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ISSN: 1732-9841 e-ISSN: 1733-4594
The impact of exposure to lead on the occurrence of insomnia in shift workers of the Zinc Steelworks in Miasteczko Śląskie
Authors: Natalia Pasierb, Karolina Magdalena Filipczyk, Przemysław Filipczyk, Łukasz Kunert, Magdalena Piegza, Robert Pudlo
DOI: 10.5603/PSYCH.a2021.0032 Article type: Research paper Submitted: 2021-06-11 Accepted: 2021-09-01
Published online: 2021-09-02
This article has been peer reviewed and published immediately upon acceptance.
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Natalia Pasierb1, Karolina Filipczyk1, Przemysław Filipczyk2, Łukasz Kunert 1, Magdalena Piegza1 , Robert Pudlo1
1Department of Psychiatry, Faculty of Medical Sciences in Zabrze, Medical University of Silesia, Katowice, Poland
2Faculty of Health Sciences, Jan Długosz University in Czestochowa, Czestochowa, Poland
The impact of exposure to lead on the occurrence of insomnia in shift workers of the Zinc Steelworks in Miasteczko Śląskie
Address for correspondence:
Karolina Filipczyk Department of Psychiatry
Faculty of Medical Sciences in Zabrze Medical University of Silesia
42–612 Katowice, Poland Tel.: +48 668 509 994 e-mail: k.filipczyk@o2.pl
Abstract
Introduction: The aim of the study was to assess the effect of lead concentration and shift work on the occurrence of insomnia in the employees of the Zinc Steelworks in Miasteczko Śląskie in Poland.
Material and methods: The study group consisted of 240 employees of the zinc smelter, reporting periodic and control examinations to the Occupational Medicine Clinic. Respondents were asked to complete a questionnaire consisting of the Athens Insomnia Scale and questions about the place and nature of work.
Results: Analyzing the results of lead and zinc proporphiryn( ZPP) levels in the shift and non-shift subgroups of the study group, a slightly higher mean level of lead and ZPP was found in the shift subgroup. Using logistic regression, the probability of insomnia was determined depending on the concentration of lead and ZPP in the organism. As the level of lead and ZPP increases in the study group, the probability of insomnia increases. It is the largest among the examined group working in shifts.
Conclusions: As the blood lead level increases, the likelihood of insomnia in steel plant workers increases.
Key words: insomia, occupational health, consequences, shift work, lead
Introduction
Harmful factors in the workplace
Occupational exposure to lead continues to be a significant problem in Poland, due to the increasing number of employees working in contact with excessive lead levels. Lead is a serious public health issue as it is one of the most widespread environmental and industrial toxins [1]. Although nowadays acute lead poisoning is rare, chronic poisoning symptoms still occur and may have uncharacteristic clinical picture.
Often, neither patients nor doctors associate symptoms such as tiredness, drowsiness and insomnia, memory disorders, general weakness, headache, constipation or elevated blood pressure [1].with exposure to lead [1]. Mandatory monitoring of lead-exposed workers aims to provide appropriate prophylaxis and protect against chronic poisoning. In Poland, the MAC (Maximum Admissible Concentration in Work Environment) value for lead and its inorganic compounds is 0.05 mg/m3 [2]. However, it is believed that in case of lead biological monitoring is essential for the proper assessment of exposure. This is due to the non-linear relationship between the blood lead concentration (PbB) (equivalent to an absorbed dose of lead) and concentration in the air, as well as the possibility to directly link the specific health effects to the PbB levels and to the possibility of excessive absorption at the workplace due to inappropriate behaviour, as well as possible additional exposure outside the workplace. Biological monitoring of the early effects of exposure includes measuring the PbB concentration and, respectively, the level of zinc proporphiryn (ZPP) in blood or the level of δ-aminolaevulinic acid (ALA) in urine [2]. An increase in the ZPP concentration in erythrocytes is delayed in relation to the beginning of exposure to lead, and as the biological half-life period of ZPP is 2–4 weeks, it is a good indicator of chronic exposure over the previous month. The proposed MAC and ACBM (Admissible Concentration in Biological Material) values are based on population-based studies of subjects exposed to lead at their workplace. The upper limit of the admissible concentration of lead in exposed people is 50 µg/dL and higher values may lead to anaemia, encephalopathy or nephropathy. In non-exposed population the maximum concentration allowed in adults is 15 µg/dL [2, 3].
