Pneumoconiosis refers to a chronic disease attributed to long-term inhalation and deposition of productive dusts in lungs during professional activities. The main pathological manifestations of pneumoconiosis include ongoing pulmonary fibrosis and other parenchymal irreversible variations that can occur after exposure ceases (1). Known pneumoconiosis consists of silicosis, coal worker pneumoconiosis (CWP), graphitosis and carbon black pneumoconiosis (2). Thus far, there has been no effective treatment available for pneumoconiosis, and prevention has been used as the main means to reduce the morbidity and mortality of pneumoconiosis. Pneumoconiosis continues to be one of the major global burdens of occupational health-related disease.
From the global perspective, in 2017, pneumoconiosis had the prevalent cases of 527,500 (3), and its prevalence had increased since 1990 from 0.0054% to 0.007% (4). As reported by Shi et al., the number of patients suffering from pneumoconiosis increased by 66.0%. However, the age-standardized incidence rates decreased by 0.6% per year on average from 1990 to 2017, as revealed by the data of the Global Burden of Disease Study 2017 (5). National Institute for Occupational Safety and Health (NIOSH) data indicated that the prevalence of progressive massive fibrosis (PMF), the most severe form of pneumoconiosis, significantly increased (6). According to 2018 study, the prevalence of pneumoconiosis in long-tenured working miners was 10% higher than that of the United States (7). According to Cynthia Lu et al., the estimated prevalence of coal workers pneumoconiosis in the United States in the 2000s exceeded that in the 1990s (8). A total of 21,488 patients suffering from pneumoconiosis died globally in 2016, and the rate of pneumoconiosis mortality decreased by 41% from 1990 to 2016 (9). Pneumoconiosis remains the critical occupational disease in China. According to the estimation of the National Health Commission of China, nearly 200 million workers were exposed to occupational hazards (10). On the whole, 17,064 new patients with occupational diseases were reported in 2020, which included 14,367 patients of pneumoconiosis that accounted for approximately 84.19% of the total reported cases (11). The Chinese central government has allocated 809 million yuan to occupational disease prevention and treatment projects in 2019. Furthermore, occupational health has been prioritized in the action plan of Healthy China 2030.
The classification of pneumoconiosis is considered an indicator to measure the severity of pneumoconiosis, as well as a main basis for cases to be compensated for occupational diseases. Pneumoconiosis is assigned as the simple (mild) or complicated (severe) level in accordance with the international classification of pneumoconiosis published by the International Labor Office (ILO, 2011). The above two types further fall to categories 1, 2 and 3 for the simple disease, as well as to categories A, B and C for the complicated disease based on radiographs, respectively (12). However, pneumoconiosis is separated into three stages (stage I, stage II and stage III) in accordance with the professional history and radiographs in China (13). There is hierarchical correlation between the three stages and the ILO radiological categories. Stage I represented ILO simple pneumoconiosis, and stage III represented complicated pneumoconiosis of ILO categories B and C; besides, stage II was between the above two stages (14). The classification of pneumoconiosis can be used as a vital factor for the survival time of cases. As reported by Ortmeyer et al., the life expectancy of miners as a whole is consistent with that of the general population (15). According to a specific 20-year follow-up study on miners and ex-miners in South Wales, the subjects with simple and category A pneumoconiosis survived, as well as those without any evidence of pneumoconiosis (16). An elevated mortality was reported only in miners with category B and C pneumoconiosis (17).
Though the government has done considerable work, pneumoconiosis remains one of the major social burdens. Since the goal of economic development is ultimately to expedite the progress of people, the survival time and quality of patients suffering from pneumoconiosis have aroused huge attention. We collected the data of cases first diagnosed with pneumoconiosis from 1961 to 2019 in Jiangsu province, to investigate the survival time and relevant factors of pneumoconiosis at different stages. We present the following article in accordance with the STROBE reporting checklist (available at https://apm.amegroups.com/article/view/10.21037/apm-21-2824/rc).
