Sampling targets to assess occupational health in occupational health and safety management system audits: a mixed-methods research in Japan

Abstract

Background: Occupational health and safety management systems (OHSMS) are the foundation of occupational health and safety activities within an organization. An important element of these systems is audits. However, OHSMS auditors often lack confidence in conducting occupational health audits compared with their proficiency in conducting occupational safety audits. For occupational health to be effectively managed by OHSMS, the sampling competence of auditors engaged in third-party audits should be improved. Therefore, we conducted this study to identify appropriate sampling targets for occupational health related to International Organization for Standardization (ISO) 45001.

Methods: We adopted a mixed methods approach to identify appropriate sampling targets. This involved conducting focus group discussions with experts in occupational health and performing systematic text condensation analysis. The validity of our findings was further reinforced through confirmation by external auditors who specialize in ISO 45001.

Results: In the qualitative phase, 6 occupational health sampling targets were identified, and of these, 5 were subsequently validated in the quantitative phase: (1) Health issues, legal requirements, and occupational health goals identified by the organization; (2) Occupational health risk assessment and control processes; (3) Processes related to occupational health and documented information showing the results of efforts; (4) Organizational roles and functions of occupational health professionals and opportunities for their professional development; and (5) Processes to ensure commitment to occupational health issues and objectives.

Conclusion: To ensure effective and comprehensive occupational health audits, auditors should review these targets. This study will enhance the competence of auditors by identifying appropriate occupational health sampling targets.

1. Introduction

Occupational health and safety management systems (OHSMS) are systems used to manage occupational health and safety (OHS) processes in accordance with an organization’s policies, plans, and operations and are used to improve these aspects of an organization continuously^1,2 and to improve OHS performance.^3,-5 As an OHSMS standard, the British Standard OHSAS 18001 was long used as an international standard, with third-party certifications conducted by private certification bodies.⁶ The ILO-OSH 2001 is a guideline on occupational safety and health adopted by the International Labour Organization (ILO) in 2001, which includes key elements such as Policy, Organizing, Planning and Implementation, Evaluation, and Action for Improvement.⁷ Although it does not require certification, it has been adapted to various national and regional levels, and in Japan it has been the basis for the OHSMS guidelines issued by the Ministry of Health, Labour and Welfare of Japan, along with the Ordinance on Industrial Safety and Health. International Organization for Standardization (ISO) 45001, published in 2018, is an international standard that combines the features mentioned above and is being used worldwide. In Japan, the ISO 45001 has been translated into the Japanese Industrial Standard (JIS Q 45001), and simultaneously, JIS Q 45100 was established to include specific items such as hazard prediction activities and the “5Ss” (Sort, Set in Order, Shine, Standardize, Sustain). The guidelines issued by Japan’s Ministry of Health, Labour and Welfare have been updated to align with these standards.⁸

The ISO 45001:2018 clause 1 scope begins with “This document specifies requirements for an occupational health and safety management system, and gives guidance for its use, to enable organizations to provide safe and healthy workplaces by preventing work-related injury and ill health, as well as by proactively improving its occupational health and safety performance.”¹ The term “occupational health and safety” is used because both occupational health and occupational safety must be properly managed.

Regarding occupational health, in 1950 the Joint International Labour Organization (ILO)/World Health Organization (WHO) issued its first definition of occupational health as follows: “Occupational Health is the promotion and maintenance of the highest degree of physical, mental and social well-being of workers in all occupations by preventing departures from health, controlling risks and the adaptation of work to people, and people to their jobs.” In 1995, the ILO/WHO Joint Committee on Occupational Health revised its aims to mandate that OHSMS effectively oversee areas pertaining not just to occupational safety but also to occupational health.⁹ Furthermore, it is estimated that the global incidence of occupational diseases resulting from occupational health risks exceeds what is reported, underscoring the growing importance of addressing these risks.^10,11 However, at least in operations in Japan, occupational health risks are often not adequately managed by OHSMS and can be a lower priority compared with occupational safety risks; thus, this situation must be improved.¹²

