Respiratory tract diseases and lung function in coke oven plant workers: a 50-year global systematic evaluation and meta-analysis

Abstract

This systematic review examines the impact of coke oven emissions (COEs) on workers’ health, with an emphasis on respiratory diseases and mortality. This study aims to investigate the relationship between exposure to polycyclic aromatic hydrocarbons (PAHs), particulate matter (PM_2.5), benzene (C₆H₆), and coal tar pitch volatiles (CTPV) among coke production workers and their association with respiratory diseases, lung function, lung cancer, and mortality from malignant respiratory diseases. A comprehensive literature search was conducted in Web of Science, Scopus, PubMed, Medline, and Embase up to December 16, 2024, based on the PECO (Population, Exposure, Comparator, and Outcomes) framework. A total of 38 studies met the inclusion criteria. PAHs were the most studied pollutants, with 47% of studies examining their effects in environments like coke oven factories, where workers are at high risk of COPD, asthma, and lung cancer. 37% of the studies focused on lung cancer, highlighting worries about the part contaminants play in the development of cancer in industrial settings. 18% of studies observed non-cancerous diseases such COPD and chronic bronchitis, as well as respiratory symptoms like chronic cough and shortness of breath. Data collection methods differed among research, with questionnaires and interviews constituting the predominant approach (66%), especially for evaluating respiratory diseases such as chronic bronchitis and asthma. This study also assesses the mortality risks associated with coke oven emissions, demonstrating a significant association between such exposure and higher mortality. However, the significant heterogeneity across studies (I-squared value of 98.9%) indicates considerable variation in methodology. This study shows that workers exposed to coke oven emissions and other industrial pollutants face significant health hazards, underlining the need for stricter occupational health and safety standards to safeguard vulnerable populations. The findings also highlight the necessity for continuous health monitoring of workers and improved management of industrial pollutants to prevent chronic respiratory diseases and enhance workers’ quality of life.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12931-026-03602-9.

Introduction

The coke-oven business is a vital segment of the global industrial sector, primarily because of its indispensable role in steel manufacturing [1]. Coke, a carbon-rich fuel and reducing agent, is essential for blast furnace processes for iron and steel production. As a result, coke manufacturing is crucial for nations with extensive steel industries, such as China, India, the United States, Russia, Japan, and South Korea [2]. Coke production involves pyrolyzing bituminous coal at 1000–1100 °C in sealed ovens under anaerobic conditions, releasing volatile components and producing metallurgical coke, coal gas, coal tar, and ammoniacal liquor [3]. The resultant coke is a dense, porous material with a high carbon content, crucial for sustaining high temperatures and creating a reducing atmosphere in blast furnaces [4]. Nevertheless, the pyrolysis process also produces notable environmental and occupational pollutants, including sulfur compounds, benzene, CTPVs, and PAHs, all of which are linked to lung toxicity and cancer [5]. Despite these concerns, coke ovens are still widely used worldwide due to the ongoing demand for steel in construction, transportation, and industrial production [6].

Occupational exposure to pollutants, especially in high-risk industries such as coal manufacturing, has considerable implications for workers’ respiratory health. Workers in these environments are often exposed to hazardous contaminants, such as polycyclic aromatic hydrocarbons (PAHs), fine particulate matter (PM_2.5), nitrogen dioxide (NO₂), sulfur dioxide (SO₂), benzene (C₆H₆), and volatile compounds from coal tar pitch (CTPV). Long-term exposure to these pollutants may result in chronic respiratory diseases, presenting significant health hazards within industrial settings [7, 8]. The most common associated diseases include lung cancer, chronic bronchitis, COPD, and other respiratory system cancers such trachea, bronchus, and lung cancer [9, 10]. Exposure to PAHs can harm lung cells, increase inflammation, and raise oxidative stress, leading to chronic respiratory conditions and decreased lung function [11, 12]. Lung cancer is a significant long-term consequence of such exposure [13].

Many cohort, cross-sectional, and case-control studies have shown that long-term exposure to industrial pollution increases chronic respiratory diseases, including lung cancer. Cohort studies are especially beneficial for comprehending long-term impacts, as they monitor lung function and disease progression over time [14]. Cross-sectional studies are effective for evaluating workers’ current health status and the association between pollutant exposure and respiratory illnesses in settings with continuous exposure.

This review aims to examine the impact of coke oven emissions on the respiratory health of workers, specifically focusing on chronic conditions such as COPD, asthma, and lung cancer. The primary objective is to investigate the correlation between exposure to coke oven pollutants and increased mortality from respiratory diseases through a comprehensive meta-analysis of existing data. Additionally, the study assesses the quality and validity of 38 current studies, identifying potential biases in their data collection methods. The review also compares various data collection techniques—such as questionnaires, interviews, and pulmonary function tests—evaluating their effectiveness in determining disease prevalence and associated symptoms. Moreover, the geographic and temporal distribution of studies is analyzed to highlight the growing recognition of this occupational health issue over recent decades. Lastly, this study offers suggestions for improving research methods and strengthening health and safety regulations to better protect workers exposed to toxic chemicals in industrial settings.

Materials and methods

Protocol of the study

This systematic review followed the methods for study selection, data extraction, and analysis outlined in the Cochrane Protocol [15] and was registered with the International Prospective Register of Systematic Reviews (PROSPERO) under registration number CRD42025649142. The literature search was conducted on December 16, 2024, using the Web of Science, Scopus, PubMed, Medline, and Embase databases, with no temporal limitations. The review procedure adhered to the principles outlined in the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) [16]. Figure 1 illustrates the study selection process. A total of 1012 articles were initially retrieved from five databases: Scopus (222), PubMed (152), Web of Science (338), Embase (151), and Medline (149). After eliminating 261 duplicates, 751 articles were retained for title and abstract screening. Of these, 64 studies were excluded – comprising review articles (12), conference papers (16), book Chaps. (5), and other formats, such as letters, editorials, short surveys, and non-English articles (31). Of the 687 full-text articles evaluated for eligibility, 649 were excluded for failing to meet the inclusion criteria. 38 studies were ultimately included in the final systematic review. The review aims were organized using the PECO (Population, Exposure, Comparison, Outcome) framework, and the study selection followed PRISMA criteria (Table 1) [17].

Fig. 1.

PRISMA flow chart for the selection of articles in the systematic search on respiratory tract diseases in coke oven plant workers

Table 1.

Overview of the PECO Framework Used in This Studye

Component	Description
P (Population)	Workers in coke oven plants.
E (Exposure)	Exposure to industrial pollutants such as Polycyclic Aromatic Hydrocarbons (PAHs), Benzene (C6H6), Coal Tar Pitch Volatiles (CTPV), Particulate Matter (PM2.5), nitrogen oxides (NOx), sulphur dioxide (SO2), and carbon monoxide (CO).
C (Comparison)	Workers with minimal or no exposure to the industrial pollutants associated with coke oven emissions.
O (Outcome)	Respiratory diseases (e.g., Chronic Obstructive Pulmonary Disease [COPD], lung cancer, and chronic bronchitis), pulmonary function (e.g., FVC, and FEV1), and mortality rates due to respiratory diseases.

