coal is a combustible mineral of organic origin , composed mainly of carbon and hydrocarbons .
the ranking of coal ( its hardness and age ) increases from peat to lignite , sub - bituminous to bituminous , and then to anthracitic . as its rank increases , the ratio of carbon to other chemicals and mineral contaminants increases .
coal mine dust contains various minerals in small amounts , such as silica ( quartz ) , iron , aluminum , pyrite , kaolin , and mica .
exposure to respirable mixed coal mine dust results in development of coal workers pneumoconiosis ( cwp ) , which is defined as the deposition of coal dust in the lungs , and the tissue s reaction to its presence .
the risk of cwp seems to depend on the concentration and duration of exposure to coal dust .
these non - palpable lesions appear as 14 mm in diameter , black areas distributed diffusely throughout the lung , but more so in the upper zone .
microscopically , the macular lesion consists of focal collections of coal dust - laden macrophages at the division of respiratory bronchioles .
it is also seen along the lymphatics within the secondary lobular septa , and beneath the visceral pleura .
focal emphysema is a subtype of centriacinar emphysema , which is known to be a histopathologic sign of cwp .
this process is mediated by antiproteases , secreted by coal dust - activated macrophages [ 24 ] , which , along with coal macules , forms the characteristic lesion of cwp .
earlier cwp studies have shown that patients with p opacities have a lower diffusing capacity than those patients with q or r opacities .
authors have attributed this to the development of focal emphysema in patients with p opacities .
cwp is classified as a simple or complicated form , depending on the presence of progressive massive fibrosis ( pmf ) .
clinically significant lung function impairment is believed to generally occur in complicated forms , predominantly in category b and c .
in contrast , the present data suggest that simple cwp can also be associated with lung function impairment .
moreover , it seems that this impairment has been principally due to emphysema and airflow obstruction , rather than underlying restrictive mechanisms .
the primary goal of the present study was to test whether coal dust exposure is associated with emphysema and/or airflow obstruction in the absence of smoking history .
we also investigated whether patients with p - type opacity have different traits regarding emphysema or airflow obstruction .
this research was observational , as well as a hospital - based cross - sectional study .
it was approved by the local ethics committee , although patient consent was not deemed necessary due to the study s retrospective nature .
the paper was written according to the  strengthening the reporting of observational studies in epidemiology ( strobe ) statement .
all coal mine workers or ex - workers seeking compensation for possible pneumoconiosis were included in the study , and were evaluated by the occupational disease diagnosis council of bulent ecevit university , zonguldak , turkey from 1 january 2013 to 31 may 2015 .
these miners were from the zonguldak coal basin , the major bituminous coal mine area in turkey .
the occupational diseases diagnosis council was composed of all faculty members working in the department of pulmonary medicine of bulent ecevit university .
the turkish institute for occupational health and safety certified that each author was an expert on pneumoconiosis ( on behalf of the international labour office , ilo ) .
evaluation of pneumoconiosis was based on chest radiography , spirometry , and high - resolution computed tomography ( hrct ) , and is mandatory according to nssi regulations .
the committee evaluated and classified each chest radiograph for perfusion , size , and shape of small and large opacities , according to the 1980 ilo .
the perfusion evaluation was based on chest radiographs , as there are no validated standards for ct scans .
the presence of the opacity perfusion subcategory , 1/0 or greater , or large opacity ( category
large opacities were classified as follows : category a : sum of the greatest diameter of one or more opacities exceeding 1 cm but less than 5 cm .
category b : sum of the greatest diameter of one or more opacities exceeding 5 cm , but their combined area not exceeding the equivalent of the right upper lung zone .
category c : one or more opacities with combined area exceeding the equivalent of the right upper zone .
five hundred thirty - two ( n=532 ) subjects were evaluated for compensation for pneumoconiosis .
we excluded smokers , former smokers , anyone with a history of tuberculosis or lung malignancy , as well as any pneumoconiosis .
the electronic database was able to collect demographics , spirometric , and procedural data , including these variables for each subject : age , tenure , smoking history , body mass index , fev1 , fvc , fef2575 , fev1/fvc , shape of small opacity , perfusion category , and potential large opacity .