The ZPP levels in the above groups should not exceed 5 and 2.5 µg/g haemoglobin (Hb), respectively [2–
4]. In our region both shift work and exposure to harmful factors in the workplace (including heavy metals) are still very common. Upper Silesia is one of the main coal mining areas not only in Poland, but in the entire Europe and is dominated by coal-based industry. Coal mines, smelters, heat and electricity power plants operate around the clock. Occupational nuisances, including exposure to heavy metals, are ubiquitous at these industrial plants, with lead as the most common harmful agent [5]. Despite the ongoing changes in employment structure, large industrial plants continue to operate in Upper Silesia, exposing their workers to special occupational hazards and diseases.
An example of an industrial plant with numerous harmful factors is the Zinc Smelter Plant in Miasteczko Śląskie. In the early 1990s, the plant introduced an environmental program, abolished the most exploited departments, installed new dust-removal devices, and modernized the old equipment. These pro- environmental activities of the smelter were recognized with certificates and prizes, and thanks to the implementation of the recovery program the company was removed from the list of eighty largest polluters in Poland.
Aim of the study
The aim of this study was to assess the impact of lead levels and shift work on the occurrence of insomnia in the employees of the Zinc Smelter Plant at Miasteczko Śląskie.
Material and methods
The assessment of occupational exposure of workers was performed on the basis of information obtained from the Central Statistical Office (GUS) and derived from the workplace conditions reports that are drawn up by entities employing more than 9 persons in workplaces with harmful factors or health- damaging conditions. The results of laboratory tests related to heavy metal exposure came from the database of the Occupational Health Centre in Miasteczko Śląskie, which carries out all the health examinations for employees of the smelter under the appropriate preventive test schemes. In case of occupational exposure to lead, the tests included: complete blood count (CBC), general urinalysis, serum creatinine, serum lead levels and at least one of the following: the ZPP concentration in erythrocytes or the ALA in urine. The above tests were carried out at the Central Laboratory of Toxicology of Heavy Metals at the Eko-Prof-Med- Medical Centre in Miasteczko Śląskie, which specializes in determination of heavy metals such as cadmium and lead in blood, as well as the ZPP and ALA levels. The Centre has specialized equipment and personnel trained to carry out such determinations. The laboratory performs tests not only for the inhabitants of Upper Silesia, but for the whole Poland.
Determination of the whole-blood PbB levels was performed using the flame atomic absorption spectrometry (FAAS) method. The ZPP levels were measured using the hemato-fluorometric method.
Study group
The study group consisted of 240 employees. 198 people worked shifts (82.5%), and 42 only daytime (17.5%). In some statistical analyzes, two subgroups were distinguished within the study group: shift work and non-shift work.
The study population was made up of male physical workers of the Zinc Smelter Plant in Miasteczko Śląskie, who reported at the Occupational Health Centre for their periodic and follow-up health
examinations. The workers are exposed to inorganic dusts (mainly metallic and mineral), carbon monoxide (CO), infrared radiation and heavy metals (mainly lead, arsenic and cadmium). Participants of the study had to meet the following criteria: at least a year-long employment at the Zinc Smelter Plant, male gender, age 18-60, informed and voluntary consent for participation in the study. People who had filled in the questionnaires incorrectly or handed in incomplete questionnaires, were excluded from the study.The workers were informed of the voluntary and anonymous character of the questionnaire. Written consent was obtained from the head of the health centre to carry out the study.
The study used the Athens Insomnia Scale (AIS), which is the first tool making it possible to assess the symptoms associated with insomnia, which has Polish validation. The studies confirmed good psychometric properties of the scale. It consists of eight items for falling asleep, waking up at night, waking up in the morning, total sleep time, sleep quality, well-being the next day, mental and physical fitness the next day, and daytime sleepiness. Each ingredient is rated on a score scale from 0 (no difficulty) to 3 (severe difficulty). The total AIS score of 8 or more points was considered to be the value, which makes it highly possible to infer inorganic insomnia according to the ICD-10 criteria. The conciseness, reliability and relevance of AIS make this tool useful in clinical practice and scientific research [6, 7]. In the study group, the questionnaires were supplemented with the information on: current PbB and ZPP laboratory results, period worked at smelter according to the date of commencement of employment.
Consent of the Bioethics Committee
According to the position of the Bioethics Committee of the Medical University of Silesia in Katowice of 10 February 2013 No. KNW/0022/KB/31/13, due to the retrospective nature of the study, it was decided at the Committee’s session of 12 February 2013 that the present study was not a medical experiment and no assessment by the Bioethics Committee of the Medical University of Silesia was required.