Search strategy

The search was conducted using a combination of keywords: Asthma OR bronch* OR laryng* OR “Kartagener Syndrome*” OR respiratory* OR lung* OR “Acute Chest Syndrome*” OR “Cystic Fibrosis” OR pulmonary* OR hemoptysis* OR “Hepatopulmonary Syndrome” OR “Scimitar Syndrome” OR “Choanal Atresia” OR epistaxis OR “Lethal Midline Granuloma” OR nasal* OR nose* OR rhin* OR pleur* OR chylothorax OR hemopneumothorax OR hemothorax OR hydropneumothorax OR hydrothorax OR pneumothorax OR “Acute Chest Syndrome” OR “Cheyne-Stokes Respiration” OR dyspnea OR “Meconium Aspiration Syndrome” OR sinusitis OR trach* OR thoracic* OR “Chronic Obstructive Pulmonary Disease” OR “COPD” OR spirometry* OR sarcoidosis OR “acute lower respiratory tract infection*” OR “ALRI” OR tuberculosis OR “Acute lower respiratory infection*” OR “Restrictive lung disease*” OR “FVC” OR “FEV*” OR “peak expiratory flow*” OR “PEFR” OR “PEF” OR “Forced vital capacity” OR “forced expiratory volume” OR pneumonia AND"coke oven*”. The selected keywords were searched across several databases, including Web of Science, Scopus, PubMed, Medline, and Embase.

Study eligibility criteria

The review encompassed English-language papers published from 1972 to 2024. Of the 1,012 papers found in electronic databases, 38 matched the inclusion criteria. The review focused on research examining occupational exposure to pollutants in industrial environments and their long-term health implications, including respiratory diseases and cancer risk, with a particular focus on lung cancer. To ensure relevance and consistency, we excluded non-English publications, conference papers, books, case reports, narrative articles, letters to the editor, and all animal, laboratory, modeling, and simulation studies.

Study selection and data analysis methods

Two researchers, A.H.Kh and S.Gh, selected and extracted pertinent keywords. A.H.Kh conducted a search of titles, abstracts, and MeSH terms (if available) using the designated keywords, and the findings were organized in EndNote X20. The systematic review concentrated on original, peer-reviewed publications in English. Following the elimination of duplicates, S.Gh and A.H.Kh independently reviewed the studies and extracted relevant data. All ambiguities or disputes were addressed through dialogue between the two reviewers. If consensus was unattainable, the third author (A.GK) was consulted for the conclusive judgment. This study employed a meta-analysis to assess the mortality effects associated with exposure to coke oven emissions (COE), integrating findings from multiple studies to explore the cumulative impacts and relationships between these pollutants and mortality from lung cancer. In this context, the effect size represents the number of deaths caused by lung cancer attributable to exposure to coke oven pollutants. The results are presented as the number of lung cancer deaths along with 95% confidence intervals (95% CI).

To visualize the cumulative effects across studies, a forest plot was generated, which illustrates the variation in effect sizes across the different studies included in the analysis. This plot highlights the overall impact of exposure to coke oven pollutants on mortality from lung cancer. Publication bias was assessed using a funnel plot, which visually inspects for asymmetry in the distribution of effect sizes. To confirm any asymmetry, the Egger test was performed, providing statistical insight into the presence of publication bias [18].

Synthesis of results

Two researchers, S. Gh and A. H. Kh, independently examined the complete texts of the remaining papers. They eliminated extraneous articles through title and abstract evaluation. Data from each eligible study included author, year, study design, sample age, gender, country, study duration, media, pollutant types, sample size, collection method, disease/symptom types (e.g., prevalence of COPD, chronic bronchitis, and respiratory symptoms), mortality data (e.g., lung cancer, nonmalignant or malignant respiratory diseases, and tuberculosis), and lung function metrics (e.g., FVC%, FEV1%, FEV₁/FVC, FVC, FEV₁, and FEF_25-75%).

The data were synthesized utilizing common formats. Table 2 delineates the characteristics of the study. The chosen papers were examined using a systematic, comparative methodology to evaluate the influence of different occupational exposures to coke ovens on respiratory diseases. The papers were classified according to the incidence of respiratory disorders (including COPD, asthma, and lung cancer), pulmonary function, and respiratory mortality rates. The impact of diverse pollutants, including PAHs, particulate matter (PM), and gaseous pollutants (NO₂, SO₂, and CO), on the disease burden of respiratory diseases was analyzed.

Table 2.

Study Characteristics and Quality Assessment of Respiratory Disease and Symptoms; - lack of data

