Association of sleep characteristics and respiratory symptoms at a smelting factory

1 Introduction

Sleep disturbance has been shown to be a major health determinant in occupational settings, especially in those at risk for occupational exposures. Numerous studies have investigated this issue and shown the mean night sleep duration of daytime workers to be from 6.36 hours to 8.22 hours [1, 2] and the prevalence of poor sleep quality based on the Pittsburgh Sleep Quality Index (PSQI) to be 61% [2]. Insomnia as another indicator of sleep disturbance has a prevalence of – 15-26.6% among general population of developing countries [3, 4] and is the leading cause of sleep complaints among adults.

Healthcare and work efficacy are two significant factors in all workplaces, including industries and healthcare centers, which could be affected by different sleep problems. Several studies have examined this issue and shown that sleep problems are related to several body dysfunctions, e.g., functional dyspepsia [5], musculoskeletal pain [6], sexual dysfunction, hypogonadal symptoms [7], and lower urinary tract symptoms [8], which are more common in male workers with poor sleep quality. There is evidence to illustrate the significant relationship between sleep quality and work performance as a prominent factor in athletes [9, 10] and medical students [11]. Meanwhile, reduces the quality of life [12], increases fatigue [13], and results in poor work performance [14].

Specific occupational exposures to hazards in most workplaces can lead to various health problems, especially sleep problems. A limited number of studies has shown a positive relationship between sleep problems and different occupational exposures, including cooking oil fumes [15], lead [16], and metal fume particulate matter [17]. A review of literature also showed exposure to air pollution increases the risk of insomnia among children [21] and adults [22]. Furthermore, some factors including respiratory symptoms [18] and smoking [19] can lead to sleep problems in occupational settings.

Although sleep problems (including poor sleep quality) are a widespread issue in shift workers and a great number of studies have discussed this issue in different workplaces, non-shift workers suffer from the same problem as well [20]. Even though a considerable number of studies have investigated the detrimental effects of shift working on sleep disturbance, a limited number of surveys have studied the harmful effects of occupational exposures on sleep problems among non-shift workers. The current study purposed to investigate sleep characteristics, including poor sleep quality and insomnia, and their relationships with work-related exposures, demographics, and other related variables in workers of a smelting factory.

2 Participants and methods

This cross-sectional study was carried out on workers in a 40-year smelting factory located in the east of Tehran province. A total of 200 male participants were included in the study; anyone found to have a history of respiratory diseases, e.g., asthma, bronchitis, emphysema, bronchiectasis, lung cancer, or any other chronic condition including sleep or mental disorders in the pre-employment assessment were excluded. The ages of the participants ranged between 20 to 65 years, and all men had at least two years of work experience in the selected factory. Among them, 51 workers were from the production process staff (furnace, sand casting, molding, surface cleaning), and the rest were office workers. Their shifts were from 6 AM to 5 PM.

All participants were asked about demographic characteristics (age, marital status, and educational status) and a few questions about smoking, exposure to respiratory pollutants, and using the mask. The weight and height of all participants were measured, and the body mass index (BMI) was calculated (kg/m²). Age and BMI were separated into two subgroups with a cut point of 30.

All participants answered validated Persian versions of the Insomnia Severity Index (ISI) [23] and the Pittsburgh Sleep Quality Index (PSQI) in order to evaluate sleep problems [24].

3 Results

A total of 200 smelter workers were evaluated. All participants were male. The means (SD) of age, weight, and BMI were 39.1 (8.9) years, 82 (14.7) kg, and 26.8 (4.5) kg/m2, respectively. Among all participants, 153 (76.5%) were married, 72 (36%) were smokers rand 145 (72.5%) had a high school diploma or higher education. Of those r51 (25.5%) experienced exposure to a respiratory pollutant e and merely 64 (32%) participants used a mask.

Among all workers r96 (48%) experienced poor sleep quality rand the mean (SD) PSQI score was 4.78 (2.5). The mean (SD) night sleep duration was 6.4 (0.96) hours rand the average proportion of hours asleep per hours in bed was 94%.

The mean (SD) ISI score among participants was 7.69 (4.3). Based on the ISI questionnaire r87 (43.5%) and 10 (5%) of workers had subthreshold and clinical insomnia rrespectively.

The current results did not show a significant relationship for the demographic variables of education level rmarital status rage rBMI subgroups rand smoking with either total ISI score or PSQI score (Table 1).

The mean age rweight rand BMI in workers with or without poor sleep quality (p-values = 0.10 r0.12 rand 0.54 rrespectively) or clinical insomnia (p-values = 0.14 r0.77 rand 0.90 rrespectively) did not show a significant difference.

Data analysis illustrated a significant positive relationship between insomnia and exposure to respiratory pollutants (p-value = 0.03) (Table 1). The relationship between insomnia and poor sleep quality was also positive rbut it was not statistically significant (p-value = 0.07). Analysis did not show a significant relationship between using the mask and either the ISI score or the PSQI score (p-values = 0.18 and 0.26 rrespectively). Further analysis with binominal regression showed participants with more exposure to respiratory pollutants were more susceptible to clinical insomnia (p-value = 0.02 rexp(B) = 0.213) rafter regressing out the confounding effect of smoking (p-value = 0.02 rexp(B) = 4.761 r95% CI = 1.281 17.699) rthe participants with exposure to respiratory pollutants remained susceptible to clinical insomnia.

