Work ability index and occupational Fatigue in the Workers of a Large Automotive company in Iran

Background Work ability and occupational fatigue are two critical topics in occupational health. We aimed to assess fatigue and the work ability and nd their association in the workers of a large Iranian car company. Methods In this cross-sectional study, we enrolled the workers of SAIPA Automotive Corporation between April and September 2019. We used the Work Ability Index and SOFI-20 questionnaires to assess the work ability and occupational fatigue in the study population, respectively. Then the correlation between work ability index and its association with fatigue dimensions and general study variables were tested. Results From a total of 400 workers, we analyzed the data of 376 workers (mean age = 37.1 ± 5.6 years). The mean WAI was 38.01 ± 7.46. Workers with regular exercise training had higher WAI (P < 0.001). Workers in the press shop had a higher WAI as compared with those who worked in the painting shop or trim shop (P < 0.001). The work ability index had a moderate indirect correlation with all of the fatigue dimensions. There was a signicant association between the work stations (P = 0.002) and WAI. As regards the fatigue dimensions, higher physical discomfort, lack of motivation and sleepiness were associated with a lower WAI (P = 0.048, P = 0.012 and P < 0.001 respectively).


Introduction
Work ability, de ned as the physical and mental ability of a worker to perform and cope with the mental and physical work demands, is an important topic in occupational health [1]. The most useful instrument for assessing is the Work Ability Index (WAI), de ned by the Finnish Institute of Occupational Health in 2006 [2]. It can actually measure whether individuals are at risk regarding their health or employability.
Consequently, WAI has been proved to be a useful predictor for work absenteeism, occupational stress, work disability, early retirement, and even mortality of the workers [3][4][5][6][7]. Therefore, there was a growing interest in recent years to assess work ability index in occupational settings and use it as a predictor or determinants for occupational health issues. Fatigue, de ned as an experience of tiredness, dislike of present activity, and unwillingness to continue performing the task at hand, is also a common nding in workers, particularly in the industrial workers [8,9]. It has been reported to be present in 14-22% of the general population [8,[10][11][12]. In a large study in the Netherlands, 25% of the Dutch workers reported fatigue during their working hours [13]. In occupational fatigue can be both acute or chronic, as well as mental or physical [14]. Fatigue in workers results in decreased ability to perform their tasks at the desired level and can be both mental and physical [15]. Consequently, it can result in mood and behavioral changes and decrease of concentration and attention as well as adverse occupational outcomes and errors, absenteeism, and physical degradation [9,14,16]. Therefore, we aimed to assess fatigue and the work ability and nd their association in the workers of a large Iranian car company.

Methods
In this cross-sectional study, we enrolled the workers of SAIPA Automotive Corporation in the West of Tehran, Iran. We conducted this study between April and September 2019. The study protocol was reviewed and approved by the research board of the Community Medicine Department at Tehran University of Medical Sciences and the medical ethics committee.
Eligible participants were those who had worked in this factory for at least one year and did not have a history of psychiatric disorder or drug abuse. The workers were informed about the study design and its objectives at their workplace. All the participants signed an informed consent form and were reassured that their data would remain con dential and published anonymously for research purposes only. All of the participants received three questionnaires: socio-demographic data, Work Ability Index (WAI), and SOFI. The workers were asked to complete these questionnaires and return them to the research team within a week.

Socio-demographic data questionnaire
The socio-demographic data questionnaire was developed by the authors from the literature review and task analysis. It included data on age, height, weight, marital status, level of education, smoking habits, years of experience, and regular physical exercise practice.

The Swedish Occupational Fatigue Inventory
The Persian version of SOFI-20 was used to assess the occupational fatigue in the participating workers [19]. This instrument consists of ve dimensions: lack of energy, physical exertion, physical discomfort, lack of motivation, and sleepiness. Each dimension is de ned by the content of four expressions related to physiological, cognitive, motor, and emotional responses. Each item can be answered using a 7-degree Likert system from zero ("not at all") to six ("to a very high degree").

Statistical Analysis
Categorical variables were shown as frequency (percentage, while quantitative variables were shown as mean ± standard deviation. The student t-test was used to compare the WAI scores between the subgroups. The normality of the data variables was checked using histograms. We used Pearson's correlation coe cients to evaluate the relationship between the SOFI dimensions and the nal scores of WAI. A leaner regression analysis was utilized to determine the association between WAI and SOFI dimensions, and the results were reported as the regression coe cient and 95% con dence interval. All the statistical analyses were performed using SPSS 21.0. (IBM, USA). The signi cance level of all the statistical tests was set at p < 0.05.