First Author & Publication Year	Study Design	Age	Gender	Country	No. of Samples	Collecting Method	Type of Disease/Symptoms	Quality
Carol K. Redmond, 1972 [19]	Cohort	-	Male	USA	3305	Data	Lung, Bronchus, and Trachea Malignant Neoplasm, Tuberculosis of Respiratory System	Q3
Charles Mittman, 1974 [20]	Cross-sectional	45.9	Male	USA	246	Questionnaire, Blood Samples	Chronic Bronchitis, Respiratory Symptoms	Q1
Hiroyuki Sakabe, 1975 [21]	Cohort	14–75	-	Japan	2178	Data	Lung Cancer	Q2
Sati Mazumdar, 1975 [22]	Cross-sectional	< 35- >54	Male	USA	4661	Data	Lung Cancer	Q3
Charles Mitiman, 1979 [23]	Cross-sectional		Male	USA	245	Questionnaire	Chronic Bronchitis	Q3
J.F. Hurley, 1983 [24]	Cohort	15-≥65	Male	UK	6608	Data	Trachea, Bronchus, and Lung Cancer, Bronchitis, Emphysema, Asthma	Q1
Carol K. Redmond, 1983 [25]	Cohort	≥ 20	Male	USA	1991	Data	Cancers of the Respiratory System, Nonmalignant Respiratory Diseases, Lung Cancer, Cancers of the Lung, Bronchus and Trachea	Q1
Roberta Madison, 1984 [26]	Cohort	≥ 45	Male	USA	3799	Blood, Urine, and Sputum Tests, Spirometry, Chest X-ray, Questionnaire	-	Q3
JP Bertrand, 1987 [27]	Cohort	62	Male	France	534	Data	Respiratory Disease, Lung Cancer	Q3
G M H Swaen, 1991 [28]	Cohort	-	Male	Netherlands	5659	Data	Respiratory System, Mouth and Pharynx Cancer, Larynx Cancer, Trachea and Lung Cancer	Q3
Sati Mazumdar, 1991 [29]	Cohort	≥ 20	Male	USA	1100	Data	Lung Cancer	Q1
N Chau, 1992 [30]	Cross-sectional	65.9	Male	France	231	Questionnaire, Spirometer, Pulmonary Radiography	-	Q3
L. Mayer, 1992 [31]	Cohort	66.6	Male	France	340	Medical Examination, Interview, Questionnaire	Chronic Bronchitis, Obstructive Syndrome, Restrictive Syndrome, Upper Respiratory and Alimentary Tract Cancer, Lung Cancer/Wheezing, Cough, Phlegm	Q1
Giorgio Assennato, 1993 [32]	Cross-sectional	40.8	Male	Italy	23	Spirometry, Questionnaire, Interview, Blood Test	-	Q1
N Chau, 1993 [33]	Cohort	64.8	Male	France	65	Data	Lung Cancer, Upper Respiratory Tract, Alimentary Tract Cancer	Q3
Carol K. Redmond, 1993 [25]	Cohort	< 20-≥30	Male	USA-Canada	2920	Data	Lung Cancer	Q1
Joseph Costantino, 1995 [34]	Cohort	-	Male	USA-Canada	5321	Data	Buccal and Pharyngeal Disease, Respiratory Disease, Non-malignant Respiratory Disease, Tuberculosis	Q1
G. M. Davies, 1977 [35]	Cohort	15-≥75	Male	Wales	250	Data	Respiratory Disease, Lung Cancer	Q3
J L Corhay, 1998 [36]	Cross-sectional	42	-	Belgium	137	Clinical Examination, Questionnaire, Pneumotachograph	Chronic Bronchitis	Q1
Suresh H. Moolgavkar, 1998 [37]	Cohort		Male	USA	4452	Data	Lung Cancer	Q1
E. Kolarzyk, 1999 [38]	Cohort	55.7	Male	Poland	31	Questionnaire, Medical Examination, Spirometer, Data	Central Bronchi Obstruction, Small Bronchi Obstruction, Emphysema	Q2
J Wu, 2002 [39]	Cohort	34.7	Male	Australia	1447	Data	-	Q1
J Wu, 2004 [40]	Cohort	38.4	Male	Australia	580	Data	-	Q1
Jian Wu M, 2004 [41]	Cohort	41	Male	Australia	764	Questionnaire, Spirometer	Cough, Phlegm, Wheeze, Shortness of Breath	Q1
Y Hu, 2006 [42]	Case-control	Case = 35.9, Control = 35.7	Male-Female	China	923 (Case = 712, Control = 211)	Questionnaire, HPLC, Spirometer, COPD GOLD Criteria	Chronic Obstructive Pulmonary Diseases (COPD), Chronic Cough or Chronic Phlegm	Q1
Yeon-Soon Ahn, 2006 [43]	Cohort	-	Male	Korea	-	Data	-	Q2
Jan Birdsey, 2007 [44]	Cohort	20–90	Male	USA	4668	Data	Lung Cancer	Q1
Brian G. Miller, 2013 [45]	Cohort	-	Male	UK	6405	Data	Lung Cancer	Q1
Suhan Wang, 2016 [46]	Cohort	40.6	Male-Female	China	1243	Questionnaire	-
Meili Shen, 2018 [47]	Case-control	Case = 36.25, Control = 33.26	Male	China	505 (Case = 390, Control = 115)	Blood Test, Questionnaire, Spirometer, GC-MS	-	Q1
Luigi Vimercati, 2020 [48]	Cohort	< 35- >65	Male	Italy	104	Questionnaire, Spectrophotometry, Gas Chromatography-Mass Spectrometry (GC-MS), HPLC	Lung Cancer	Q1
Wei Wei, 2020 [49]	Cohort	40.6	Male-Female	China	1243	Questionnaire, Spirometer, Chemistry Analyzer, GC-MS	-	Q1
Wei Wei, 2020 [49]	Cohort	40.6	Male-Female	China	1243	Questionnaire, Spirometer, Chemistry Analyzer, GC-MS	-	Q1
Xiulong Wu, 2021 [50]	Cross-sectional	41.3	Male-Female	China	1762	Interview, Questionnaire, Spirometer		Q1
Lu Liu, 2022 [51]	Cohort, Cross-sectional	35	Male-Female	China	Cross-sectional = 629, Cohort = 304	Questionnaire, Interview, HPLC-MS, Lung Function Analyzer	-	Q1
Liqiong Guo, 2022 [52]	Case-control	Case = 36.3, Control = 35.4	Male-Female	China	768 (Case = 558, Control = 210)	Blood Test, Questionnaire, Spirometer, GC-MS, HPLC	-	Q1
Lu Liu, 2023 [53]	Cohort	40	Male-Female	China	307	HPLC with Fluorescence Detector, Questionnaire, Interview, Lung Function Analyzer	-	Q1
Jiajun Wei, 2024 [54]	Cohort	40	Male-Female	China	307	HPLC with Fluorescence Detector, Questionnaire, Interview, Lung Function Analyzer	-	Q1

Risk of bias assessment

Two independent reviewers (S.Gh and A.H.Kh) appraised the risk of bias (RoB) in the chosen non-randomized studies utilizing the Risk of Bias in Non-randomized Studies – of Exposures (ROBINS-E) instrument, designed to assess bias in observational exposure studies. Any disputes between reviewers were handled through discussion and consensus. The ROBINS-E instrument evaluates bias in seven key areas: (1) confounding, (2) participant selection, (3) exposure classification, (4) deviations from intended exposures, (5) missing data, (6) outcome measurement, and (7) selection of reported results. Each domain was assessed as possessing “low,” “moderate,” or “high” risk of bias according to the predetermined criteria specified in the ROBINS-E guidance [55]. The overall risk of bias was classified as “high” if any domain was assessed as high risk. Studies were classified as having “moderate” risk of bias if they did not satisfy the criterion for low risk but also did not reach the threshold for high risk. Only studies assessed as “low” in all dimensions were classified as possessing an overall low risk of bias (Appendices A1 and A2).

Quality control

The methodological quality of the selected research was evaluated using the standards established by the Joanna Briggs Institute (JBI) [56]. This 22-item checklist assesses multiple facets of study design, encompassing the identification and management of confounding variables, the suitability of statistical analyses, the clarity of research objectives, reliable measurement of variables, and the accurate evaluation of exposure and outcomes. Each item received a rating of “Yes,” “No,” “Unclear,” or “Not Applicable.” Studies were classified into three quality levels—high (Q1), moderate (Q2), and low (Q3)—based on the quantity of “Yes” responses. Studies were deemed high quality if more than half of the replies were indicated as “Yes.” Low quality ratings were given to those that had a large number of “No” answers. If roughly 50% of the replies were deemed “Unclear,” the study was classified as moderate quality. All included studies had high or moderate quality and were hence retained for further analysis (Appendix 3).

Results

Geographic distribution of studies

The studies examined in the present review were conducted across many nations, illustrating the international reach of research on occupational exposure-related respiratory diseases. Of the 38 studies, the majority were from China, comprising 10 publications (Fig. 2), primarily concentrating on lung cancer, COPD, and other chronic respiratory ailments. Additionally, the United States published nine studies that focused on respiratory conditions such as chronic bronchitis and lung cancer.

Fig. 2.