A lower night sleep duration was observed among participants with exposure to inhalational material (p-value = 0.05) (Table 2). Analysis also showed significant relationships between both marital status and educational level and air pollutant exposure. Higher exposure rates were observed in workers who were married and those who had higher education levels (p-values = 0.04 and 0.03 rrespectively) (Table 2).

The most prominent finding of the current survey was that workers exposed to respiratory pollutants are more vulnerable to sleep disturbance respecially insomnia. This exposure rate among all workers was 25.5%. This study also illustrated that participants with exposure to inhalational materials had lower night sleep duration. Overall rit can be inferred that exposure to repiratory pollutants during work can lead to sleep disturbance in workers.

Among all participants in the current study rthe mean PSQI and ISI scores were 4.78±2.5 and 7.69±4.3 rrespectively. The prevalence of poor sleep quality was 48% rand subthreshold and clinical insomnia were detected in 43.5% and 5% rrespectively. The average duration of night sleep was 6.4 hours.

In line with the current results, several studies have shown a higher rate of insomnia in workers who experienced a specific material exposure to aluminium [27], lead [28], CO [29], or particulate matter with a diameter <10 μm [30]. Additionally, similar to the outcomes reported herein, several studies have investigated the relationship between sleep quality and hazardous material exposures including lead exposure among solderers in the electronics industry [16], metal fume particulate matter exposure in shipyard welders [17], and Chinese cooking oil fumes [15]. According to the current study, using a mask had no significant useful effect on sleep health. This result might be due to non-fitting masks or inappropriate usage. The strengths of the current study are that workers with a specific medical history related to sleep problems were excluded.

In a cohort study in Mexico City, Bose et al. showed fewer sleep hours during the night among children exposed to air pollutants; their result is similar to the current outcome [21]. Likewise, another study showed, Long term exposure to air pollution with a higher concentration of PM1, PM2.5, PM10 (particulate matter with aerodynamic diameters of≤1μm,≤2.5μm and 10μm, respectively), O3 (ozone) increases the risk of insomnia symptoms in adults [22].

The average PSQI score in 1171 daytime workers at a manufacturing workshop in South Korea was 4.46 (men scored 4.4 and women scored 5.48), and they slept 6.36 hours at night. These results are sufficiently close to those of the current study [31]. Moreover, Madrid-Valero et al. in Spain showed that the mean PSQI score was 5.74, and 38.2% of subjects suffered from poor sleep quality and night sleep duration which, was reported to be 6.43 hours. Their results were the same as those of the current study [32]. In China, the maximum mean PSQI score and minimum night sleep duration in civil servants were reported as 4.87 and 7.85 hours, respectively. The minimum PSQI score and maximum night sleep duration in farmers were reported to be 3.74 and 8.22 hours, respectively; overall, 26.9% experienced poor sleep quality (1). Musa et al. reported the PSQI score and poor sleep quality in their study as 5.45 and 61%, respectively (2). Despite the similar results, a small difference is due to the diversity of occupational settings, including shift times, occupational exposure, industry type, lifestyle, and cultural variety. In addition, participants with a specific medical history related to sleep problems were excluded from the current study.

In this study, only 5% of participants had signs of clinical insomnia; however, if subclinical insomnia had been considered, this figure would have reached 48.5%. In line with the current study, Deguchi et al. showed that 36.1% of Japanese workers suffer from insomnia [33], and the prevalence of insomnia among workers with different occupations and working hours in Brazil measured 57.3% [34]. These small differences may originate from the usage of different insomnia questionnaire, for example, the Athens Insomnia Scale (AIS) [33] or the Karolinska Sleepiness Questionnaire (KSQ) [34], and different cut points for each questionnaire to assess insomnia.

5 Limitations

First, in the current study, workers’ sleep status was evaluated by using self-reported standard sleep questionnaires, which may not be the best tool to study sleep quality and insomnia. Objective assessment of sleep duration and problems by full-night polysomnography and actinography would be more informative. Second, the present study was a cross-sectional one; thus, the results are not generalizable to other populations, and the methodology limits the assessment of causal relationships and bidirectional effects, so more investigations are needed in this regard. Third, the respiratory pollutants in the current study were not analyzed and or extracted to emphasize which hazardous materials could cause sleep disturbances, so further studies with analyzes of workspace environment’s hazardous materials (including inhalational ones) are recommended. Fourth, this study is conducted in a single smelting factor; more comprehensive studies in different workspaces with various hazardous exposures would clarify the importance of work exposures to sleep disturbances of workers.

6 Conclusions

Occupational exposures that disturb sleep are not limited to known risk factors such as shift work. Occupational exposures to hazardous material, including inhalational exposures, could cause sleep disturbance, which warrants more attention paid by sleep specialists. Further studies are recommended in this area and on the role of occupational exposures (including inhalational ones) on the sleep health of workers.