Results
In the current study, from a total of 400 workers, we collected and analyzed the data from 376 workers with complete data. The mean age of the study population was 37.1 ± 5.6 years. The mean WAI was 38.01 ± 7.46. Comparison of WAI between the subgroups of the demographic and personal characteristics showed that those who had regular exercise training had a higher WAI (P < 0.001). In addition, those who were working in the pressing shop had a higher WAI as compared with those who worked in the painting or trim shops (P < 0.001). These comparisons are summarized in Table-1. while 'lack of motivation' had the lowest score.
In the linear regression analysis, there was a signi cant association between the work station (P = 0.002) and WAI. As regards the fatigue dimensions, higher physical discomfort, lack of motivation and sleepiness were associated with a lower WAI (P = 0.048, P = 0.012 and P < 0.001 respectively). There was also a negative borderline association between smoking and WAI (P = 0.052), whereas regular exercise training had a borderline association with WAI (P = 0.059) ( Table-4).  The work ability index had a moderate indirect correlation with all of the fatigue dimensions. However, there was no correlation between WAI and the personal characteristics of the workers (Table-3).

Discussion
In this cross-sectional study, we found that work ability of the workers in a large automotive company was signi cantly affected by the fatigue dimensions. Also, the work station, smoking, and regular exercise training were associated with WAI.
Work ability is an important issue in occupational health as it can affect the e ciency of the workers, and WAI is used as a standard assessment tool worldwide [20]. It can increase employees' sustainable employability by examining their workability and whether they are at risk. Therefore, it helps them to become more employable and reduce levels of sickness and absence from work.
Several studies have discussed variables affecting work ability in various occupational settings. Sociodemographic and psychosocial factors were related to work ability among 168 cemetery workers, and the main factor was the quality of leadership [21]. In a study in Iran on 280 workers, occupational injuries were the strongest predictors of WAI scores and was a strong association between WAI scores and supervisor support, skill discretion, occupational training, sleep quality, work nature, and educational level [20]. However, none of their determinants was similar to our variables. In another study, the authors evaluated the effect of work-related and sociodemographic factors on work ability among 167 employees of a company [22]. Their results showed that age, sex, smoking, regular exercise activity, and sleep quality were signi cantly associated with the WAI. From their variables, smoking and regular exercise training were similar to ours and con rmed our ndings.
On the other hand, fatigue is also a common condition among workers, and its prevalence can even reach 75% in some speci c careers [11]. Various factors have been recognized to in uence fatigue in the general population, such as age, sex, physical activity, and socioeconomic status [12]. A study in a car factory in China showed that the workers generally felt more fatigue and less satisfaction than the engineers [23]. Also, work-related factors, such as working environment, duration of work, and monotony, were signi cantly correlated with fatigue.
Moreover, fatigue can result in a decrease of attention, alertness, and vigilance, prolonged reaction time, distorted judgment, lack of motivation, disturbed memory, reduced eld of vision, and drowsiness in the workers [24]. All these together mean a higher rate of injury and events in the workers at their workplace.
Some studies have discussed the association between fatigue and work ability. In a much-related study to ours that investigated the association between fatigue and work ability in cancer survivors, a reduction of ve points on general fatigue was associated with an improvement of one point in perceived WAI.
Additionally, changes in physical and mental fatigue were signi cantly inversely associated with a change of perceived work ability [25]. These results were con rmed in a comparable longitudinal study [26]. A similar study on breast cancer patients, there was a signi cant relationship between self-reported attentional fatigue and perceived work ability. Although these studies are conducted on cancer patients and do not re ect occupational fatigue or work ability, they are valuable clues to show how much fatigue is related to work ability. In a study on 110 nursing professionals, 43% of the participants had residual fatigue, which was signi cantly associated with a reduction in work ability [27]. One study showed that fatigue was more frequent in unmarried workers, and they also had higher WAI scores than the other marital status workers [28]. Also, those who had mental work tasks had a lower rate of fatigue and higher WIA scores. Overall, fatigue scores were negatively correlated to the WAI scores, which is similar to our ndings.
Our ndings highlight the fact that fatigue is a critical occupational health issue in industrial workers that should be monitored regularly in the workers. Upon diagnosis of fatigue in a worker, proper interventions could help to solve the problem and reduce the burden on both the worker himself and the workplace by reducing the harms and adverse events.

Study limitations
This study included workers from a single company, so its results could not necessarily be generalized to other workers with a different job setting. Moreover, this study is affected by the healthy worker effect because not all the studied workers had a major health problem that could affect the work ability index.

Conclusion
In this study on the workers of a large automotive corporation in Iran, we observed that fatigue dimensions and the WAI. This confers occupational fatigue in industrial workers can signi cantly in uence their output and e ciency. Therefore, the use of fatigue monitoring programs at workplaces can help much to reduce fatigue in the workers and improve safety and e ciency. Future studies should focus on the preventive measures for occupational fatigue or treat it in those who already have it. We declare that this manuscript is original, not previously published, and not under concurrent consideration elsewhere.

AVAILABILITY OF DATA AND MATERIALS:
Data is available upon request.

COMPETING INTERESTS:
The authors declare that no con ict of interest exists.