the perfusion of small opacities was scored on a linear scale as follows : 1/0=2 , 1/1=3 , 1/2=4 , 2/1=5 , 2/2=6 , 2/3=7 , 3/2=8 , and 3/3=9 .
spirometric measurements were performed with the masterscreen pneumo - device ( flow - based ) at rest ( i.e. , while in a sitting position ) by a trained technician , according to guidelines of the american thoracic society and european respiratory society .
spirometric predictive values were based on the european community for coal and steel reference values .
all lung function data , except fev1/fvc values , are expressed as percentages of the predicted values .
hrct scans were obtained with the activision 16-row ct scanner ( toshiba medical systems , otawara , japan ; 1.5 mm slice thickness , tube voltage 140 kv , tube current 175 ma ) .
emphysema was quantified by the percentage of low - attenuation areas , using a hounsfield unit ( hu ) threshold < 950 and perc15 ( hu point below which 15% of the voxels are distributed ) .
after determination of appropriate hrct scan data ( n=53 of n=57 ) for analysis in an electronic database , we decided to use a control group for emphysema comparability because ct densitometric analysis is affected by multiple factors .
found the emphysema index ( % , sd ) to be 5.4 ( 66 ) and 30.2 ( 248 ) for non - smoking non - miners and never - smoking coal miners , respectively .
we used this value for sample size calculation along with an allocation ratio ( n2/n1=5.7 ) .
we accepted a 0.05 level for type 1 error (  ) and 0.2 for type 2 errors (  ) .
nine age- and sex - matched control cases ( n=9 ) with no history of working in a mine or ever having smoked were chosen randomly from the radiology database .
descriptive statistics of categorical variables are given as numbers or percentages ; continuous variables are given as medians ( interquartile range ) .
the mann - whitney u - test was used to compare the medians of variables when appropriate . the relationship between emphysema and spirometric variables
was evaluated with a partial correlation analysis , controlling for age and body mass index ( bmi )
. multiple linear regression analysis ( backward method ) was used to determine independent factors associated with the percent of emphysema and fev1/fvc , respectively .
the following variables were included in the regression models : age , bmi , tenure , fev1/fvc , perfusion score and large opacity for percent of emphysema ( dependent variable ) and age , bmi , tenure ( underground working years ) , percent of emphysema , perfusion score and large opacity for fev1/fvc ( dependent variable ) , respectively .
it was reported in previous studies that a - type large opacity shows similarity to simple cwp , rather than b- and c - opacities showing physiological impairment .
the presence of large opacity was defined as category b or c and was entered into the regression analysis as indicator variables .
statistical analysis was performed using spss version 18.0 for windows ( spss , ibm inc . , chicago , il ) and medcalc for windows , version 12.2.1.0 ( medcalc software , ostend , belgium ) .
p values were two - sided , and values less than 0.05 were considered statistically significant .
this research was observational , as well as a hospital - based cross - sectional study .
it was approved by the local ethics committee , although patient consent was not deemed necessary due to the study s retrospective nature .
the paper was written according to the  strengthening the reporting of observational studies in epidemiology ( strobe ) statement .
all coal mine workers or ex - workers seeking compensation for possible pneumoconiosis were included in the study , and were evaluated by the occupational disease diagnosis council of bulent ecevit university , zonguldak , turkey from 1 january 2013 to 31 may 2015 .
these miners were from the zonguldak coal basin , the major bituminous coal mine area in turkey .
the occupational diseases diagnosis council was composed of all faculty members working in the department of pulmonary medicine of bulent ecevit university .
the turkish institute for occupational health and safety certified that each author was an expert on pneumoconiosis ( on behalf of the international labour office , ilo ) .
evaluation of pneumoconiosis was based on chest radiography , spirometry , and high - resolution computed tomography ( hrct ) , and is mandatory according to nssi regulations .
the committee evaluated and classified each chest radiograph for perfusion , size , and shape of small and large opacities , according to the 1980 ilo .
the perfusion evaluation was based on chest radiographs , as there are no validated standards for ct scans .
the presence of the opacity perfusion subcategory , 1/0 or greater , or large opacity ( category
large opacities were classified as follows : category a : sum of the greatest diameter of one or more opacities exceeding 1 cm but less than 5 cm .