Number of studies included in each country

Australia, France, Italy, Japan, and the United Kingdom (UK) all made significant contributions in this area, each producing two to four research papers.These studies primarily investigated various respiratory diseases, including lung cancer, chronic bronchitis, and asthma. The regional differences in focus reflected the predominant environmental or occupational exposures in each area. The analyzed studies find that respiratory diseases such as lung cancer, COPD, and chronic bronchitis occur with high frequency among coke oven workers, with occupational exposure to hazardous industrial pollutants being the primary contributing factor.

Analysis of data collection methods, study quality, and their relationship with respiratory diseases in coke oven workers

There is significant variation in data collection methods used across the 38 studies, with many utilizing multiple approaches (Table 2). Questionnaires and interviews were the most common methods, used in 66% (n = 25) of studies. These techniques are particularly useful for diagnosing respiratory diseases like asthma and chronic bronchitis, which rely on observable symptoms such as coughing, wheezing, and shortness of breath. Spirometry tests were used in 39% (n = 15) of studies, essential for assessing conditions such as COPD, emphysema, and lung cancer. In addition, 29% (n = 11) of studies employed chemical analysis and High-Performance Liquid Chromatography (HPLC) to assess environmental contaminants and biomarkers. Blood tests and imaging techniques, such as chest radiography, were used less frequently, mainly for diagnosing and monitoring pulmonary carcinoma.

According to a data analysis, the majority of studies (30 out of 38) are categorized as Q1 quality, with a primary focus on severe respiratory conditions such as chronic bronchitis, COPD, and lung cancer (Table 2). Q2 and Q3 studies, each comprising four investigations, largely examine problems such as chronic bronchitis, asthma, and non-malignant respiratory diseases, particularly among workers with short-term exposure to industrial pollution. These studies typically use basic data-gathering techniques, such as questionnaires and clinical tests, which are proficient at identifying overarching patterns in respiratory symptoms but less accurate at evaluating disease progression or the specific effects of individual pollutants.

In terms of study design, the 38 studies included in the review varied in structure: 65.6% (141) were retrospective, 73.68% (28) were cohort, 18.42% (7) were cross-sectional, and 7.89% (3) were case-control studies. These designs correspond to the different data collection techniques used and the variety of health outcomes investigated, ranging from longitudinal cohort studies to more cross-sectional and retrospective approaches.

Temporal distribution of studies

The papers considered in this analysis were published throughout several decades, from 1972 to 2024 (Table 2). The distribution of studies by publication year demonstrates a clear shift in the focus of respiratory disease research. From the 1970s to the 1990s, most research was done on long-term illnesses like lung cancer, bronchitis, emphysema, and tuberculosis. This study focused primarily on the epidemiological and clinical aspects of these disorders, which are frequently encountered in industrial settings, including coal, tar, and related sectors.

Since the 2000s, especially in the past decade (2010–2024), shifting toward more complex diseases such as COPD and lung cancer, particularly among workers exposed to industrial toxins like coke oven emissions. This shift reflects increased awareness of the global impact of respiratory diseases, advancements in diagnostic techniques, and more research funding. Overall, the trend indicates a growing focus on severe conditions related to industrial exposures, emphasizing the need to address the health risks of industrial pollution, especially in high-risk workplaces.

Prevalence of respiratory diseases

The reviewed studies show a strong link between exposure to industrial pollutants such as PAHs, benzene, and CTPV and higher rates of lung disease among workers in coke-oven factories. These pollutants are frequently associated with elevated incidences of respiratory diseases, including COPD, chronic bronchitis, and lung cancer. Specifically, PAHs and benzene were identified as significant factors in the decline of lung function and the heightened incidence of respiratory diseases (Table 3). Long-term exposure to these industrial pollutants seriously harms health. Coke oven plant workers are especially at risk for lung cancer and chronic respiratory diseases such as chronic bronchitis and emphysema, as shown in the analyzed studies. The findings underscore the need to minimize exposure to these toxic compounds by instituting enhanced safety protocols and workplace rules to safeguard workers’ health.

Table 3.

Prevalence of Respiratory Diseases; - lack of data

First Author (Publication Year)	Study design	Prevalence: chronic obstructive pulmonary diseases (COPD) (%)	Prevalence: chronic bronchitis (%)	Prevalence: Respiratory symptoms (%)	Pollutant
Charles Mitiman (1974) [20]	Cross-sectional	-	17	33	-
L. Mayer (1992) [31]	Cohort	-	30.6	16.5	Gas, fumes, benzene, phenol, sulfates
J L Corhay (1998) [36]	Cross-sectional	-	9.5	21.2	NO2, SO2, benzene, phenols, ammonia, PAH, CO, carbon dioxide
E. Kolarzyk (1999) [38]	Cohort	-	-	33.4	CO, NOX, SO2, total dust SiO2, H2S, BaP
Jian Wu. M (2004) [41]	Cohort	-	-	17	-
Y Hu (2006) [42]	Case-control	21.9	-	27.2	Benzene soluble fraction (BSF)

Mortality due to respiratory diseases

The reviewed research indicates that exposure to industrial pollutants, including PAHs, benzene, and CTPV, is significantly correlated with elevated mortality rates from respiratory disorders among workers in coke oven facilities. Numerous studies have demonstrated substantial increases in mortality rates, particularly for lung cancer, nonmalignant respiratory disorders, and respiratory cancer diseases (Table 4). Other research has found a significant association between pollution exposure and mortality from nonmalignant respiratory diseases, in addition to lung cancer. These findings underscore the significant health risks faced by workers exposed to industrial pollutants.

Table 4.

Mortality Due to Respiratory Diseases; - lack of data

First Author & Publication Year	Study Design	Mortality: Lung Cancer (per 100,000)	Mortality: Nonmalignant Respiratory Diseases (per 100,000)	Mortality: Malignant Respiratory Diseases (per 100,000)	Mortality: Tuberculosis (per 100,000)	Pollutant
Carol K. Redmond (1972) [19]	Cohort	-	30	69	8	-
Hiroyuki Sakabe (1975) [21]	Cohort	15	-	-	-	-
Sati Mazumdar (1975) [22]	Cross-sectional	69	-	-	-	Coal Tar Pitch Volatiles (CTPV)
G. M. Davies (1977) [35]	Cohort	8	14	-	-	-
J.F. Hurley (1983) [24]	Cohort	-	79	167	-	-
Carol K. Redmond (1983) [25]	Cohort	113	90	131	-	Coal Tar Pitch Volatiles (CTPV)
JP Bertrand (1987) [27]	Cohort	17	3	-	-	PAH
Sati Mazumdar (1991) [29]	Cohort	50	-	-	-	Coal Tar Pitch Volatiles
G M H Swaen (1991) [28]	Cohort	-	100	178	-	PAH
Carol K. Redmond (1993) [25]	Cohort	50	-	-	-	Coal Tar Pitch Volatiles (CTPV)
N Chau (1993) [33]	Cohort	25	6	4	-	-
Joseph Costantino (1995) [34]	Cohort	-	125	-	15	Coal Tar Pitch Volatiles (CTPV), PAH, Benzene Soluble Fraction (BSF)
Suresh H. Moolgavkar (1998) [37]	Cohort	255	-	-	-	-
Jan Birdsey (2007) [44]	Cohort	494	-	-	-	-
Luigi Vimercati (2020) [48]	Cohort	3	-	-	-	PAHs, Benzene, Aliphatic Amines, Aromatic Amines