OHSMS have multi-layered mechanisms for enacting continuous improvement. Among them, audits stand out as a significant opportunity for improving entire systems. An audit refers to both internal and third-party audits and is a process that verifies the effectiveness of the entire management system. The results are scrutinized by top management, which serves as a strong motivation to address and improve OHS risks. Therefore, it is important to properly conduct audits and to use the audit findings to formulate and execute improvement plans.^1,13 Audits customarily concentrate on specific elements within an OHSMS to determine the system’s effectiveness. A commonly used method in this process is known as “sampling.” This technique involves selecting a portion of the management system, either through statistical means or based on the auditors’ judgment, to assess its effectiveness.¹⁴ Given the constrained time frame for audits, appropriate sampling is crucial for conducting an effective audit. Previous literature on OHSMS audits includes studies discussing the overall audit process,^15,16 warnings against the ritualization of audits,¹⁷ and observations that external audits often fail to properly integrate psychosocial factors related to occupational health.¹⁸ Additionally, it is often reported that audits of occupational health are inadequate, which likely leads to insufficient management of occupational health within the OHSMS.¹⁹ The reason why adequate audits may not be conducted could be that the numerous requirements of ISO 45001 might consume a significant amount of time for document audits, which could have affected the adequacy of the audits. Alternatively, the fact that there are numerous requirements could be one of the reasons why adequate audits are not conducted.

Occupational health risks are complex, necessitating the expertise of professionals and adherence to established guidelines.^12,20 Therefore, ISO has published ISO 45003 and ISO 45006 as guidelines to address psychological and biological hazards, respectively, which need enhanced understanding, especially in the context of ISO 45001.^21,22 To improve occupational health processes in OHSMS in response to an audit’s findings, it is essential that occupational health sampling is conducted appropriately during the audit and that the competence of the auditor is ensured. This competence is supposed to satisfy the requirements outlined in ISO 17021,^23,24 which describes the competence standards that audit organizations and auditors should achieve. However, OHSMS auditors working at third-party audit organizations in Japan are highly aware of their shortcomings when conducting sampling related to occupational health.¹⁹

To ensure effective management of occupational health by OHSMS, it is crucial to enhance the sampling skills of auditors involved in both internal and external audits. To achieve this objective, it would be beneficial to establish guidelines for the appropriate sampling of occupational health. Accordingly, we employed a mixed methods approach to ascertain suitable sampling targets for occupational health in the context of ISO 45001. Focus group discussions (FGDs) were conducted with professionals who possessed expertise in occupational health and practical experience with OHSMS. The findings from the FGDs were then validated through corroboration with external auditors specializing in ISO 45001.

2. Methods

2.1. Study design

In this study, we adopted a mixed methods approach, encompassing both qualitative and quantitative research paradigms.^25,26 For the qualitative aspect, we selected a research design that included focus group discussions (FGDs) and content analysis for data collection and analysis.²⁷ Additionally, a questionnaire survey was conducted as part of the quantitative research to further validate the findings from the qualitative component.

The purpose, methods, publication, and ethical considerations (eg, protection of personal information) of the study were explained to all participants in written format, and their willingness to participate was confirmed by signing a consent form. The participants were informed that they could withdraw their consent at any time. All of the study procedures were conducted in accordance with relevant guidelines and regulations. This study was undertaken with the approval of the Research Ethics Review Committee of the University of Occupational and Environmental Health (Approval No. R3-012 and Approval No. ER23-025).

2.2. Qualitative research

We undertook this study in accordance with the consolidated criteria for reporting qualitative research.²⁸ In this study, we determined that the occupational health experts in the audited organization were best suited to assess whether the occupational health management system was effective for the audited organization. Therefore, we conducted FGDs with occupational health physicians (OHPs) and industrial hygienists (IHs) who had OHSMS experience to investigate their views on appropriate sampling targets to assess the system’s effectiveness for managing occupational health risks within the audited organization. All authors were experienced occupational physicians. S.K. and A.F., who are certified ISO 45001 auditors, also took part in the FGDs. This study was additionally supported by K.M., who provided training about ISO 45001 for internal auditors, and S.I. and H.S., who have experience in OHSMS practice.

The participants of the FGDs were OHPs and IHs with at least 3 years of experience in occupational health and with experience in OHSMS implementation support, internal audits, or third-party audits. The participants were selected by snowball sampling. FGDs were conducted for each job category. The participants were offered a gift card worth 5000 yen as an honorarium for participating in the study.

FGDs were conducted using an online conferencing system (Zoom Video Communications, Inc, San Jose, CA, USA). Each FGD interview spanned between 90 and 120 minutes. The interviews were audio-recorded with the participants’ consent. We asked them “What sampling should be emphasized in an OHSMS audit to assess an organization’s occupational health activities?” The interviews were conducted between October 2 and October 6, 2021.

