise zone was identified at (-4) dBA intervals [Figure 1].
Figure 1: Noise map of the black tea processing factory under study 1, withering trough; 2, crushing–tearing–curling machine; 3, continuous fermenting machine; 4, drier; 5, sorting vibro-machine; 6, packing machine; 7, conveyor; 8, blower; 9 fans; A – withering area, B – crushing–tearing–curling (CTC) room, C – drying area, D – sorting area, E – packing area, F – unloading area

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Employees of the factory work in three 8-hour shifts every working day. After identifying noise zones, working stations of these employees throughout their shift were identified by observation and marked on the noise map [Figure 1].

SPLs were measured at each of these stations for an entire shift by observations at 15-minute intervals. During each observation, SPL (dBA) was measured for 5 seconds at 1-second intervals, and the readings were averaged. Data thus generated for the working day were then fed in a web-based 'Noise Exposure Calculator' of the 'NoiseMeters' web application to obtain the 8-hour time-weighted equivalent continuous noise pressure level (Leq. 8hr dBA) at each of the working stations.[11] For each working station, such measurements were carried out for the three different shifts on three non-consecutive days so as to account for inter- and intra-day variations, and the results were averaged. Leq8hr data at individual working stations were combined with the time spent by each worker in those stations to find the daily Leq8hr dBA noise exposure to the employee [Figure 2].
Figure 2: Data collection process for daily 8-hour time-weighted equivalent noise exposure (dBA)

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In the next step, the participants were interviewed using the questionnaire to obtain sociodemographic and job-related data. Finally, the presence of NIHL was measured using the portable audiometer.[12],[13]

Data analysis

AdobeTM Illustrator was used to generate the noise map by superimposing the noise data on the schematic diagram of the factory. Participants' data were entered in a spreadsheet, analyzed using IBM SPSS v. 25 statistical software, and reported using frequencies and percentages. Multi-variate logistic regression analysis was performed to calculate association between the presence of NIHL (dependent variable) and age of the participants, their sex, educational and socio-economic status, number of years worked at the factory, and daily Leq. 8hr noise exposure (independent variables). The results were expressed as adjusted Odds ratios (AORs) ± 95% confidence intervals. A P value of <0.05 was considered to be statistically significant.

Ethical considerations

The study was conducted after obtaining ethical clearance from the Institutional Ethics Committee of the North Bengal Medical College and Hospital and necessary permissions from the respective authorities. Written informed consent was obtained from each of the participants before interview, and privacy, confidentiality, and anonymity were maintained for all of the study participants.

  Results Top

In the current study, the SPL at workers' ear level at different parts of the factory was found to range from 58.7 dBA (sorting and packing areas) to 90.3 dBA (CTC room). Regarding the sources of noise, the highest was made by blowers in the CTC room (90.3 dBA) and the lowest by sorting vibro-machines of the sorting room (62.7 dBA) [Figure 1] and [Table 1].
Table 1: Noise generated by various machinery in different parts of the tea factory

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Of the 54 total workers, eight (8) did not meet the inclusion criteria, yielding a total sample size of 46. Of them, 13 (28.3%) were women. The age of the participants ranged from 22 to 36 years (mean 27.3 ± 2.9 years). Most had completed their secondary education (39.1%) and belonged to a lower socioeconomic status (Class IV and V) as per the modified BG Prasad scale (updated AICPI, July 2021).[14]

The prevalence of NIHL was found to be 28.3%. The highest prevalence was seen among employees working in the CTC room (47.6%), followed by those working in the withering area (40%) and unloading area (25%). None of the employees working in the drying, sorting, and packing areas had NIHL. Most of the participants suffered from moderate levels of NIHL (41–60 dB HL), followed by slight hearing impairment (26–40 dB HL), as per the World Health Organization criteria[15] [Table 2].
Table 2: Prevalence of noise-induced hearing loss (NIHL) among the participants (n=46)

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The participants had been working in the factory for 3–13 years (mean 6.2 ± 1.7 years) [Table 3]. Most of them had their working stations located in the CTC room (45.6%), and the least had them in the unloading area (8.7%) [Figure 3].
Figure 3: Comparison of 8-hour time-weighted equivalent noise exposure (Leq. 8hr dBA) of participants according to the location of their work (n = 46) CTC – Crushing-tearing-curling, Leq. 8hr – 8-hour time-weighted equivalent noise exposure

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Table 3: Sociodemographic determinants of occupational noise-induced hearing loss (ONIHL) among the participants (n=46)

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It was observed that none of the participants used any hearing protection equipment during work. They were not provided with any hearing protection equipment by the factory administration as well. Furthermore, when asked, none of the participants were aware of the use and utility of such personal protective equipment for hearing protection.