Pulmonary function

The reviewed studies show a clear relationship between exposure to industrial pollutants and changes in pulmonary function, as measured by metrics such as Forced Vital Capacity (FVC), Forced Expiratory Volume in 1 s (FEV₁), and the FEV₁/FVC ratio. Significant findings emerged from research on workers in coke-oven factories and individuals exposed to pollutants such as PAHs, BSF, and various industrial emissions (Table 5). A general trend of declining lung capacity (FVC%) was observed, with studies showing a reduction in pulmonary function among workers exposed to PAHs. Similar patterns were observed for FEV1%, reflecting impaired expiratory volume, a characteristic of obstructive pulmonary conditions such as COPD. Furthermore, reduced values in the FEV₁/FVC ratio suggested early-stage restrictive or obstructive lung disease, as reported in some studies.

Table 5.

Pulmonary Function; FVC - Forced Vital Capacity, FEV₁ - Forced Expiratory Volume in 1 s, FEF₂₅₋₇₅ - …

First Author & Year of Publication	Study Design	FVC%	FEV1%	FEV1/FVC (%)	Pollutant
ROBERTA MADISON (1984) [26]	Cohort	-	100.83	-	Coal Tar Pitch Volatiles
N Chau (1992) [30]	Cross-sectional	89.4	89.4	99	-
Giorgio Assennato (1993) [32]	Cross-sectional	101.4	90.8	-	PAH: PAH-DNA Adducts
J L Corhay (1998) [36]	Cross-sectional	-	96.8	80.4	Coal Dusts, Coal Tar, NO2, SO2, Benzene, Phenols, Ammonia, PAH, CO, CO2
E. Kolarzyk (1999) [38]	Cohort	66.25	66.29	73.13	CO, NOX, SO2, Total Dust, SiO2, H2S, BaP
J Wu (2002) [39]	Cohort	83	97.7	95.59	-
J Wu (2004) [40]	Cohort	86	84.7	97.7	-
Jian Wu. M (2004) [41]	Cohort	89	87.3	97.8	-
Y Hu (2006) [42]	Case-control	99.38	93.3	81.7	Coke Oven Emissions (COE): Benzene Soluble Fraction (BSF)
Suhan Wang (2016) [46]	Cohort	87.5	89.9	89.5	PAHs
Meili Shen (2018) [47]	Case-control	105.9	101.2	79.4	PAHs, PM2.5
Wei Wei (2020) [49]	Cohort	89.7	91.9	88.3	Arsenic, PAH
Wei Wei (2020) [49]	Cohort	87.6	90	-	Lead (Pb), Cadmium (Cd), PAHs
Xiulong Wu (2021) [50]	Cross-sectional	72.5	88.57	89.5	-
Lu Liu (2022) [51]	Cohort, Cross-sectional	88.9	91.8	-	PAH
Lu Liu (2023) [53]	Cohort	81.8	88.7	-	PAH
Jiajun Wei (2024) [54]	Cohort	83.5	84.1	-	PAH

Pollutant categories and frequency of studies

A review of 34 studies on pollutants in coke oven factories reveals significant variability in pollutant prevalence and their link to respiratory diseases. PAHs were the focus of 16 studies (47%), making them the most frequently studied pollutant due to their carcinogenic properties and strong association with chronic respiratory conditions in coke oven workers (Fig. 3). CTPV was investigated in 9 studies (26%), with a significant link to lung cancer and other chronic respiratory disorders, underlining its occupational health risks. Benzene was the subject of 5 studies (15%), highlighting its contribution to lung cancer and severe respiratory diseases, although it remains less studied than PAHs and CTPV. PM_2.5 was examined in 4 studies (12%), emphasizing its negative effects on conditions like COPD and bronchitis. Additionally, 6 studies (18%) assessed pollutants such as NO_x, SO₂, and CO, which are commonly found in industrial environments, especially in coke-oven plants, and are associated with diseases like asthma, COPD, and lung cancer.

Fig. 3.

Percentage Distribution of Pollutants Studied in Coke Oven Plants

Disease categories and frequency of studies

The 38 studies in this review show significant variation in respiratory disease rates and pollution-related fatalities, especially in coke-oven factories. Of the six studies on respiratory illnesses, one reported a COPD prevalence of 21.9%, and chronic bronchitis rates ranged from 9.5% to 33.4% (Table 2). Respiratory symptoms such as coughing, wheezing, and dyspnea were documented in all 6 studies. Regarding mortality, 15 studies on respiratory disease deaths found lung cancer as the leading cause, with some studies reporting up to 69 deaths (Table 3). Nonmalignant conditions like asthma and chronic bronchitis were also common. These findings highlight the high prevalence of respiratory diseases in polluted industrial environments.

Regarding lung function, 16 studies found that pollutant exposure was consistently linked to reduced FVC and FEV₁ (Table 4). For example, studies on PAHs reported average FVC of 81.8% and FEV₁ of 84.1%, indicating significant lung function decline. Chronic exposure to industrial pollutants like PAHs and CTPV appears to cause lasting lung damage.

Mortality effects associated with coke oven emissions

This study examined the impacts of exposure to coke oven pollutants (COE) on mortality across several different studies. The pooled effects from these studies were calculated using meta-analysis methodology (Fig. 4). The I-squared value of 98.9% indicates considerable heterogeneity among the studies. This suggests that while there is an association between pollutants and mortality, the data show significant variability, which may stem from differences in methodology, participant characteristics, and environmental conditions.

Fig. 4.

Meta-Analysis of the Association Between Coke Oven Emissions and Respiratory Disease Mortality

In the subgroup analysis, mortality due to lung cancer among coke oven workers showed significant differences based on income level, geographic region, and study period. In high-income countries, the mortality rate was 49.02, indicating a stronger impact of pollutants in these regions. In Europe, the mortality rate was 20.42, while in North America, it reached 164.37. Furthermore, studies conducted before 2000 reported a mortality rate of 45.61, whereas studies after 2000 reported a mortality rate of 243.60, indicating an increased risk in recent years (Table 6).

Table 6.