The interview transcripts were analyzed using qualitative content analysis with systematic text condensation, which is a descriptive, cross-case analysis strategy.²⁷ The analysts (S.I., S.K., H.S., A.F., and K.M.) read all of the transcripts to obtain an overall impression. All texts were imported into Microsoft Excel, divided into clauses, and then supplemented and simplified as much as possible with peripheral information (eg, the aforementioned explanations designated by indicative words). This process allowed the background of the statements to be understood using only the divided clauses (S.I. and H.S.). We conducted a detailed search for meaning units. Meaning units are short or long pieces of text that help answer a research question.²⁷ The identified meaning units were coded and then sorted into categories that summarized what the meaning units jointly described (S.I., S.K., H.S., A.F., and K.M.). The categories were annotated with illustrative quotations (S.I., S.K., and K.M.). A descriptive narrative of the categories based on meaning units was developed to contextualize the analysis (S.I., S.K., and K.M.). Finally, to ensure the trustworthiness of the data, the relationships between the results and the transcripts were carefully checked according to the Gibbs method to ensure that they did not contain obvious misinterpretations.²⁹ Quotations were translated from Japanese to English by the first author and edited by a professional language-editing service.

2.3. Quantitative research

The questionnaire survey was conducted for auditors affiliated with international certification bodies that specialize in ISO 45001.

This survey was conducted among all 21 auditors affiliated with one ISO 45001 certification body that agreed to participate following our appeal to several ISO 45001 certification bodies, based on the results of qualitative research and the development stage of training and questionnaires. They participated voluntarily, acting on their own accord.

This survey was conducted between January 22 and February 22, 2024. To evaluate its effectiveness, assessments were conducted both before and after the training using a 3-point scale (1, not very important; 2, somewhat important; 3, very important) for each item discussed during the FGDs.

We evaluated the significance of each item to the participants and its deviation from the theoretical median. Initially, we employed the Shapiro-Wilk test to assess the normality of the data distribution for each item. Depending on the distribution, we applied a 1-sample t test for normally distributed data and a Wilcoxon signed-rank test for non-normally distributed data. Both tests were 2-tailed, aiming to compare the observed differences with a theoretical median value of 2.0. All statistical analyses were performed using Stata Statistical Software (Release 16; StataCorp LLC, College Station, TX, USA).

3. Results

3.1. Qualitative research

The characteristics of the participants in the FGDs are summarized in Table 1. Five OHPs and 7 IHs consented to participate in the study. All participants were men, and the average number of years worked in the occupational health field was 21 years for OHPs and 20 years for IHs. The percentage of time spent on OHSMS per year showed considerable variability. No participants dropped out of the study or withdrew their consent during the study.

We extracted 64 meaning units from the OHPs and 66 from the IHs for a total of 130 meaning units. We identified the following 6 categories of sampling targets: (1) Occupational health issues, legal requirements, and occupational health goals identified by the organization; (2) Occupational health risk assessment and control processes; (3) Processes related to occupational health and documented information showing the results of efforts; (4) Organizational roles and functions of occupational health professionals and opportunities for their professional development; (5) Processes to ensure commitment to occupational health issues and objectives; and (6) An organization’s key considerations when conducting an audit. The content and supporting citations for each category are presented below. The extracted categories and the number of meaning units are shown in Table 2.

3.1.1. Health issues, legal requirements, and occupational health goals identified by the organization

This category included, as occupational health sampling targets, the specific issues that the organization has identified, the organization’s priorities, legal requirements, and goals. OHSMS is based on the concept of Plan-Do-Check-Act (PDCA), and it is important to sample input information that contributes to “Plan.” In this vein, it is recommended to confirm the existing health-related issues identified in the organization and objectives focused on the issues in the management system.

The organization determines external and internal issues in accordance with ISO 45001 4.1 (Understanding the organization and its context). (OHP 4).

Health issues for each organization must be determined. Sampling should be tailored to the challenges of each individual organization. (OHP 5).

I ask about the legal requirements in occupational health. They are identifying and dig deeper from there. (IH 6).

I confirm the occupational health objectives of the management system. (IH 5).