A total of nearly 20,000 cases were collected in the follow-up and retrospective investigation of pneumoconiosis in Jiangsu Province in 2019. Excluding cases lacking the time of onset, survival status cannot be investigated, and no exact death date of death cases, etc., 15,402 pieces of relatively complete data were screened out. The study population consisted of 15,402 workers who were first diagnosed with pneumoconiosis from 1961 to 2019 in Jiangsu province. The patients were diagnosed by an institution qualified for pneumoconiosis diagnosis in Jiangsu Province that was performed by at least three radiologists in accordance with the Diagnostic Criteria of Pneumoconiosis. Since the diagnostic criteria were constantly revised, “silica dust workers medical preventive measures”, “GB 5906-1986 pneumoconiosis X-ray diagnostic criteria and principles of treatment”, “GB 5906-1997 X-ray diagnosis of pneumoconiosis”, “GBZ 70-2002 diagnostic criteria for pneumoconiosis”, “GBZ 70-2009 diagnostic criteria and GBZ 70-2015 diagnostic criteria for pneumoconiosis” have been successively employed for pneumoconiosis diagnosis from 1961 to 2019. Pneumoconiosis was significantly separated into three stages. The study was conducted based on the Declaration of Helsinki (as revised in 2013). The study was approved by the Ethics Committee of the Jiangsu Province Center for Disease Control and Prevention (No. 2012025) and individual consent for this retrospective analysis was waived.
The information of patients suffering from pneumoconiosis was acquired and submitted by diagnosis institutions via Network Direct Report System of Occupational Diseases. In addition, the follow-up of pneumoconiosis cases was conducted by the occupational disease prevention and control department of CDC each year to ensure that their information was updated timely. The data acquired involved basic demographic information, types of work, industry categories, economic levels of companies, initial dates of dust exposure, end dates of dust exposure, dates of the first diagnosis of pneumoconiosis, types of pneumoconiosis, diagnosis of the stages, dates of diagnosis of each stage outcome variable, date of death, etc. The missing data were checked to ensure the data to be integral and reliable. The data of pneumoconiosis cases were collected from 1961 to 2019 in accordance with the summary of the most recent follow-up information in Jiangsu province.
Death cases: died of pneumoconiosis or its complications (e.g., tuberculosis and pulmonary heart disease) were defined as death cases.
Censored cases: death from other causes (e.g., accidents, liver failure, renal and autoimmune disease), or still alive at the end of observation were classified as censored data.
Descriptive and survival analyses were conducted with the R Programming Language. Categorical variables were analyzed by performing the χ2 tests. For survival analysis, Kaplan-Meier method and Cox proportional hazards regression model were used in accordance with the partial maximum likelihood estimation of the forward method [forward: likelihood ratio (LR)]. Table 1 lists the regression variable assignment.