Results of subgroup analysis for respiratory disease mortality in coke oven plant workers

Subgroup analysis
Subgroup	Category (number of studies)	Pooled mortality (n) [95% CI]	I2 (%)	Q statistic (df )	p of heterogeneity
Income level	High income (11)	49.02 [40.02, 58.01]	98.9	10	< 0.001
	LMICs (-)	-	-	-	-
Region	Europe (5)	20.42 [12.17, 28.66]	98.8	4	< 0.001
	East /southeast Asia (1)	15.00 [12.35, 17.64]	-	0	-
	Middle East (-)	-	-	-	-
	Americas (5)	164.37 [112.94, 215.81]	98.0	4	< 0.001
Study date	In or before 2000 (8)	45.61 [34.36, 56.86]	98.4	7	< 0.001
	After 2000 (3)	243.60 [2.75, 489.96]	99.2	2	< 0.001

After excluding studies with a high risk of bias, the overall effect estimates remained largely unchanged, demonstrating the robustness of the results even after their removal. However, publication bias was detected among the included studies (Egger’s test, P < 0.05) (Fig. 5). This suggests that studies with positive or significant results are more likely to be published, potentially leading to an overrepresentation of positive findings in the final analysis. Further analysis of confounding factors and biases in the studies is provided in Appendix A2 and Fig. 6. Most studies were classified as having a low risk of bias, while only a few exhibited moderate or high risk, particularly regarding deviations in exposure, exposure assessment, and outcome measurement. In the overall RoB evaluation, 21 studies were classified as high risk, 1 as moderate risk, and 16 as low risk.

Fig. 5.

Funnel plot for assessing publication bias with pseudo 95% confidence limits

Fig. 6.

Assessment of RoB in the included studies related to respiratory tract diseases in coke oven plant workers (%)

Discussion

Geographic distribution of studies

The geographical distribution of research indicates that nations such as China and the United States have predominantly led investigations in this field, highlighting the increasing awareness of emissions from coke oven operations and their lasting impact on pulmonary health.

In China, fast industrialization and emissions from coke oven operations have led to a notable rise in respiratory ailments among workers, especially lung cancer, COPD, and chronic bronchitis. Research conducted by Liu et al. (2022) [51] and Wei et al. (2020) [49] underscores the association between PAHs, CTPV, and other pollutants in the etiology of these serious diseases. The expansion of the coke oven sector in China poses significant health risks to workers [47]. The United States has highlighted the respiratory risks of PAHs, gas emissions, and smoking among coke oven workers. Research by Redmond et al. (1972) has consistently demonstrated a strong association between lung cancer and non-malignant respiratory conditions, such as chronic bronchitis, resulting from continuous exposure to deleterious compounds in the coke oven environment [19]. The notable findings from this research underscore workplace pollution as a primary contributor to the elevated prevalence of lung cancer among these workers [45].

France and Australia have made significant contributions to this domain, focusing on the impact of industrial pollutants on respiratory health. Chau et al. (1992) [30] and Wu et al. (2021) [50] discovered similar trends in COPD, lung cancer, and chronic bronchitis among coke oven workers, highlighting the worldwide scope of the problem. Research conducted in France specifically highlights the effects of prolonged exposure to coke oven emissions and underscores the necessity for enhanced laws and occupational safety protocols [38].

Methodological trends, data collection methods, and study quality in respiratory disease research among coke oven workers: current insights and future directions

The studies reviewed use a variety of methods to assess respiratory diseases in workers exposed to industrial pollutants, especially from coke ovens. Questionnaires and interviews, used in 66% of studies, are effective for diagnosing observable diseases like chronic bronchitis and asthma. Spirometry, used in 39% of the studies, is essential for monitoring COPD and emphysema, as highlighted by Wei et al. (2021) [57]. Moreover, HPLC and chemical analysis (29%) are key for detecting pollutants such as CTPV and PAHs, and assessing their effects on respiratory health, as demonstrated by Liu et al. (2022) [51] and Miller et al. (2000) [45]. The study by Liu et al. (2023) [53], focusing on coke oven workers, underscore the importance of these sophisticated methodologies for identifying early lung function deterioration and evaluating long-term health risks [45].

Blood tests and imaging techniques, though less frequent, are critical for identifying lung cancer and tracking lung damage when combined with spirometry. Recent studies show a growing trend of integrating multiple methods. While questionnaires are cost-effective, combining them with spirometry and HPLC provides a more comprehensive assessment. The use of chemical analysis is increasingly significant for measuring pollutant exposure, and imaging tools like X-rays remain essential for tracking disease progression, particularly in lung cancer. Future research should continue to combine non-invasive methods with advanced techniques like chemical analysis and imaging to better understand the impact of pollutants on pulmonary health.

The findings indicate that Q1 studies, concentrating on severe disease such as lung cancer and COPD, are essential for coke oven workers who experience significant exposure to detrimental pollutants such as PAHs and CTPV. Q2 and Q3 studies often focus on asthma, chronic bronchitis, and other non-malignant illnesses. These studies use questionnaires and clinical exams to directly assess symptoms such as chronic cough, wheezing, and dyspnea in coke oven workers. These more straightforward methods continue to yield significant insights, particularly for research involving workers with minimal exposure or for detecting early-stage respiratory problems. Nonetheless, they do not provide the same degree of specificity as more sophisticated diagnostic instruments. This analysis underscores the need for advanced methodologies in Q1 studies to assess long-term health effects of pollutants on workers in high-risk settings such as coke ovens. Simpler methods remain useful for monitoring general respiratory health trends, particularly in less severe cases.

Evolution of research focus over time

Earlier studies (1970–1990 s) mostly focused on non-malignant respiratory disorders; however, recent research has clearly shifted towards more serious conditions such as lung cancer and COPD. This alteration might be ascribed to advancements in diagnostic methodologies and a growing awareness of the long-term effects of industrial exposure, as exemplified by coke oven workers [19, 22]. The growing knowledge of how long-term exposure to certain pollutants, such as PAHs and CTPV, causes the development of chronic and malignant respiratory diseases is a major contributing reason to this change [47, 53].

Furthermore, the increase in studies about respiratory symptoms, including chronic cough and phlegm, indicates that scholars are expanding their scope beyond life-threatening diseases. This trend is largely due to a better understanding of the various effects of workplace contaminants on lung health [20, 36]. Research on respiratory conditions, from minor to serious, underscores the importance of early detection and prevention in occupational health. This advancement in research underscores the need for ongoing research into the enduring impacts of industrial pollution on lung health. Enhancing diagnostic and preventive techniques will more effectively alleviate the effects of these exposures on public health [49].

Impact of exposure to pollutants in coke oven plants on the prevalence of respiratory diseases in workers

This study confirms the significant impact of industrial pollutants, particularly PAHs, benzene, and CTPV, on the respiratory health of coke oven workers. Long-term exposure to these pollutants is associated with higher rates of COPD, chronic bronchitis, and lung cancer. Studies by Miller et al. (2013) [45] and Liu et al. (2020) [51] show that prolonged exposure leads to deterioration in lung function and increased disease risk. PAHs and benzene are carcinogenic and cause chronic inflammation in the lungs.

Exposure to mixtures of pollutants, such as CO, NO_X, SO₂, and BaP, also exacerbates respiratory issues, increasing the incidence of diseases like COPD and chronic bronchitis [38]. Studies by Hu et al. (2006) [42] and Kolarzyk et al. (1999) [38] highlight that pollutant concentration and exposure duration are critical factors in disease progression. Hu et al. (2006) [42] found a high prevalence of COPD (21.9%) and respiratory symptoms (27.2%) among workers exposed to BSF. This affirms substantial health concerns associated with even minimal exposure to these chemicals.