3.1.2. Occupational health risk assessment and control processes

This category included methods for identifying occupational health hazards and assessing and controlling risks. It is important to sample whether the efforts that have been taken are in accordance with the Plan. It is recommended to ensure that existing occupational health hazards, namely, physical, chemical, biological, ergonomic, and psychosocial factors, are managed in an appropriate manner.

How many are there and how they are evaluated must be determined. (OHP 5).

I think sampling should focus on whether health hazards have been identified and risks have been assessed. From there, check control methods and work environment measurement results. (IH 2).

To assess risk, I will ask about high risks, such as control class-III (the worst category) and ask how occupational health risk measures and reassessment are being conducted, and check the site. (IH 3).

3.1.3. Processes related to occupational health and documented information showing the results of efforts

This category included sampling documented information that provides evidence of how measures are implemented for occupational health as a process. In addition to the PDCA cycle, ISO 45001 requirements provide a Support section related to documented information, human resources, and competence. In this vein, it is recommended to confirm documented information on occupational health processes and results of efforts.

We can’t evaluate performance in audits, but if we say “Show me the documentation” we can check the process. (OHP 2).

I check the health officer’s patrol records and ask them what the comments indicate and what the results were. (IH 6).

3.1.4. Organizational roles and functions of occupational health professionals and opportunities for their professional development

This category included what responsibilities and authorities are assigned to the professionals in charge of occupational health activities (such as OHPs, IHs, and occupational health nurses) and what professional growth opportunity initiatives are in place in the organization. It is important to sample their resources and competence.

I ask how occupational health professionals are assigned in the management system. (OHP 2).

It’s important for the improvement of the whole system to improve occupational health professionals’ competence, so I ask whether the organization is offering continuing education for them. (IH 3).

3.1.5. Processes to ensure commitment to occupational health issues and objectives

This category included sampling to assess whether the PDCA cycle is being followed in response to the occupational health goals and issues in the Plan and whether consistent operations are correctly executed as “Do” items, whether they are verified, and whether they lead to continuous improvement. In this vein, it is recommended to confirm the Do items and cycle continuation on occupational health.

I would check the process—whether the organization understands the issues, has chosen the right methods, and who is implementing them to ensure the occupational health objectives are fulfilled. (OHP 4).

I ask whether the PDCA cycle is working—that the organization has created and is operating an occupational health system, and that it is leading to performance results. (IH 7).

3.1.6. An organization’s key considerations when conducting an audit

This category of sampling targets refers to an audited organization’s checklists that are presented to auditors in advance of an audit. Such checklists including occupational health should be considered important for sampling.

I give the auditors a list of questions that I want my company to ask. (OHP 2).

I have created checklists of about 300 items, including occupational health, and have the site check those items in advance. Then, after checking the lists during the internal audit, I visit the site and mutually check the lists and the site. (IH 7).

3.2. Quantitative research

The characteristics of the participants in the questionnaire survey are summarized in Table 3. Eleven individuals consented to participate and offered their responses.

There were 6 categories within this study. The score distributions for Categories 1, 2, 3, and 5 deviated from a normal distribution, whereas Categories 4 and 6 followed a normal distribution pattern. Using the Wilcoxon signed-rank test, Categories 1, 2, 3, and 5 showed a significant divergence from the theoretical median of 2.0. In contrast, the 1-sample t test revealed that Category 4 also significantly deviated from this median. However, when Category 6 was subjected to the 1-sample t test, it did not exhibit any significant variation from the theoretical median of 2.0. The details of the results are presented in Table 4.

4. Discussion

We conducted this study to identify appropriate occupational health sampling targets that are in accordance with ISO 45001 and found 6 main categories of targets. We recruited participants from 2 fields of experts—OHPs and IHs—to avoid bias toward one of these areas. The external auditors identified all categories, with the exception of Category 6 (An organization’s key considerations when conducting an audit), as significant components of the audit process. To our knowledge, this is the first study to specifically identify sampling targets for occupational health in OHSMS. The results were consistent with either ISO 45001 or ISO 17021-1for the treatment of occupational health risks, except for Category 6.

Organizations have different occupational health issues, legal requirements, and occupational health goals, and each organization needs to decide which issues to address. Inputting OHS policies, program attributes, and hazards into the OHSMS is a key element for OHSMS to function,³⁰ and auditors should properly understand and sample these aspects. In addition, ISO 45001 requires top management to provide a framework for setting OHS objectives, and relevant departments should establish concrete actions to achieve them.¹ Auditors should ensure that the occupational health objectives have been adequately established.