|Variable name||Assignment description|
|Dust exposure years (X1)||Continuous variable|
|Age of onset (X2)||Continuous variable|
|First diagnostic stage (X3)||Stage I =0, stage II =1, stage III =2|
|Types (X4)||Silicosis =0, Coal worker’s pneumoconiosis =1, Welder’s pneumoconiosis =2, others =3|
|Industry category (X5)||Mining =0, manufacturing =1, public management and social security =2, others =3|
|Upgrade of stages of pneumoconiosis (X6)||Stable stages =0, upgrade I to II =1, upgrade II to III =2, upgrade I to III =3|
|Economic level (X7)||High level =1, medium level =2, low level =3|
Demographic and occupational characteristics
Silicosis (10,549 cases, 68.49%), CWP (2,990 cases, 19.41%) and welder pneumoconiosis (591 cases, 3.84%) were the top three of the total reported pneumoconiosis. The data of demographic and occupational characteristics fell into three groups based on the pneumoconiosis stage. There were 12,322 (80.00%) patients first diagnosed at stage I, 2,386 (15.49%) at stage II, and 694 (4.51%) at stage III among all the 15,402 subjects. Most of the patients (13,847, 89.90%) remain the first diagnosed stages and keep stable, 1,107 (7.19%) upgrade from stage I to stage II, 232 (1.51%) upgrade from stage II to stage III and 216 (1.40%) upgrade from stage I to stage III. The average age of the three stages was 69.42±10.73, 70.59±12.11, and 68.46±13.01 years, respectively. The age of onset at stage III was higher than that at stage I and stage II. In contrast to the above result, the average dust exposure period at stage III was less than that at stage I and stage II, and the dust exposure period generally ranged from 1 to 58 years. The overall average survival time of all the subjects was 14.74±9.57 years, and the survival time at stage III was 10.91±9.12 years, significantly less than that at stage I (14.92±9.60 years) and stage II (14.90±9.34 years). The total mortality of patients suffering from pneumoconiosis at stage I, stage II and stage III was 19.89%, and 17.5%, 28.5% and 32.7%, respectively, and the mortality rate tended to increase with the upgrade of the stage. All the variables of the three stages were statistically significant (P<0.001, Table 2).
|Variables||Stage I (n=12,322)||Stage II (n=2,386)||Stage III (n=694)||F||P|
|Age (years), mean ± SD||69.42±10.73||70.59±12.11||68.46±13.01||14.832||<0.001|
|Age of onset (years), mean ± SD||54.14±11.18||54.96±11.47||56.82±11.53||22.376||<0.001|
|Dust exposure time (years), mean ± SD||16.24±9.89||17.05±9.88||15.02±9.48||12.986||<0.001|
|Survival time (years), mean ± SD||14.92±9.60||14.90±9.34||10.91±9.12||58.512||<0.001|
SD, standard deviation.
Mean survival time of patients suffering from pneumoconiosis in different age groups
Table 3 lists the cumulative survival rate and the mean survival time of different age groups. Ten age groups were classified with an interval of 5 years old in the respective group. According to the corrected observed patients, the death probability of pneumoconiosis cases in different age groups was calculated. The cumulative survival rate tended to decrease with age. The mean survival time of patients suffering from pneumoconiosis in 40–45 and over 65 years of age was longer than that of others. The deceased cases are listed in Table 3.
|Age groups||Observed patients||Surviving patients||Died patients||Cumulatively observed patients||Corrected observed patients||Mortality rate||Survival rate||Cumulatively survival rate||Cumulatively mortality rate||Hazard rate||Median survival time (years)|
Age-specific mortalities for pneumoconiosis cases at the respective stage
Age-specific mortalities and mean survival time at the respective stage are listed in Table 4. There were considerable observed cases and dead patients in the older age group of the three stages. The high mortality rate and decreasing mean survival time were found at stage II and stage III compared with those at stage I. However, all the variables had no obvious trend variations from the perspective of age groups.
|Age||Stage I||Stage II||Stage III|
|Observed patients||Died patients||Mortality rate (%)||Median survival time (years)||Observed patients||Died patients||Mortality rate (%)||Median survival time (years)||Observed patients||Died patients||Mortality rate (%)||Median survival time (years)|
Comparison of survival curves between the three-stage groups
Figure 1 illustrates that the survival curve of patients suffering from pneumoconiosis at stage I was higher than those at the other two stages, and the survival curve of pneumoconiosis cases at stage II was higher than that at stage III. Moreover, a significant difference was identified in the survival time of the three stages (P<0.001). The cumulative survival rate at the three stages tended to decrease as the survival time was prolonged. The whole life expectancy of all the subjects reached 34.324 years. The life expectancy regardless of age at stage I stage II and stage III reached 35.354, 31.045 and 24.926 years, respectively, and the chi-square value of Kaplan-Meier was 289.383. A statistically significant difference was identified between the above three groups (P<0.001).
