The impact of noise exposure and work posture on job stress in a food company

2.1 Participants

This cross-sectional study was conducted in an Iranian food company with 255 workers. The inclusion criteria for workers were: 1) age > 18 years, and 2) having at least 2 years of work experience. Exclusion criteria were: 1) having background diseases or accidents affecting workers’ musculoskeletal system, and 2) using sedatives. In total, 200 workers were selected by convenience sampling, including 69 males and 131 females, aged from 19 to 61 years (mean = 33.31, SD = 8.63). The workers’ age was classified into three categories (Table 1).

A demographic questionnaire was used for collecting demographic characteristics, including age (years), sex (male or female), education level (under diploma, diploma or AD, BSc or higher), marital status (single, married), work experience (years), and medical background.

The Osipow job stress questionnaire was used to evaluate the level of occupational stress. This questionnaire consisted of 60 questions in a 5-item Likert Scale (never = 1, sometimes = 2, usually = 3, often = 4, and very often = 5). These six scales are [18]:

The first aspect was the role overload (lack of the needed support for heavier work tasks) related to how one responds to the demand of the workplace.

The total score from all the questions was interpreted as follows: score ranges 50–99, 100–149, 150–199, and 200–250 were referred to as low stress, low-moderate stress, moderate-high stress, and high stress, respectively. The validity and reliability of Osipow questionnaire have been determined in Iranian populations with a Cronbach’s alpha of 0.83 [18, 19]. The results of the Osipow questionnaire for job stress among the workers in this study are shown in Table 2.

Quick Exposure Check (QEC) was used for the assessment of work-related musculoskeletal risks, which is an observational method developed by Li and Buckle at the Roben’s Centre for Health Ergonomics [18]. QEC analysis provides scores for four main body parts (back, shoulder/arm, wrist/hand, and neck). This method assess movements (static/dynamic), frequency, time spent on observed task, weight handled, vibration /visual demands, and the stressfulness of the work [20]. There are four Preliminary Action Levels (PAL) for the QEC that are classified as: acceptable (≤40 %), further investigation (41–50 %), further investigation and change soon (51–70 %), urgent investigation and changes (>70%). Finally, the results are classified into four risk levels: low, moderate, high and very high risk level [21]. The Quick Exposure Check (QEC) results for work-related musculoskeletal risks among the workers in this study are shown in Table 1.

2.4 Measures

Personal noise exposure was measured by TES-1354 noise dosimeter (TES Electrical Electronic Corp., Taiwan), according to ISO 9612 [22]. Before the measurements, TES-1356 calibrator (TES Electrical Electronic Corp., Taiwan) was applied to calibrate the noise dosimeter. Noise dosimetry is a reliable method for measuring sound pressure levels in an environment with the aim of estimating the total amount of noise to which a worker is exposed over an 8-hour work shift [21, 23]. In the present study, workers had 30 minutes’ break during the 8-hour shift. Thus, exposure to noise was measured for 7.30 h (while they were working) and 30 minutes (while they were in the break room).

The measurement of workers’ exposure to noise and equivalent sound level during the 8-hour shift was calculated by the formula:

Measurement of the dose of workers’ exposure to noise: D ( % ) = 8 Ta × 100

Where D is the noise dose (%) and T_a is the permissible exposure time based on the environmental sound pressure level (hr).

The standard dose of noise exposure is calculated through the following equation, which is based on the Iran Occupational Exposure Limit (OEL): D ( % ) = 12.5 ∑ i = 1 n ti anti log ( SPLi - 85 10 )

Where D represents the received dose of noise, ti is the duration of noise exposure, SPL_i is the noise pressure level and SPLstd is the standard threshold of noise exposure, which based on the occupational standards in Iran is 85 dB.

The following formula was used to measure 8 h equivalent sound level: Leq . 8 h ( dB ) = 10 Log [ 1 8 ∑ i = 1 n ti × 10 lpi 10 ]

Where, Leq.8 h = the equivalent sound level for 8 h exposure [dB], t_i is the exposure time (hr), and Lp_i is the measured equivalent sound level in the rest room and the work environment (dB).