category b : sum of the greatest diameter of one or more opacities exceeding 5 cm , but their combined area not exceeding the equivalent of the right upper lung zone .
category c : one or more opacities with combined area exceeding the equivalent of the right upper zone .
five hundred thirty - two ( n=532 ) subjects were evaluated for compensation for pneumoconiosis .
we excluded smokers , former smokers , anyone with a history of tuberculosis or lung malignancy , as well as any pneumoconiosis .
the electronic database was able to collect demographics , spirometric , and procedural data , including these variables for each subject : age , tenure , smoking history , body mass index , fev1 , fvc , fef2575 , fev1/fvc , shape of small opacity , perfusion category , and potential large opacity .
the perfusion of small opacities was scored on a linear scale as follows : 1/0=2 , 1/1=3 , 1/2=4 , 2/1=5 , 2/2=6 , 2/3=7 , 3/2=8 , and 3/3=9 .
spirometric measurements were performed with the masterscreen pneumo - device ( flow - based ) at rest ( i.e. , while in a sitting position ) by a trained technician , according to guidelines of the american thoracic society and european respiratory society .
spirometric predictive values were based on the european community for coal and steel reference values .
all lung function data , except fev1/fvc values , are expressed as percentages of the predicted values .
hrct scans were obtained with the activision 16-row ct scanner ( toshiba medical systems , otawara , japan ; 1.5 mm slice thickness , tube voltage 140 kv , tube current 175 ma ) .
emphysema was quantified by the percentage of low - attenuation areas , using a hounsfield unit ( hu ) threshold < 950 and perc15 ( hu point below which 15% of the voxels are distributed ) .
after determination of appropriate hrct scan data ( n=53 of n=57 ) for analysis in an electronic database , we decided to use a control group for emphysema comparability because ct densitometric analysis is affected by multiple factors .
found the emphysema index ( % , sd ) to be 5.4 ( 66 ) and 30.2 ( 248 ) for non - smoking non - miners and never - smoking coal miners , respectively .
we used this value for sample size calculation along with an allocation ratio ( n2/n1=5.7 ) .
we accepted a 0.05 level for type 1 error (  ) and 0.2 for type 2 errors (  ) .
nine age- and sex - matched control cases ( n=9 ) with no history of working in a mine or ever having smoked were chosen randomly from the radiology database .
descriptive statistics of categorical variables are given as numbers or percentages ; continuous variables are given as medians ( interquartile range ) .
the mann - whitney u - test was used to compare the medians of variables when appropriate . the relationship between emphysema and spirometric variables
was evaluated with a partial correlation analysis , controlling for age and body mass index ( bmi )
. multiple linear regression analysis ( backward method ) was used to determine independent factors associated with the percent of emphysema and fev1/fvc , respectively .
the following variables were included in the regression models : age , bmi , tenure , fev1/fvc , perfusion score and large opacity for percent of emphysema ( dependent variable ) and age , bmi , tenure ( underground working years ) , percent of emphysema , perfusion score and large opacity for fev1/fvc ( dependent variable ) , respectively .
it was reported in previous studies that a - type large opacity shows similarity to simple cwp , rather than b- and c - opacities showing physiological impairment .
the presence of large opacity was defined as category b or c and was entered into the regression analysis as indicator variables .
statistical analysis was performed using spss version 18.0 for windows ( spss , ibm inc . , chicago , il ) and medcalc for windows , version 12.2.1.0 ( medcalc software , ostend , belgium ) .
p values were two - sided , and values less than 0.05 were considered statistically significant .
all coal mine workers or ex - workers seeking compensation for possible pneumoconiosis were included in the study , and were evaluated by the occupational disease diagnosis council of bulent ecevit university , zonguldak , turkey from 1 january 2013 to 31 may 2015 .
these miners were from the zonguldak coal basin , the major bituminous coal mine area in turkey .
the occupational diseases diagnosis council was composed of all faculty members working in the department of pulmonary medicine of bulent ecevit university .
the turkish institute for occupational health and safety certified that each author was an expert on pneumoconiosis ( on behalf of the international labour office , ilo ) .