Statistical methods
Statistical analysis of the obtained results was performed using the MS Office Excel 2007 application and the STATISTICA v.10PL software. The result of the statistical analysis was considered statistically significant if the obtained level of significance p was less than or equal to 0.05 [7–9].
The distributions of the analyzed quantitative variables were statistically significantly different from the normal distribution (tested with the Shapiro-Wilk test). The Mann-Whitney U test was used for statistical comparative analysis between the two groups, and the Kruskal-Wallis test for a larger number of groups.
Logistic regression analysis was used to determine the probability of exceeding the threshold value of 8 points, obtained in the Athens Insomnia Scale, depending on the concentration of lead and ZPP in the body. Logistic regression describes the conditional probability P that the dichotomous dependent variable
(assuming a specific endpoint) will take the value equal to 1 for specific realizations of the independent variables.
Results
The impact of lead concentration on the incidence of insomnia in the smelter plant workers.
In the study group we collected data on the current laboratory test results carried out under the periodic and follow-up examinations at the Occupational Health Centre. Based on the analysis of PbB and erythrocyte ZPP levels we determined the mean values of the results, standard deviation (SD), median, maximum and minimum, as well as the 5th and 95th percentile in the whole study group. The results are shown in Table 1. An analogous method was used for the two subgroups created within the study group, namely the shift workers and the non-shift workers. No statistically significant differences either in PbB or ZPP levels were detected between the subgroups, although the PbB and ZPP were slightly higher in the participants working shifts.
Table 1. Concentration of lead in whole blood and ZPP in erythrocytes in the study group
Statistical parameter Lead level ZPP level
Population 240 240
Average value 36,7 5,2
Standard deviation 10,9 2,8
Median 38,0 4,6
Minimum 4,7 1,0
Maximum 71,9 19,8
5th percentile 17,2 2,0
95th percentile 53,2 11,0
Spearman’s rank correlation coefficient was used to determine the strength of correlation between the following pairs of variables: the AIS score ― PbB and the AIS score ― erythrocyte ZPP. The results are presented in Table 2. Correlation coefficients in the analysed pairs of variables were, respectively, R = 0.21 and R = 0.25, which indicates a positive but weak correlation, i.e. an increase in the value of one variable weakly accompanies the increase of the second variable. It was observed that as the AIS score increased, the PbB concentrations in participants were also slightly higher. Similarly, higher AIS scores corresponded to slightly higher ZPP concentrations. By comparison, when examining the correlation between the erythrocyte ZPP and the PbB, the dependence between which is well-known, we obtained the correlation coefficient of R = 0.56, which means a high correlation.
Table 2. Spearman's correlation coefficients in the study group.
Correlation Type Correlation
coefficient
Statistical Test result
Insomnia scale ― lead concentration in whole blood
R = 0,21 p = 0.0009
Insomnia scale ― ZPP concentration in erythrocytes
R = 0,25 p = 0.0001
ZPP concentration in erythrocytes ― total blood lead concentration
R = 0,56 p < 0.000001
Analogous calculations were performed within the study group depending on whether the participants worked shifts. Correlation between the variable pairs: AIS ― PbB and AIS ― ZPP was also weak, with the correlation coefficient of R = 0.25 and R = 0.23, respectively and with a high correlation between the variable pair PbB ― ZPP.
Logistic regression analysis was used to determine the likelihood of exceeding the threshold score of 8 points obtained in the AIS, depending on lead and ZPP concentration in the body. Based on the determined regression coefficients, the probability curves shown in Figures 1 and 2 were plotted.
Figure 1. The probability of exceeding the threshold value of 8 points. AIS scale depending on lead concentration in whole blood, taking into account shift work
Figure 2. The probability of exceeding the threshold value of 8 points. the AIS scale depending on the concentration of ZPP in erythrocytes, taking into account shift work
By analysing the obtained logistical regression results for the probability of exceeding the threshold of 8 points in the AIS depending on the PbB and the erythrocyte ZPP levels in the study group in total, and taking into account shift work, it was demonstrated that the probability of exceeding said threshold and of a diagnosis of non-organic insomnia increased together with the increase of the PbB and erythrocyte ZPP levels. Also, having analysed the Spearman's rank correlation coefficient for the AIS score and the years worked at the smelter plant, the correlation strength was determined as weak (R = 0.08). Thus, there was no statistically significant relationship between the period of work and the AIS score.