The number of participants who were exposed to standard noise levels (less than the threshold≤85 dB) and non-standard noise level (over the threshold > 85 dB) is shown in Table 1.

2.5 Statistical analysis

The data distribution normality was evaluated by the Kolmogorov–Smirnov test. Standard descriptive statistics (mean±standard deviation) and frequency (percentage) were determined for directly measured and derived variables. Statistical outliers were checked using Grubb’s test, which is based on the difference of the mean of the sample and the most extreme data, considering the standard deviation [24]. Two-way analysis of variance (ANOVA) was used to compare the mean of stress scores between levels of posture and noise conditions, and the interaction between them. Post-hoc LSD analyses were used for multiple comparisons between posture levels. Also, two-way ANOVA was used to compare the mean of stress scores between gender and age groups. All statistical analyses were performed with SPSS version 23.0 (IBM Corporation, New York, United States). P values smaller than 0.05 were considered statistically significant.

3.1 The first hypothesis: Noise exposure and work postures affecting the job stress

There were a significant main effects for noise conditions (F (1,194) = 6.90, p = 0.009, Power = 0.74) and posture levels (F (2,194) = 23.81, p < 0.001), indicating a significant differences between the two different noise conditions and three levels of posture on stress scores. A post-hoc pairwise comparison indicated that there is a significant difference in mean stress scores among the three levels of posture, such that the mean stress scores in the moderate risk level group (160.84 +–22.99) was lower than the same values in the other two groups [high risk (170.06 whether +–21.75), very high risk (203.0 +–21.57)].

3.2 The second hypothesis: An interaction effect between noise exposure and work postures on the job stress

The results demonstrated no significant interaction effects between “noise conditions” and “posture levels” (F (2,194) = 1.34, p = 0.26, Power = 0.28) in term of job stress (Fig. 1).

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Fig. 1

Interaction effect between posture levels and noise conditions on stress scores (non-significant interaction, p > 0.05).

Also, there was no significant main effect of “age group” on stress scores (F (2, 194) = 1.15, p = 0.31, Power = 0.25). However, there was a significant main effect for “gender” on stress scores (F (1, 194) = 11.10, p < 0.001, Power = 0.91), suggesting that the mean stress scores were significantly higher in females than males. Moreover, there was no significant interaction effect between “age group” and “gender” for stress scores (F (2, 194) = 0.61, p = 0.54, Power = 0.15).

4.1 General discussion

In line with our hypothesis, there is a significant relationship between occupational noise and job stress. In our study, 55.5% of workers were exposed to non-standard noise levels greater than 85 dB. According to Abbasi et al., exposure to these high levels of occupational noise have a positive significant effect on job stress over years of work experience [10]. This result is in keeping with Lazarus’s “Transactional Theory of Stress and Coping (TTSC)”. Based on this theory, stress arises from an imbalance between environmental demands and the person’s ability to cope with these demands. Therefore, it can be argued that prolonged exposure to non-standard ambient noise, as an adverse environmental factor, could exceed an individual’s tolerance threshold and cause stress [25, 26]. Another possible explanation is that exposure to excessive noise leads to the secretion of stress hormones, such as adrenaline, noradrenaline, and adrenal steroids as well as simulating the sympathetic nervous system by releasing epinephrine [27]. These processes could lead to elevated blood pressure, heart rate cardiac output, and thus increased stress levels in workers [28]. Although several studies have indicated the significant impact of noise exposure on job stress [10, 29], the outcome from the study by Nuaim et al. is in conflict with the result of this study. The study demonstrated no significant relationship between noise condition and stress. The main reason for this inconsistency is probably that in Nuaim et al. study workers were wearing ear plug or other hearing protection devices, and therefore were exposed to low noise intensity [30].