evaluation of pneumoconiosis was based on chest radiography , spirometry , and high - resolution computed tomography ( hrct ) , and is mandatory according to nssi regulations .
the committee evaluated and classified each chest radiograph for perfusion , size , and shape of small and large opacities , according to the 1980 ilo .
the perfusion evaluation was based on chest radiographs , as there are no validated standards for ct scans .
the presence of the opacity perfusion subcategory , 1/0 or greater , or large opacity ( category a , b , or c ) was considered as evidence of cwp .
large opacities were classified as follows : category a : sum of the greatest diameter of one or more opacities exceeding 1 cm but less than 5 cm .
category b : sum of the greatest diameter of one or more opacities exceeding 5 cm , but their combined area not exceeding the equivalent of the right upper lung zone .
category c : one or more opacities with combined area exceeding the equivalent of the right upper zone .
five hundred thirty - two ( n=532 ) subjects were evaluated for compensation for pneumoconiosis .
we excluded smokers , former smokers , anyone with a history of tuberculosis or lung malignancy , as well as any pneumoconiosis .
the electronic database was able to collect demographics , spirometric , and procedural data , including these variables for each subject : age , tenure , smoking history , body mass index , fev1 , fvc , fef2575 , fev1/fvc , shape of small opacity , perfusion category , and potential large opacity .
the perfusion of small opacities was scored on a linear scale as follows : 1/0=2 , 1/1=3 , 1/2=4 , 2/1=5 , 2/2=6 , 2/3=7 , 3/2=8 , and 3/3=9 .
spirometric measurements were performed with the masterscreen pneumo - device ( flow - based ) at rest ( i.e. , while in a sitting position ) by a trained technician , according to guidelines of the american thoracic society and european respiratory society .
spirometric predictive values were based on the european community for coal and steel reference values .
all lung function data , except fev1/fvc values , are expressed as percentages of the predicted values .
hrct scans were obtained with the activision 16-row ct scanner ( toshiba medical systems , otawara , japan ; 1.5 mm slice thickness , tube voltage 140 kv , tube current 175 ma ) .
emphysema was quantified by the percentage of low - attenuation areas , using a hounsfield unit ( hu ) threshold < 950 and perc15 ( hu point below which 15% of the voxels are distributed ) .
after determination of appropriate hrct scan data ( n=53 of n=57 ) for analysis in an electronic database , we decided to use a control group for emphysema comparability because ct densitometric analysis is affected by multiple factors .
found the emphysema index ( % , sd ) to be 5.4 ( 66 ) and 30.2 ( 248 ) for non - smoking non - miners and never - smoking coal miners , respectively .
we used this value for sample size calculation along with an allocation ratio ( n2/n1=5.7 ) .
we accepted a 0.05 level for type 1 error (  ) and 0.2 for type 2 errors (  ) .
nine age- and sex - matched control cases ( n=9 ) with no history of working in a mine or ever having smoked were chosen randomly from the radiology database .
descriptive statistics of categorical variables are given as numbers or percentages ; continuous variables are given as medians ( interquartile range ) .
the mann - whitney u - test was used to compare the medians of variables when appropriate . the relationship between emphysema and spirometric variables
was evaluated with a partial correlation analysis , controlling for age and body mass index ( bmi )
. multiple linear regression analysis ( backward method ) was used to determine independent factors associated with the percent of emphysema and fev1/fvc , respectively .
the following variables were included in the regression models : age , bmi , tenure , fev1/fvc , perfusion score and large opacity for percent of emphysema ( dependent variable ) and age , bmi , tenure ( underground working years ) , percent of emphysema , perfusion score and large opacity for fev1/fvc ( dependent variable ) , respectively .
it was reported in previous studies that a - type large opacity shows similarity to simple cwp , rather than b- and c - opacities showing physiological impairment .
the presence of large opacity was defined as category b or c and was entered into the regression analysis as indicator variables .
statistical analysis was performed using spss version 18.0 for windows ( spss , ibm inc . , chicago , il ) and medcalc for windows , version 12.2.1.0 ( medcalc software , ostend , belgium ) .
p values were two - sided , and values less than 0.05 were considered statistically significant .
the study population consisted of 57 non - smoking men ( n=57 ) with cwp .