Developing and implementing OHSMS risk control strategies are key elements of occupational health risk assessment and control processes.³⁰ These correspond to “Plan” and “Do” in the PDCA cycle. For example, in the management of chemical substances in Japan, the risk assessment and control methods are not clearly defined by the law, and organizations can choose the method of implementation. It is important for auditors to sample and confirm that the methods are appropriate for the issues identified by the organization and that if the audit identifies that the occupational health risk is unacceptable, priorities for risk reduction should be considered and appropriate actions taken.

Documenting information about processes related to occupational health and the results of efforts that have been made is important for 2 main reasons. One is to ensure process continuity; the other is to provide proof that an action has been completed.³¹ The auditor can evaluate the continuity of processes and the results of occupational health activities by sampling information about the procedures and related processes, as well as information that proves that the management system is operating as intended and activities have been completed.

The organizational roles and functions of occupational health professionals should be defined and opportunities for professional development given to ensure that these professionals are highly trained because occupational health activities have become more complex owing to the increasing variety of risks. In Japan, occupational hygienists are certified by the Japan Association for Working Environment Measurement. This certification is also accredited by the International Occupational Hygiene Association. Occupational physicians hold a specialty certification as occupational health physicians, accredited by the Japan Society for Occupational Health. Both certifications require fulfilling specific criteria within 5 years to maintain their qualification. In organizations, the roles and responsibilities of these positions are similarly sophisticated and require a high level of professional knowledge. ISO 45001 requires that top management ensure that the responsibilities and authorities of relevant roles are clarified and that appropriate human resources are provided. Auditors can assess an organization’s commitment to occupational health by sampling whether appropriate positions, responsibilities, and authorities are defined and the necessary education provided.

Regarding processes to ensure commitment to occupational health issues and objectives, it is important to confirm that the operation is correctly verified against objectives and issues and that a cycle of continuous improvement is in place. The PDCA concept is an iterative process used by organizations to achieve continuous improvement,¹ and the processes related to “Check” and “Act” must be sampled. For example, in the assessment of OHS risks posed by chemical substances, “prevention of explosion and fire of chemical substances” is listed in the Plan for occupational safety, and “prevention of health hazards such as carcinogenesis and dermatitis” is listed in the Plan for occupational health. According to these plans, regarding safety, “Do” states to optimize use and storage methods to prevent explosions and fires, whereas regarding health, “Do” states to optimize work methods to prevent harmful effects on workers. To properly implement these actions, an OHS monitoring, measurement, analysis, and performance evaluation process should be established and maintained, and any incidents or nonconformities reported and corrected. Thus, “Check” and “Act” in occupational health can be accomplished through processes commonly used with occupational safety. Therefore, auditors should focus on sampling that the OHS processes include operations consistent with the occupational health objectives and issues and that there is an iterative cycle of continuous improvement.

To make audits more effective, the organization may present checklists including occupational health for the auditors to review in advance. The effectiveness of the OHSMS within each organization is shaped by various factors, such as the organization’s historical background, corporate culture, leadership objectives, and current operational conditions. By using materials provided by the organization, auditors can more precisely focus on the occupational health initiatives most critical to the organization, thereby increasing the audit’s impact. Consequently, our qualitative research advises auditors to observe the organization’s checklist. Nonetheless, our quantitative analysis did not identify this sampling approach as notably impactful, potentially due to the exclusion of Category 6 from the ISO 45001 and ISO 17021-1 standards, suggesting that this category should merely serve as a supplementary reference.

In summary, both internal and external audits underscore the necessity for organizations to integrate measures for managing occupational health risks into the PDCA cycle, mirroring the approach taken for occupational safety risks. The above categories were organized such that Category 1 was “Plan,” Category 2 was “Plan” and “Do,” Categories 3 and 4 were “Support,” and Category 5 was “Do,” and confirming the continuation of the PDCA cycle. In other words, the results showed that all elements of the PDCA cycle required by ISO 45001 were met. Additionally, the results were in line with occupational safety sampling perspectives. However, it is based on the premise that occupational health risks are not adequately managed by OHSMS, unlike those of occupational safety.¹² “Check” and “Act” are the processes that occur after Plan and Do; if Plan and Do do not address occupational health issues, then “Check” and “Act” will not be useful. Therefore, the organization needs to fully include occupational health in the “Plan,” and auditors need to fully understand the occupational health issues and legal requirements. For auditors to properly sample these issues, we believe that it is necessary for them to have an understanding of these issues and confidence in resolving them. Thus, auditors should acquire the knowledge necessary to conduct appropriate audits related to occupational health.

