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 .