Multivariate Cox regression analysis of pneumoconiosis cases
As demonstrated by the results of the Cox regression analysis in Table 5, dust exposure years, first diagnostic stage (stage II and stage III), upgrade stages of pneumoconiosis (upgrade I to II, upgrade I to III), economic level (low level) were correlated with the death of pneumoconiosis. First diagnosed to stage III [relative risk (RR) =2.371, 95% confidence interval (CI): 2.056–2.733], upgrade I to III (RR =1.673, 95% CI: 1.346–2.079), first diagnosed to stage II of pneumoconiosis (RR =1.612, 95% CI: 1.461–1.779) were reported as the top three risk factors, followed by upgrade I to II (RR =1.232, 95% CI: 1.099–1.379), low economic level (RR =1.103, 95% CI: 1.004–1.212) and dust exposure years (RR =1.081, 95% CI: 1.077–1.085).
|Variables||b||Standard error||Wald c2||P||RR||95% CI|
|Dust exposure years||0.078||0.002||1,592.840||<0.001||1.081||1.077–1.085|
|Age of onset||−0.014||0.002||43.878||<0.001||0.986||0.981–0.990|
|First diagnostic stage||190.108||<0.001|
|Coal worker’s pneumoconiosis||−0.273||0.088||9.628||0.002||0.761||0.640–0.904|
|Public management and social security||−0.825||0.093||78.178||<0.001||0.438||0.365–0.526|
|Upgrade of stages of pneumoconiosis||31.008||<0.001|
|Upgrade I to II||0.208||0.058||12.945||<0.001||1.232||1.099–1.379|
|Upgrade II to III||0.077||0.116||0.440||0.507||1.080||0.860–1.355|
|Upgrade I to III||0.514||0.111||21.522||<0.001||1.673||1.346–2.079|
CI, confidence interval; RR, relative risk.
Pneumoconiosis is one of the most common occupational diseases worldwide. In the proposed study, a survival analysis was conducted with 15,402 patients suffering from pneumoconiosis diagnosed in one of the major industrial provinces in China. The types of pneumoconiosis here mainly covered silicosis, CWP, welder pneumoconiosis and other types (e.g., asbestosis, cement pneumoconiosis, founder pneumoconiosis, potter’s pneumoconiosis). The survival time and life expectancy at stage III were significantly shorter than those at stage I and stage II. Dust exposure years, first diagnosed at stage II or stage III, stage I upgrade to stage II, stage I upgrade to stage III and low economic level had correlations with the death of pneumoconiosis cases.
The three most common types of pneumoconiosis included asbestosis, silicosis and CWP. To be specific, silicosis is more frequent than asbestosis in developing nations, whereas this trend is reversed in developed nations (2,18). CWP and silicosis were listed as the top two reported new cases of pneumoconiosis as indicated from the national occupational disease report from 2015 to 2016 in China (19). Silicosis, CWP, welder pneumoconiosis and asbestosis were listed as the top four of pneumoconiosis types in this study due to the developmental non-ferrous metals, metallurgy and coal mining industries in Jiangsu province. Stage I (80%) still accounted for the majority of the total patients suffering from pneumoconiosis, followed by stage II and stage III, which was consistent with the reports of characteristics of pneumoconiosis in Shanghai, Hubei and Tianjin (20,21). Stage of pneumoconiosis would upgrade with the progression of lung pathological variations since the occurrence of pneumoconiosis is considered a gradual process. Pneumoconiosis stages were correlated with the first diagnosis time. Most cases were diagnosed at stage I, thereby indicating that the occupational health monitoring policy (e.g., regular physical examinations) may exert a certain effect. There were statistically significant differences in the average age of onset and dust exposure period at different stages, which were correlated with the time, concentration, free silica content and individual sensitivity of pneumoconiosis cases, and whether they were checked and diagnosed timely (22).