Also, the present results demonstrated a significant relationship between body posture during work and job stress. Candan et al. showed that the severity of musculoskeletal pain of hazelnut factory workers was associated with stress in the workplace. Bulduk also reported that job-related stress was significantly associated with moderate to severe upper back pain. Choobineh et al. found that job stress dimensions were predictors for musculoskeletal pain. De Almeida demonstrated that the prevalence of MSD symptoms among the workers was 66% and the symptoms were significantly associated with perceived stress [16, 31–33]. Consistent with mentioned studies, the significant association between the high levels of exposure to musculoskeletal risks (71.5%) and job-related stress in the present study could support that working in an improper sitting posture without a sufficient break time is a risk factor for prevalence of WMSDs and job stress [16]. Because the workers in the current study had to work for seven and a half hours in a day with only a half hour break in an awkward sitting position. In addition, lack of individual working adjustments of the bench and desks, the workers have to bend their neck excessively. It seems that awkward postures, as a physical risk factor, may affect psychological stress and vice versa. On the one hand, workers with musculoskeletal pain caused by inappropriate postures are more prone to job stress, presumably because pain renders workers less tolerant of the psychological demands of the workplace [34]. Furthermore, pain can possibly cause psychological stress by activating the sympathetic nervous system and the hypothalamic-pituitary-adrenal system [32]. On the other hand, stress might cause the continuous activation of small and low-threshold motor units, which are normally activated merely at low levels of contraction. This, in turn, may result in degenerative processes, damage, and pain in the muscles [35]. Stress may also elevate the awareness of musculoskeletal disorders symptoms, or decrease the ability to manage the symptoms [36]. Moreover, according to a hypothesis suggested by Schleifer and Ley [37], hyperventilation caused by psychological stress reduces peak CO₂ levels and elevates the blood pH level (beyond 7.45 causing alkalosis), in a way which results in increased muscular tension and heightened sensitivity to sympathetic activity.

Although noise and posture independently have significant impact on job stress, the interaction effects between these physical factors does not cause a significant change in stress scores. In other words, with noise increasing, job stress scores similarly increased in all posture levels. It is worth noting that the non-significant result for the interaction is likely to be the reflection of a small sample size (Power < 0.8) in the present study; a significant difference may emerge with a larger sample.

Although many studies have found a significant relationship between age and job stress [38, 39], the present results did not show a significant relationship between these variables, which is inconsistent with the results of a study by Rauschenbach et al. Based on this study, it is likely that different age-related losses and gains may compensate for one another, and eventually nullify each other in the overall relationship between age and stress [38]. In the present study, gender had a significant correlation with job stress; female workers had significantly higher stress levels than male workers. This finding is in agreement with the results of previous studies [40–42]. According to these studies, females with lower-status jobs were more vulnerable to work-related stress. In similar job positions females usually paid less than males and experience greater challenges in order to obtain a higher job position. Thus, this effort-reward imbalance is likely to cause additional psychological stress [41]. Furthermore, in comparison to males, females are often responsible for largest amounts of unpaid work (childcare and household tasks), and they are more likely to be exposed to a greater total workload (unpaid duties + paid) than males. This extra burden could be a significant contributor to stress [40]. Finally, higher levels of work-related stress among females could also occur when there is a mismatch between demands at the workplace and general female physical characteristics.

4.2 Theoretical and practical implications

According to the findings, to reduce job stress levels in the food industry workers studied, the following corrective and supportive measures are recommended:

Reducing non-standard noise in the workplace by applying engineering controls [place a barrier between the noise source and workers (e.g. sound walls or curtains) and enclose the noise source].

4.3 Limitations and suggestions for future research

There are a number of limitations in current study worth mentioning, and it is suggested that they will be considered in future studies. First, there is the possibility of selection bias, due to the cross-sectional nature of the study. Second, in addition to the studied variables, job stress is also affected by other physical environment factors (light, temperature), workload, psycho-social factors, workers’ health status, and lifestyle factors (smoking, alcohol consumption and eating habits) which are beyond the scope of this study. Additionally, in the present study, job stress was not assessed using objective methods. The findings of the study may be more conclusive if objective measures are considered.