In addition to exposure, factors such as personal sensitivity, pre-existing health conditions, and use of protective gear play a role. Recent research, such as studies by Wei et al. (2021) [57] and Shen et al. (2018) [47], indicates that although the risks associated with PAHs and benzene exposure are significant, they can be mitigated through appropriate safety measures. Furthermore, Vimercati et al. (2020) [48], emphasized that improved surveillance and routine health assessments for employees can facilitate early identification and intervention, which are essential for improving long-term health outcomes in high-risk sectors such as coke-oven factories. These findings stress the need for stricter safety regulations and better protective measures in coke oven plants to prevent chronic respiratory diseases and safeguard worker health. To reduce worker exposure to hazardous contaminants and prevent the onset of chronic respiratory diseases, enhanced occupational health measures, along with continuous research into the long-term effects of these pollutants, will be essential for safeguarding employees’ health in high-risk sectors.

Impact of pollutant exposure on mortality due to respiratory diseases in coke oven workers

This study highlights the significant impact of industrial pollutants, including PAHs, benzene, and CTPV, on workers’ mortality from respiratory diseases, particularly in coke oven factories. Many studies show increased death rates, linking long-term exposure to these chemicals to severe health effects.

The association between CTPV exposure and lung cancer mortality, as reported by Mazumdar et al. (1975) [22] and Redmond (1983) [25], emphasizes the carcinogenic potential of these pollutants. For example, Redmond et al. (1972) [19] documented a mortality rate of 30 per 100,000 workers attributable to lung cancer and a mortality rate of 69 per 100,000 workers resulting from nonmalignant respiratory diseases. Similarly, Mazumdar et al. (1975) [22] reported a mortality rate of 69 per 100,000 for lung cancer among laborers exposed to CTPV. Sakbe et al. (1975) [25] documented a lung cancer mortality rate of 15 per 100,000, further substantiating the association between industrial pollution exposure and elevated mortality risk. These findings underscore the elevated risk of cancer linked to industrial pollutants.

Additionally, mortality rates from nonmalignant respiratory diseases, like chronic bronchitis and emphysema, are concerning. Studies by Swaen et al. (1991) [28] and Redmond et al. (1983) [25] demonstrate a high mortality rate from such diseases among workers exposed to PAHs, reinforcing the long-term respiratory issues caused by industrial exposure. The research by Costantino et al. (1995) [34] documents higher mortality rates, especially from malignant respiratory disorders. These findings highlight the need for focused actions to reduce exposure to pollutants in high-risk locations.

Differences in mortality across studies may be due to varying exposure levels, durations, and other confounding factors. However, the consistent trend highlights a strong link between industrial pollution and higher mortality rates from respiratory diseases. This study’s findings corroborate current research highlighting the detrimental impact of industrial pollutants on workers’ respiratory health, particularly in coke-oven factories. The elevated mortality rates identified in the analyzed research underscore the need to implement more stringent occupational safety regulations, improve workplace safety protocols, and minimize exposure to hazardous substances to alleviate health concerns for workers.

Impact of pollutant exposure on pulmonary function in coke oven workers

This review highlights the considerable influence of industrial pollutants, including PAHs, coke oven emissions, and other chemical contaminants, on pulmonary function. The reduction in pulmonary metrics, including FVC, FEV₁, and the FEV₁/FVC ratio, among workers exposed to these pollutants indicates that long-term exposure may result in both restrictive and obstructive lung dysfunction.

The studies reviewed indicate a clear trend of diminished FVC% and FEV1% in workers exposed to PAHs, leading to a decrease in lung capacity and airflow [49, 51, 53]. This suggests that workers may be at a higher risk of developing chronic respiratory diseases like COPD and restrictive lung disease. However, a study by Hu et al. (2006) [42] shows an anomaly where FEV1% is elevated despite a reduced FEV₁/FVC ratio, indicating early signs of airflow obstruction. Similarly, workers exposed to PAHs and PM_2.5 in the study by Shen et al. (2018) had unexpectedly high FVC% (105.9%) and FEV1% (101.2%), which may reflect compensatory mechanisms or reduced exposure [47]. Alternatively, these workers might be at an earlier stage of exposure before pulmonary function declines.

In contrast, studies by Kolarzyk (1999) [38] and Corhay et al. (1998) [36] found significant declines in lung function, indicating that exposure to pollutants such as CO, NO_X, SO₂, and total dust, highlighting the cumulative effect of these exposures. Additionally, research by Assennato et al. (1993) [32], suggests that genetic damage from chemical exposure can contribute to long-term lung dysfunction. In summary, these findings indicate that long-term exposure to industrial pollutants, including coal dust, coke oven emissions, and PAHs, significantly reduces workers’ lung function. These findings underscore the importance of further research and the need for better protective measures and environmental regulations to reduce health risks for workers.

Analysis of pollutant categories and their frequency of impact on respiratory diseases in coke oven workers

This research highlights substantial health hazards associated with exposure to industrial pollutants in coke-oven factories, with PAHs and CTPV among the most commonly examined contaminants.The observation that 47% of studies focused on PAHs underscores their significant contribution to the etiology of respiratory ailments, including COPD, asthma, and lung cancer, among coke oven workers. The carcinogenic nature of PAHs and their long-term impact on respiratory health make them a primary concern in occupational health.

The fact that 26% of the research focused on CTPV further underscores their danger in coke-oven environments. Exposure to CTPV has been repeatedly associated with the incidence of lung cancer, specifically among workers with extended exposure to these substances. Benzene, though examined in only 15% of studies, is also a significant cause of lung cancer and respiratory disorders. Despite its lower prevalence in studies, benzene’s toxicological properties warrant further attention. The presence of PM_2.5 in 12% of studies underscores the growing recognition of fine particle pollution as a significant contributor to respiratory diseases, notably COPD and bronchitis. Although PM_2.5 is less frequently examined than other contaminants, such as PAHs and CTPV, its inclusion in studies underscores the importance of evaluating airborne particulate matter in industrial settings.

In 18% of the studies, other pollutants, such as CO, SO₂, and NO_x, were discussed, underscoring their continued importance in industrial settings, particularly in coke-oven plants. These pollutants are a major cause of respiratory diseases and are produced by combustion activities. In conclusion, PAHs and CTPV are significant contributors to lung cancer and chronic respiratory conditions, but other pollutants like benzene, PM_2.5, and combustion-related toxins also play a vital role in worker health. Strengthening safety protocols and reducing exposure to these pollutants is critical for safeguarding the health of coke oven workers.

Discussion on disease categories and respiratory symptoms in coke oven workers

The findings from the studies examined in this review reveal a significant impact of industrial pollutants, particularly PAHs, benzene, and CTPV, on the prevalence of respiratory diseases among coke oven workers. The high prevalence of chronic bronchitis and respiratory symptoms such as persistent cough, wheezing, and dyspnea is evident among workers exposed to these pollutants. For example, studies by Mitiman (1974) [20], Mayer (1992) [31], and Corhay (1998) [36] report substantial rates of chronic bronchitis (17%–30.6%) and respiratory symptoms (16.5%–33.4%) in workers exposed to a variety of industrial chemicals, including NO₂, SO₂, benzene, PAHs, and other fumes. These findings highlight the significant role of industrial pollutants in exacerbating respiratory conditions and the importance of improving workplace safety protocols to reduce exposure to these harmful substances.