The mortality rate in 15,402 patients suffering from pneumoconiosis was 19.89%, higher than 13.4% (366 of 2,738) in a survival analysis conducted in a cohort of 2,738 cases with simple CWP in the Huai-Bei coal mine (17). The difference above might further confirm that the death rate of silicosis is higher than that of CWP. Silicosis is generally known as the most common, fastest- progressing and most harmful type of pneumoconiosis (23). Silicosis (68.49%) was confirmed as the most part of pneumoconiosis in this study. First diagnosed in stage III was correlated with shorter dust exposure time. The proportion of silicosis in each stage may be the important reason for this difference. There were 66.37% (8,178/12,322) silicosis patients in stage I, 75.44% (1,800/2,386) in stage II, 81.70% (567/694) in stage III. Silicosis progresses to stage III was short than other types. Although their dust exposure time was not long, a proportion of silicon dust exposed workers were directly classified as stage III pneumoconiosis in the first diagnosis. There were a highest proportion of silicosis in stage III in this study, the dust exposure time of stage III may short than other two stages. The overall average survival time of all the subjects, correlated with the first diagnosed age, reached 14.74±9.57 years. As reported by Han et al., 459 CWP patients diagnosed from 1963 to 2014 in a state-owned coal mine in China had an average life span of 12.1 years (24). Short survival time could reveal the great harm of dust to people and correspond to the high first diagnosed age of pneumoconiosis. The age of onset of novel patients suffering from pneumoconiosis in China showed an overall upward trend. The first diagnosed age of 2,738 CWP patients from 1963 to 1989 in Huai-Bei coal mine reached 47.6±7.0 years (17), and the average age of onset of 838 CWP cases from 1970 to 2011 in the Kailuan Colliery Group was 52.0±4.3 years (25). The median age of onset of novel cases of pneumoconiosis from 1997 to 2009 in China reached 51.00 years. Furthermore, age of onset of silicosis was elevated from 51.0 years in 1997 to 54.5 years in 2009 (26).
As revealed from the life table, overall mortality probability and mean survival time increased with the age of 15,402 patients suffering from pneumoconiosis. The mortality probability ascending trend with age existed in considerable survival analysis reports. As illustrated by a study on global occupational-attributable deaths and disability-adjusted life years (DALYs) from pneumoconiosis, the age-standardized death rates were the highest in the older age groups, and the highest rate of DALYs was identified in 75–84-year age group (9). The increase in mean survival time may be determined by the majority of observed cases in older age groups. As indicated from existing research, the survival time of stage I pneumoconiosis cases can reach the level of normal people since programmatic treatment was arranged for free under the diagnosis of pneumoconiosis (15,27). Given the higher age of onset and 80% of cases first diagnosed at stage I, numerous subjects were involved in the older age groups here.
Pneumoconiosis was classified into three stages in China. Stage I represented simple pneumoconiosis, and stage III represented categories B and C of complicated pneumoconiosis; stage II was between the above two stages (14). The mortality rate tended to increase at the three stages of pneumoconiosis, followed a declining life expectancy. Stage III with the most severe lung function damage had a notable ascent in death risk over cases at stage II or I (28). As indicated by the cumulative survival curve, the cumulative survival rate of patients suffering from pneumoconiosis decreased with the upgrade of pneumoconiosis. The prevention and delay of the progression of pneumoconiosis were found to significantly prolong the survival time of patients.