the control group included 9 ( n=9 ) non - smoking men without any known pulmonary disease .
the average age of subjects with cwp was 71 ( iqr , 6279 ) years .
densitometric analysis was performed for 54 of 57 cases due to inappropriate hrct scan data in 3 cases .
subjects were categorized into simple cwp ( 28 patients ) and complicated cwp ( pmf ) ( 29 patients ) groups .
table 1 shows demographics , spirometry , and radiologic measurements in cwp and control subjects .
percent of emphysema and perc15 values were significantly higher in the cwp group compared with the control group ( p<0.001 ) .
there was also a significant difference in terms of emphysema and perc15 between simple cwp and the control group ( p<0.001 , p=0.001 ) , as well as between the pmf group and the control group ( p<0.001 , p<0.001 ) , respectively .
spirometric evaluation revealed obstructive lung disease ( fev1/fvc<0.7 ) in the majority ( n=26 , 51% ) of patients with cwp .
we used the global initiative for chronic obstructive lung disease , and defined 25/51 ( 49% ) subjects at stage 0 , 3 ( 6% ) at stage i ( mild ) , 16 ( 31% ) at stage ii ( moderate ) , 6 ( 12% ) at stage iii ( severe ) , and 1 ( 2% ) at stage iv ( very severe ) .
the number and percent of subjects demonstrating the fev1/fvc ratio was less than 0.7 and that the fev1 was less than 80% of the predicted value was found to be 7 ( 26% ) in simple cwp and 16 ( 67% ) in the complicated cwp group .
restrictive impairment in lung function ( fev1/fvc 0.70 and fvc < 80% as predicted ) was seen in 9 patients ( 18% ) , all of which had complicated cwp .
correlation between spirometric findings and densitometric emphysema values was investigated . in patients with cwp , partial correlation analysis ( controlling for age and bmi ) revealed that the percent of emphysema and perc15 were correlated with fev1/fvc ( r=0.45 , p=0.002 , r=0.47 , and p=0.001 ) , and fef2575 ( r=0.36 , p=0.015 and r=0.56 , p<0.001 ) , respectively .
fev1 was only significantly correlated with perc15 ( r=0.38 , p=0.01 ) , but no correlation was found between fvc and percent of emphysema or perc15 . in partial correlation analysis , a significant correlation ( p<0.05 ) was observed between tenure and percent of emphysema ( r=0.36 ) , perc15 ( r=0.39 ) , fev1/fvc ( r=0.35 ) , fef2575 ( r=0.38 ) , fev1 ( r=0.33 ) , but not with fvc ( r=0.18 ) .
to evaluate factors associated with percent of emphysema and fev1/fvc , we performed multiple linear regression analyses . in the first model , factors associated with percent of emphysema were fev1/fvc and the presence of large opacity ( b and c ) ( =0.24 , p=0.009 and =3.97 , p=0.079 , respectively ) .
other factors not retained in the models were age , bmi , tenure , and the perfusion score . in the second model ,
emphysema ( =0.51 , p=0.018 ) and tenure ( =0.63 , p=0.044 ) remained independently associated with fev1/fvc .
other factors ( age , bmi , and perfusion score ) did not reach statistical significance in this model .
last , we evaluated whether the p - type ( n=18 ) and other type of opacities ( q , r , s , t , u ) ( n=7 ) have different traits in terms of emphysema or airway obstruction in cases with simple cwp . regarding the topics of age , perfusion score ( median ) , and tenure , there were no differences between groups .
percentage of emphysema and ratio of fev1/fvc were compared between groups using the mann - whitney u - test . although percentage of emphysema in subjects with p - type opacity was higher than it was in subjects with other type opacities , it was not statistically significant ( figure 2 ) .
in contrast , there was a statistically significant difference in fev1/fvc between subjects with p - type opacities and other types ( figure 2 ) .
the present study shows that cwp is closely associated with both emphysema and airflow obstruction in non - smoking subjects .
the average percent of emphysema and percent of subjects with a fev1/fvc ratio was less than 0.7 , and was 15% and 51% , respectively , in subjects with cwp . in a multivariate analysis ,
the percent of emphysema was associated with fev1/fvc and the presence of large opacity ( category b and c ) .