When organizations have multiple management systems, such as ISO 9001 for quality and ISO 14001 for environment, integrated audits are sometimes conducted. Although these audits offer advantages like time savings and cost reduction, they also present potential drawbacks. These include the carryover of nonconformities across different systems and a reduction in the time allocated for occupational health audits due to overall audit time reduction.³² This necessitates more precise sampling in occupational health audits. Therefore, enhancing auditors’ expertise is crucial to maintaining audit quality. This study predicts that the importance of this will further increase in integrated audits. Furthermore, many ISO 45001 auditors are also qualified to audit ISO 9001 and ISO 14001, with their careers commonly initiated in the quality and environmental sectors. This career trajectory might contribute to a deficit in confidence when conducting occupational health audits. To address highly specialized audits like those in occupational health, it is crucial to offer auditors opportunities to enhance their competence in this specific area.

The results of this study provide a framework for sampling that can be easily understood by users who are not specialists in the occupational health field. However, a limitation to this study is that the results do not comprehensively cover the topic of occupational health sampling because the study was organized with the purpose of improving auditor competence. Furthermore, these findings are pertinent to evaluating ISO 45001 implementation or certification-seeking workplaces. However, they may not completely encompass management system considerations for all workplaces exposed to health risks.

These limitations should be kept in mind when using the results of our study. Although this qualitative study included experts in Japan with accomplishments and qualifications in the field of occupational health, not all of them were qualified as external auditors of OHSMS, and thus their statements are not necessarily in line with the standards. Therefore, 2 qualified auditors participated in the study group to ensure conformity with the standards (S.K. and A.F.) and quantitative studies were conducted with external auditors to ensure validity. Recruitment was carried out using snowball sampling, but the number of participants was limited. This was due to the limited total number of experts in occupational health and OHSMS in Japan. Consequently, there are limitations in the scope and depth of the data collected, and caution is needed when generalizing the results. Although we appealed to multiple certification bodies for the quantitative study, it ultimately involved auditors from only a single body, resulting in a limited sample size. These external auditors specialize in OHSMS; however, they may not necessarily possess expertise in occupational health. Conducting surveys with more certification bodies and external auditors would likely yield more reliable results. Future research should evaluate the effectiveness of these sampling targets across various industries or auditing contexts. Additionally, developing practical guidelines for auditing ISO 45001 processes based on these findings could help auditors enhance occupational health assessments.

5. Conclusion

For occupational health to be effectively managed in OHSMS, that is, for the organization’s occupational health issues to be properly assessed and continually improved, auditors should review the occupational health issues, legal requirements, and occupational health goals identified, the occupational health risk assessment and control processes, the documented information about the processes and results, the position of the occupational health professionals, the processes that meet the objectives, and whether the process is being continuously improved. Auditors are expected to have a general knowledge of occupational health to be able to verify these sampling categories. We believe that auditors in Japan often lack confidence in auditing occupational health issues; thus, it is necessary to provide them with appropriate educational opportunities.

Acknowledgments

We thank Katherine Thieltges from edanz (https://jp.edanz.com/ac) for editing a draft of this manuscript.

Author contributions

Instruction was provided by S.K. and K.M., who had experience in qualitative research. S.I., S.K., and K.M. designed the study and contributed to writing the manuscript. All authors performed data collection, and read and approved the final manuscript.

Funding

This study was supported by Komatsu Ltd and a UOEH Research Grant for Promotion of Occupational Health (R2-R3).

Conflicts of interest

S.I. is an occupational health physician at Komatsu Ltd. S.K. is a health and safety consultant at Komatsu Ltd. The other co-authors have no conflicts of interest to disclose.

Data availability

The data underlying this article will be shared on reasonable request to the corresponding author.

Present address: Department of Occupational Health Practice and Management, Institute of Industrial Ecological Sciences, University of Occupational and Environmental Health, 1-1 Iseigaoka, Yahatanishi-ku, Kitakyushu 807-8555, Japan

References