As indicated by the Cox model, dust exposure years, first diagnosed at stage II or stage III, stage I upgrade to stage II, stage I upgrade to stage III and low economic level were found as the vital risk factors of the survival of patients suffering from pneumoconiosis. Han et al. reported that the RR of death was 0.902, 0.943, 1.897 for the initial dust exposure year, age of onset and first diagnostic stage, respectively (24). Moreover, the RR of progression from stage I to III was 2.360 in another survival analysis of 2,738 cases with simple pneumoconiosis (17). Economic level will affect the preventive measures against dust in the region, the regulations and policies adopted for dust enterprises, the medical expenses of pneumoconiosis patients, nursing expenses, nutrition expenses, and wages for lost work. etc., which will indirectly affect the diagnosis and treatment of patients with pneumoconiosis. As demonstrated by the multivariate analysis results above, the prognosis of pneumoconiosis could be a complex process dependent of multiple factors. Lung tissue was more severely damaged as the patients were more exposed to occupational dust, which indirectly caused a shortened survival time of patients (29). Furthermore, as confirmed by the three survival analyses of pneumoconiosis whether the patients were first diagnosed at stage II or stage III or subsequently upgraded to stage II or stage III, the case’s survival time would be directly affected. Delaying the progression of pneumoconiosis and staying at stage I could significantly impact the survival of patients suffering from pneumoconiosis.
This study might have the following shortcomings. The analysis of factors affecting the survival time of patients suffering from pneumoconiosis may be insufficient. The survival and progression of pneumoconiosis is indeed affected by many factors. Due to the limitation of the collected data, we have included all influencing factors which can be analyzed. Dust exposure years, age of onset, first diagnostic stage, types of pneumoconiosis, industry category, upgrade of stages of pneumoconiosis, and economic level were included in the Cox proportional hazards regression model.
There might be misclassification bias since the study used medical diagnostic records preserved over the past few decades though the medical records for every case were checked during the annual regular follow-up. In addition, information bias (e.g., retrospective bias or investigator bias) were inevitable in this long-term retrospective study. A series of measures were employed to control the above bias. In the follow-up phase, questions were designed as objective indicators to reduce the survey bias (e.g., dust-exposure age and diagnosis time). Newly diagnosed patients suffering from pneumoconiosis have been preferentially selected during the screening stage. Cox proportional hazards regression model has used to analyze multiple and complex factors that affect the survival time of patients suffering from pneumoconiosis. Meantime, as impacted by the limited data, there was still a lack of many factors from the onset to death of pneumoconiosis cases, further accumulation and improvement of epidemiological data are needed in the future.
Existing studies showed that the data of occupational disease reported less than 7% of the actual incidence of occupational diseases (1,30). The pneumoconiosis cases investigated in this study were just the tip of the iceberg. The incidence of pneumoconiosis might still increase subsequently with the development of industry and the improvement of diagnostic technology (7). Elimination of dust and personal protection of pneumoconiosis workers are of high significance. A considerable number of measures have been employed to protect workers against dust inhalation. However, dust concentration in the workplace seriously exceeds the standard, and insufficient occupational health supervision, limited dust prevention technology and low coverage rate of occupational health examination and others still reduce the incidence of pneumoconiosis. Pneumoconiosis refers to an incurable chronic disease. Social support and care significantly impact the quality of life for cases. Much effort is required to reduce the incidence of pneumoconiosis and prolong the survival time of patients suffering from pneumoconiosis.
We would like to express our gratitude to all participants for their contributions to the study.
Funding: This study was supported by the General Project of Jiangsu Natural Science Foundation (No. BK20201485) and Key Medical Research Project of Jiangsu Provincial Health Commission (No. K2019026).
Reporting Checklist: The authors have completed the reporting STROBE reporting checklist. Available at https://apm.amegroups.com/article/view/10.21037/apm-21-2824/rc
Data Sharing Statement: Available at https://apm.amegroups.com/article/view/10.21037/apm-21-2824/dss
Peer Review File: Available at https://apm.amegroups.com/article/view/10.21037/apm-21-2824/prf
Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://apm.amegroups.com/article/view/10.21037/apm-21-2824/coif). The authors have no conflicts of interest to declare.
Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. The study was conducted based on the Declaration of Helsinki (as revised in 2013). The study was approved by the Ethics Committee of the Jiangsu Province Center for Disease Control and Prevention (No. 2012025) and individual consent for this retrospective analysis was waived.
Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.
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