airflow obstruction ( fev1/fvc < 0.7 ) was associated with percent of emphysema and tenure .
we attempted to exclude effects of cigarette smoking and concomitant pulmonary pathology on the lungs , which is a major advantage of our study .
we did not find a correlation between ilo perfusion score and impairment of lung function or emphysema , which was similar to previous studies .
additionally , in subjects with simple cwp , p - type opacities showed a higher percent of emphysema , although not significant , but a higher fev1/fvc value than other opacities .
airflow obstruction , spirometrically , is diagnosed by decreased fev1/fvc , which is the result of decreased elastic recoil ( i.e. , emphysema ) of the lungs , promoting airflow and increased resistance ( airway wall thickening and bronchiolar obliteration ) of the airways , which limits airflow . concerning the relationship between cwp and emphysema ,
autopsy studies also have some limitations , such as the possibility of subjects with severe disease ( selection bias ) and a long interval between spirometry and death .
additionally , the effect of smoking on the development of emphysema or airflow obstruction can not be eliminated , which is the most important problem in these studies [ 2022 ] . despite this
, autopsy studies have strongly demonstrated that coal mine dust exposure is associated with emphysema , and its severity increases with increasing lung dust retention .
the best evidence was reported by kumpel et al . , who found that coal mine dust exposure and cigarette smoking presented similar problems in the prediction of emphysema severity .
furthermore , this study showed that the largest difference in emphysema severity was between non - smoking miners and non - smoking non - miners ( about 6-fold ) .
they found that the average percent of emphysema in non - smoking miners was 30% .
we found the percent of emphysema in non - smoking subjects with cwp to be 15% ( median , about 4-fold compared with the control group ) .
our study showed that large opacities ( category b and c ) were associated with emphysema in the regression model .
coal dust inhalation can initiate toxic and inflammatory processes in the airways and alveolar tissue .
moreover , it is well - documented that the prevalence of chronic bronchitis is higher in coal miners .
available evidence indicates that functional impairment is associated with small airways , rather than central or large airways .
we found that the prevalence of airway obstruction ( fev1/fvc < 0.7 ) in subjects with simple cwp was 37% .
similarly , kibelstis et al . reported that the prevalence of airway obstruction in non - smoking coal miners was 37.6% ( 10 ) . in our study , the percent of subjects with fev1/fvc ratio was less than 0.7 , and fev1 was less than 80% of the predicted value , or 26% in the simple cwp group and 67% in the pmf group , respectively .
the relationship between the ilo perfusion score and lung function ( or emphysema ) is not clear .
we did not find a significant relationship between the perfusion score and pulmonary function parameters , or the percent of emphysema .
gevenois et al . suggest that the presence of micronodules in coal miners has no effect on lung function , and that it should only be considered as a marker of exposure .
did not find an association between perfusion score and pulmonary function or blood gas parameters . on the other hand ,
it is also known that the presence of emphysema may reduce accuracy of reading of perfusion on chest radiographs . in our study ,
furthermore , morphologic changes at the level of respiratory bronchioles are not detected by conventional spirometry .
previous reports noted that the shape of small opacities is associated with impairment of pulmonary function .
it was shown that individuals with p opacities have a lower diffusing capacity than patients with q and r opacities .
there is additional evidence that p - type opacity is associated with increased air space size , which is a result of development of focal emphysema . in our study ,
emphysema more common in individuals with p - type opacity than with other types , although this difference was not statistically significant .
in contrast , other opacities were associated with airway obstruction ; obstructive disease of the small airways ( less than 1 mm in diameter ) are often not detected by conventional spirometry or measurements of airway resistance , and are ultimately responsible for less than 15% of total airway resistance .
this may be a reason for normal spirometric findings in individuals with p - type opacity .
pathologically , q and r opacities ( formerly micronodular and nodular , respectively ) were pathological palpable lesions .
it was similarly demonstrated in previous reports that these lesions were associated with increased airway resistance .
furthermore , in some studies , irregular opacities , which are considered to be related to tissue reaction to inhaled coal dust , were reported to be more strongly associated with impaired pulmonary function than were round opacities . in the group classified as  other type opacities ,  the reason for impaired pulmonary function may be the combined effects of these opacities mentioned above . however , the limited number of cases in this group makes interpretation difficult .