Daily Leq. 8hr noise exposure among participants ranged from 63.4 to 87.6 dBA (mean 77.4 ± 8.5 dBA). The highest daily Leq. 8hr noise exposure occurred during the curling process in the CTC room (82.4-87.6 dBA, mean ± SD = 85.2 ± 1.7 dBA), followed by withering, drying, and unloading of the tea leaves. The lowest noise exposure occurred during packaging in the packing room (63.4–67.3 dBA, mean ± SD = 66.3 ± 1.5 dBA) [Figure 3].

Multi-variate analysis showed that NIHL was significantly associated with increasing Leq. 8hr noise exposure (AOR 1.68, P value = 0.018). Other predictor variables were not found to be significantly associated. As the working station of the participants determined their noise exposure, their location was not considered for multi-variate analysis [Table 3].

  Discussion Top

The Center for Disease Control and Prevention states that noise levels above 80 dBA can cause permanent hearing damage and recommends noise levels in workplace be kept below this level.[16] However, most Indian agricultural industries have noise levels that are significantly higher than 80 dBA.[17] This was also true for the tea industry as several machines in the factory, especially those in the CTC room, produced noise levels of ≥80 dBA, going as far as 90.3 dBA. Old machinery and the lack of regular maintenance, noise-proofed infrastructure, and enforcement of industrial noise standards might be the reasons behind such high occupational noise levels.

Conventionally, Leq. 8hr is utilized to calculate the total dose of noise a person is exposed to based on the SPL of individual areas the person has been in and the duration of stay there.[18] The National Institute of Occupational Safety and Health (NIOSH) has recommended an 'action limit' for remedial measures when the occupational Leq. 8h noise exposure exceeds 85 dBA, with a 3 dBA exchange rate.[19] Pertinently, the daily Leq. 8hr noise exposure among employees was found to exceed this action limit at several locations of the factory. These findings are similar to those reported by Çiçek et al.[8] They further reported that curling the process generated the highest daily noise exposure, which was also corroborated in the current study.

Of all the causative factors of hearing loss, being exposed to high levels of noise for prolonged periods is probably the most well-established.[20] To this effect, NIHL was expected to be a problem among the workers, an assumption that was confirmed by the high prevalence (28.3%) of the condition. The idea that occupational noise exposure was the primary driver behind the NIHL observed among the workers was affirmed by some factors. First, there was a high prevalence of NIHL in high-noise locations in the factory and a positive association was observed with increasing Leq. 8hr noise exposure as well as years of work, results similar to which have been observed in other Indian industries.[21],[22] Additionally, all the participants lived in a village within the garden itself, around 300 meters away from the factory. The nearest source of high levels of environmental noise was a highway, which was 1 km away. Furthermore, none of the participants were observed to be exposed to any other noise sources excepting those of the factory machinery during their work hours, including noise sources such as music systems or using headphones to listen to music while working. Since the employees were full-time workers, it can be surmised that the noise exposure experienced by them was from the factory itself and thus occupational in nature.

Participants belonging to higher socioeconomic and educational statuses were found to be better protected against ONIHL, which can be explained by the fact that workers hailing from such backgrounds often work at the least risky areas.[23] Strikingly, a negative association was observed between the presence of ONIHL and the age of the participants. This might have resulted from a 'healthy worker effect', where older employees suffering from severe hearing impairment might have left their jobs, leaving only their more resilient peers.[24]

Personal protective equipment (PPE) such as earplugs, earmuffs, and helmets have been well-established as the definitive measure against NIHL.[25] The non-use of PPE might have also played a role in the high prevalence ONIHL as observed in the present study.


The cross-sectional design and the small sample size were major limitations. Furthermore, Leq. 8hr was measured and calculated using a sound level meter. Resource constraints prevented the researchers from utilizing more precise instruments such as personal integrating noise dosimeters. Possible confounding factors such as sensorineural hearing loss because of diabetes mellitus could also not be assessed. However, a larger study including more tea factories in the region is currently in the works to address these lacunae.

  Conclusions Top

It was observed in the current study that high levels of occupational noise exposure were present in various parts of the black tea processing factory. This was associated with a high observed prevalence of ONHIL among workers. Upgradation of the existing infrastructure and mandating the use of PPE during work can help mitigate this problem.

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.

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  [Figure 1], [Figure 2], [Figure 3]

  [Table 1], [Table 2], [Table 3]


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