Additionally, a notable percentage of workers also show symptoms of COPD. For example, in the study by Hu et al. (2006), 21.9% of workers exposed to the benzene soluble fraction (BSF) exhibited symptoms of COPD [42]. This is consistent with other research indicating that prolonged exposure to pollutants such as PAHs, benzene, and CTPV leads to chronic inflammation and long-term damage to the respiratory system, resulting in diseases such as COPD and chronic bronchitis. These findings support the need for enhanced protective measures and environmental regulations to reduce exposure to industrial pollutants.

Research focus across different countries

The study’s findings show a significant association between higher mortality rates among those exposed to COE and exposure to these contaminants. Although the aggregate association between COE and mortality is statistically significant, the high I-squared value of 98.9% indicates substantial heterogeneity across the included studies. This heterogeneity may arise from discrepancies in methodologies across studies, variations in study participants’ characteristics, or differences in environmental conditions and the types of contaminants examined. For example, studies such as Miller et al. (2013) [45] and Birdsey et al. (2007) [44] report higher effect sizes, while other studies like Mazumdar et al. (1975) [22] show more moderate effects. This considerable variation underscores the need for additional research employing standardized, controlled study designs to enhance understanding of the association between COE exposure and mortality [35, 45, 51]. Additionally, the study highlights the role of publication bias in the existing literature, which may overstate the effects of pollutant exposure on respiratory diseases. This bias emphasizes the need for transparent reporting and methodological rigor in research. To improve the accuracy of future findings, it is essential for studies to implement strategies to minimize publication bias.

Assessment of bias across domains

These results emphasize the importance of understanding significant differences in mortality rates due to respiratory diseases in coke oven workers, considering factors such as income level, geographic region, and study period. Additionally, biases across various studies play an important role in the effects of pollutants on respiratory health [58]. Among the most significant biases are deviations in exposure assessment and biases resulting from inaccuracies in measuring exposure, which may either underestimate or overestimate the effects of pollutants, particularly in COE (coke oven emissions) studies [59].

Based on the findings of this study, most research falls under a low-risk category in terms of outcome measurement and selection bias. However, biases in exposure assessment—especially in COE-related research—can lead to either under- or overestimation of health impacts [37, 45]. The deviations in exposure assessment and departures from it, particularly in high-risk studies, can alter the associations between COE exposure and respiratory diseases, potentially leading to incorrect conclusions.

Regarding confounding biases and missing data, the results showed that most studies fall under a low-risk category in these areas. However, the presence of studies with moderate risk, particularly concerning confounding variables and missing data, highlights the need for more precise management of these factors in future research [60]. Additionally, publication bias was observed in the studies, indicating that studies with positive or significant results are more likely to be published, while studies with negative or insignificant results are less likely to receive attention [61]. This may result in an unbalanced representation of positive findings in the final analysis [62].

The findings of this study underline the need for better strategies to minimize biases related to exposure deviations, exposure assessment, and confounding variables. Such improvements would enhance the scientific validity and reliability of research findings and reduce the influence of biases in health impact analyses [63].

Strengths and weaknesses

This study has numerous significant strengths that augment the validity and robustness of its findings. Its systematic approach to gathering and analyzing data from many research studies and meta-analyses to estimate worker health impacts of coke oven emissions is a strength. The use of multiple approaches and stringent quality evaluation of the studies enhances the reliability of the conclusions. Moreover, the systematic review provides a comprehensive perspective on the issue, considering multiple factors that affect respiratory health outcomes among exposed workers.

However, certain limitations warrant careful consideration. The methods used to measure pollutants in the included studies are inconsistent, which represents a significant weakness. Lack of standards in measuring pollutants and differences in follow-up times could cause biases that hurt the accuracy of the results. Moreover, several studies lacked comprehensive details regarding the specific pollutants, hence constraining the evaluation of the direct correlation between individual pollutants and health outcomes. The variety of study conditions, encompassing geographic locations, exposure levels, and environmental factors, significantly led to heterogeneity in the data. This variability underscores the need for further research to standardize exposure assessments and develop clearer, more reliable methods for assessing the long-term health effects of coke oven emissions.

Conclusion

This systematic review evaluated 38 studies to examine the effects of several contaminants, particularly those related to coke oven emissions, on respiratory health. Regarding disease classifications, lung cancer emerged as the most frequently reported condition, accounting for 37% of the studies, indicating a significant concern about the contribution of pollutants to cancer development in industrial settings. Non-cancerous conditions including COPD and chronic bronchitis accounted for 18% of studies, whereas respiratory symptoms like chronic cough and shortness of breath were common. Most data were collected through questionnaires and interviews (66%), followed by spirometry (39%) and advanced chemical tests such as HPLC (29%). These techniques were essential for evaluating lung function and associating contaminants with particular diseases.

This study assessed the effects of exposure to coke oven pollutants on mortality and demonstrated a substantial association between exposure and elevated mortality. However, the significant heterogeneity (I-squared score of 98.9%) suggests that research methods vary. The risk of bias evaluation indicates that although the majority of studies exhibit a low risk of bias, there are problems related to exposure measurement and deviations from exposure that may affect the validity of some findings. The risk of bias evaluation indicates that although the majority of studies exhibit a low risk of bias, some studies exhibit problems with exposure measurement and deviations from exposure that may affect the validity of some findings.

This review unequivocally indicates that workers in industrial settings exposed to coke oven emissions and other pollutants face substantial health hazards. It emphasizes the necessity for enhanced steps to safeguard these at-risk populations. The methodologies employed in this research illustrate the complexity of assessing pollution exposure and its enduring impacts on respiratory health.

Suggestions for future research

Future studies should focus on standardizing data collection methods, using advanced tools like spirometry and HPLC-MS for more accurate results. Longitudinal studies tracking workers’ respiratory health would provide deeper insights into the long-term effects of exposure. Analyzing geographic and environmental variations in disease prevalence could help identify high-risk areas for targeted interventions. Additionally, employing multivariate models to assess combined pollutant effects will enhance our understanding of their impact on respiratory health. Finally, strengthening safety protocols, improving protective equipment use, and raising awareness about the long-term health risks in high-risk industries like coke ovens will be crucial in reducing disease and mortality rates.

Authors’ contributions

AHKh: Investigation/Writing Original draft/ systematic search/Supervision/Review and Editing/Data Curation, SGH: Investigation/systematic search/extraction/Writing Original draft/Data Curation/Review and Editing, MR: systematic search/extraction, AG-K: Review& Editing, AY: Review& Editing.

Funding

The author(s) stated that there is no financial support linked to the work presented in this paper.

Data availability

The corresponding author can provide the datasets that were used and/or analyzed for this work. (Safiye Ghobakhloo: sa_ghobakhloo@yahoo.com)

Declarations

Ethics approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.