Discussion
The results of the presented study confirmed the hypothesis that the existence of a correlation between the concentration of lead and the occurrence of insomnia.
There are several publications confirming this relationship. One of them is a 2019 study conducted in the city of Neyshabur among soldering workers, which confirmed that people exposed to the lead have poor sleep quality and gastrointestinal disorders [10].
There are a number of publications describing negative effects of lead on various functions of the human body, even where the admissible limits have not been or have been only slightly exceeded. Such results were obtained in several studies of the semen of men occupationally exposed to lead [11–13]. A study by Kasperczyk et al. confirmed the influence of lead exposure on arterial hypertension [14] as well as on the blood levels of ceruloplasmin, haptoglobin, copper and selenium [15]. A study using a population of Chinese children found that elevated lead levels in early childhood were associated with an increased risk of sleep problems and excessive daytime sleepiness in later childhood [16]. Prevention of exposure, including the identification of community-based resources to assist families and landlords in lead-hazard abatement, is the most effective public health strategy, requiring the concerted efforts of health care providers, local, state and Federal public health officials, health policy makers, and relevant community- based services and advocacy groups [17]. Most likely for the above reasons, in recent decades the exposure standards for heavy metals have been becoming more and more stringent, both in the general and occupationally exposed population, and a tendency can be seen towards lowering the permissible limits. It is surprising, however, that despite the widespread conviction that high lead concentrations have a toxic effect on the central nervous system and the great number of studies carried out in this area, the minimum level of lead that causes impaired brain function has yet to be established. Even Goodman et al. in their meta-analysis failed to definitively determine the relationship between the occurrence of subclinical neurobehavioral disorders and occupational exposure to lead [18]. Our study is the first publication that has demonstrated the quantitative influence of PbB on the risk of insomnia.
It should be borne in mind that the study group consisted of zinc smelter employees for whom the permissible lead level is more than 3 times higher than for the general population (50 and 15 μg/dL). The mean value of PbB in the study group was 36.7 µg/dL, which was more than twice the permissible limit for occupationally non-exposed population. A similar pattern applies to the concentration of ZPP, which is considered indicative of lead exposure. While for the general population the permissible ZPP level is up to 2.5 μg/g Hg, for occupationally exposed workers the limit is four times higher. In the study group the mean ZPP value was 5.2 µg/g Hg. Literature contains also studies confirming the occurrence of more severe cognitive disorders in people with long-term occupational exposure to lead. Weuve et al. [19] wrote about the negative effects of lead on cognitive functions in elderly subjects. Weisskopf et al. [20] studied adult pairs of monozygotic twins and concluded that lead-exposed twins showed greater changes on Magnetic Resonance Imaging (MRI) scans. In their study, Sikora and Langauer-Lewowicka [21] described the deterioration of cognitive functions in people working in contact with metallic mercury, inorganic lead and carbon disulfide.
The age at which, the participants of our study began working at the zinc smelter in contact with lead ranged from 18 to 51 years. However, most of the participants started working under these conditions in early adulthood; mean age of commencement of work was less than 26 years. There was a low correlation coefficient between age at starting exposure and the AIS score. No studies have been found in the available literature on the relationship between the age of commencement of occupational exposure to lead and the incidence of insomnia. However, we have determined a statistically significant weak correlation between the age at starting occupational exposure to lead and lead levels. There were slightly higher levels of lead in the blood of workers who had started working in exposure to this element at an earlier age. Again, currently available literature has no reports on the above subject. Certain studies only confirm that higher PbB levels have been observed in a group of people chronically exposed to this metal [22]. As exposure to harmful agents in the workplace is of combined nature, it should be remembered that overlapping occupational hazards may mutually modify one another’s effects on human body, and a simultaneous influence of many harmful factors may not have equal effects to the sum of the actions each of them would have separately.
Recent research shows that workers can be adjusted the constant changes of schedules established by their institutions, and are able to carry out their work, however, individuals with 6 or more years of work in the present-day regime shifts already feel some symptoms of physical and psychological wear, and thus speeding up their ag-ing [23]. In view of the observed results, it can be strongly concluded that the workers comprise the risk group and adequate safety, precautionary and preventive measures could only minimize exposure and the related health hazards.
Conclusion
The risk of developing insomnia increases with the concentration of lead in the blood. Both shift work and exposure to lead affect the risk of insomnia
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