there are several limitations in our study , including a recall bias due to the retrospective design and small sample size , affected by strict inclusion criteria .
a large proportion of the study population was composed of retired coal miners who had already applied for compensation , so
selection bias was possible . underlying pulmonary disease ( e.g. , alpha 1-antitrypsin deficiency , asthma , or bronchial hyperreactivity ) may similarly change study outcomes .
another limiting factor was that the pre - bronchodilator spirometric assessment was performed , which can influence prevalence of airway obstruction .
the quantification of emphysema using ct densitometry can be influenced by multiple factors , including scanning protocol , scanner calibration , and variation in inspiratory level .
exposure to dust varies widely from mine to mine , and , in certain places , within a given mine .
multiple etiologic factors other than coal may be involved in individual cases , which makes causal interpretation difficult .
there are several limitations in our study , including a recall bias due to the retrospective design and small sample size , affected by strict inclusion criteria .
a large proportion of the study population was composed of retired coal miners who had already applied for compensation , so selection bias was possible .
underlying pulmonary disease ( e.g. , alpha 1-antitrypsin deficiency , asthma , or bronchial hyperreactivity ) may similarly change study outcomes .
another limiting factor was that the pre - bronchodilator spirometric assessment was performed , which can influence prevalence of airway obstruction .
the quantification of emphysema using ct densitometry can be influenced by multiple factors , including scanning protocol , scanner calibration , and variation in inspiratory level .
exposure to dust varies widely from mine to mine , and , in certain places , within a given mine .
multiple etiologic factors other than coal may be involved in individual cases , which makes causal interpretation difficult .
some authors have expressed doubt as to whether coal mine dust can cause clinically significant loss of lung function in the absence of complicated pneumoconiosis .
however , available evidence indicates that the presence of simple cwp contributes significantly to the impairment of lung function in coal miners , which also seems to be related to obstructive lung disease .
lifelong non - smokers offer a unique opportunity to evaluate the association between coal dust exposure and obstructive lung disease . despite all of these limitations
, this study highlights the association of obstructive lung disease with coal dust exposure in non - smoking coal miners with pneumoconiosis .
 backgroundaccumulating evidence shows that functional impairment in subjects with coal workers pneumoconiosis ( cwp ) is principally due to emphysema and airflow obstruction , rather than underlying restrictive mechanisms . 
 however , cigarette smoking has remained a major confounder . 
 the aim of this study was to assess whether coal dust exposure was associated with emphysema and/or airflow obstruction in the absence of smoking history.material/methodthe subjects evaluated for possible pneumoconiosis between 2013 and 2015 were retrospectively enrolled into this study . after excluding those with history of smoking , tuberculosis , or lung cancer , the study population was a total of 57 subjects . 
 the emphysema severity and airflow obstruction were quantified by computed tomographic densitometry analysis and spirometry , respectively . for comparability regarding emphysema , 9 age- and sex - matched nonsmoker ( n=9 ) control subjects without known lung disease 
 were randomly selected from a radiology database.resultsemphysema severity was significantly higher in the cwp group compared with the control group ( 15% vs. 4% , p<0.001 ) . 
 the median percent emphysema and percentage of those with fev1/fvc < 0.7 was 13% and 37% in subjects with simple cwp and 18% and 67% in subjects with complicated cwp , respectively . 
 percent emphysema and perc15 ( 15th percentile of the attenuation curve ) was correlated with fev1/fvc ( r=0.45 , r=0.47 ) and fef2575 ( r=0.36 , r=0.56 ) , respectively , but not with perfusion score . 
 a linear regression analysis showed that factors associated with emphysema were fev1/fvc ( =0.24 , p=0.009 ) and large opacity ( =3.97 , p=0.079 ) , and factors associated with fev1/fvc were percent emphysema ( =0.51 , p=0.018 ) and tenure ( =0.63 , 
 p=0.044).conclusionsour results support the observation that coal dust exposure is associated with emphysema and airflow obstruction , independent of smoking status . 
Background
Material and Methods
Study design
Participants and setting
Data source and measurement
Data analysis
Results
Discussion
Limitations
Conclusions