| Industrial silica dust is the most common occupational hazard in China.Long-term exposure to silica dust could cause serious damage to the health of the occupational population.Several stuides have been conducted on this topic;however,we still found some shortcomings:(1)Systematic reports of silicosis incidence due to long-term industrial silica dust exposure are insufficient;(2)Smoking is very common among industrial workers,but it is not clear whether smoking directly affects the relationship between silica dust exposure and silicosis morbidity or mortality among workers;(3)Does silica dust exposure and cigarette smoking have a joint effect on cardiopulmonary disease mortality or total mortality,whose impact on workers' mortality risk is greater than the impact of simply industrial silica dust exposure or simply cigarette smoking?In order to evaluate the health effect of long-term industrial silica dust exposure,we used an already established entire cohort of iron miners(totally 7665 workers)who had been registered and worked for more than one year from January 1st,1960 to December 31 st,1974,and were followed up until December 31 st,2012.We collected historical airborne dust monitoring data for each position of the iron mine and conducted actual dust concentration measurement for evaluating the industrial dust exposure level.The cohort members‘ historically physical examination and silicosis diagnosis data were collected from staff hospital,the death cause information from the deceased members were regularly updated and the smoking information from the smokers were obtained through questionnaires.Quality control was performed during the data collection process,and systematically logic checks were performed on the aggregated data.The purpose of this study is to clarify the historical variation trends of the industrial dust concentration in iron mine workplaces,to analyze the relationship between long-term silica dust exposure and morbidity risk of silicosis as well as mortality risks of various diseases among workers,and to explore the incidence law of silicosis caused by industrial silica dust,as well as the impact of silica dust exposure and cigarette smoking on deaths of various diseases,so as to provide a practical and scientific basis for the development of occupational site silica hazard prevention measures and occupational workers health monitoring measures.This study is divided into three parts as follows.Part ? Evaluation on exposure level of iron mine industrial dustObjective Respirable dust can better reflect the damage of dust on health,but in China the determination of respirable dust concentration had not been carried out before 2010.Meanwhile,in order to facilitate the comparison between the research results in China with similar research results abroad,in this part,we conducted parallel measurement and analytical comparison of dust measurement methods of Chinese total dust concentration and Western respirable dust concentration,and established a functional equation between Chinese total dust and Western respirable dust.At the same time,by using the measurement results of the industrial dust concentration in different positions of iron mine over the years,the long-term variation trend will be analyzed..Methods 1.We selected sampling points in the workplace according to relevant standards.Several samplers were parallelly placed at the sampling point,they were the American and Europe total dust and respirable dust samplers(complying to American Conference of Governmental Industrial Hygienists(ACGIH)and British Medical Research Council(BMRC)formulated particle separation curves,respectively)and Chinese total dust sampler.Western total dust and respirable dust was continuously sampled for more than 2 hours,and Chinese total dust was sampled at multiple times among different periods,15 minutes for each time.The dust concentration was determined by the filter mass method.The functional equation between Chinese total dust and Western respirable dust was fitted through a large amount of parallel sampling results.2.The above functional equation was used to convert the mine airborne total dust concentration monitored from 1953 to 2012 into the respirable dust concentration,and the airborne respirable dust concentration in the whole mine as well as underground pit,locomotive transportation and ore-separation workshop during different monitoring periods was analyzed for variation trend,especially changes after taking dust control measures.3.The whole mine as well as underground pit,locomotive transportation and ore-separation workshop were evaluated for whether the respirable dust had met the national occupational health limit requirements during different monitoring periods.Results 1.The consistency of dust determination results between China and Western country was good.The correlation coefficient between Chinese total dust concentration and European total dust concentration was 0.949(P < 0.001),and it was 0.965(P < 0.001)between Chinese total dust concentration and American total dust concentration.As to the respirable dust concentration,the correlation coefficient was 0.781(P < 0.001)between China and Europe,and it was 0.974(P < 0.001)between China and America.After fitting,the functional equation between total dust concentration and respirable dust concentration in the iron mine was: European method: Respirable dust concentration = 0.1878 × Total dust concentration;American method: Respirable dust concentration =-0.0007 × Square of Total dust concentration + 0.1432 × Total dust concentration.Since the European method was adopted as respiratory dust measurement method in China,we used the former method to conduct the conversion of respiratory dust concentration.2.The airborne respirable dust concentration of the iron mine had generally declined yearly.Among them,the underground pit respirable dust concentration had steadily declined,while the locomotive transportation and ore-separation workshop respirable dust concentration had fluctuated during the decline.Comparing the respirable dust concentration in different work types,we found the results as follows: underground pit types: excavation > supporting,locomotive transportation types: operation > auxiliary,ore-separation workshop types: casting > crushing/sorting.3.The airborne respirable dust concentration of the whole mine,underground pit,locomotives transportation,and ore-separation workshop was generally decreased yearly.In the 1950 s,the respirable dust concentrations in the whole mine,underground pit,locomotives transportation,and ore-separation workshop were 1.89 ± 1.16 mg/m3,2.35 ± 1.31 mg/m3,1.61 ± 0.99 mg/m3 and 1.58 ± 0.92 mg/m3,respectively.Compared with national occupational exposure limit of 0.7 mg/m3,they exceeded the national occupational health standard by about 1~4 times.After the ventilation and dust removal measures were adopted from 1958 to 1960,the respirable dust concentration in the workplace decreased yearly,and the over-standard range gradually narrowed.After 1980 when the dust removal reform was carried out through the whole mine,the respiratory dust concentrations in the whole mine,underground pit,locomotives transportation,and ore-separation workshop were 0.42 ± 0.21 mg/m3,0.48 ± 0.15 mg/m3,0.42 ± 0.23 mg/m3 and 0.36 ± 0.23 mg/m3,respectively,already meeting the national occupational health requirements.Conclusion The dust determination method in China is basically consistent with that in Western country,with high correlation.The respirable dust concentration in iron mine workplace far exceeded the national occupational health standards in the early 1950 s.After the dust removal measures,the respirable dust concentration in all positions of the iron mine decreased year by year.After 1980,all the positions have met the national occupational health standards.Part ? Characteristics of silicosis morbidity among iron workers exposed to silica dustObjective In order to assess the relationship between long-term industrial silica dust exposure and silicosis morbidity,in this part,we explored the silicosis morbidity characteristics among iron mine silica-dust-exposed workers with cohort study method,by establishing a job-silica dust exposure matrix to estimate the individual cumulative silica dust exposure level and to analyze the relationship between cumulative silica dust exposure and silicosis morbidity,as well as the role of cigarette smoking on silicosis morbidity.Our aim is to provide a reliable scientific basis for the development of silicosis prevention and control measures.Methods 1.Research subjects: A total of 3658 industrial silica-dust-exposed workers were selected from the cohort.Patients diagnosed of silicosis before 1960 were excluded.They were followed up until the end of 2012.The workers‘ basic data,work history,smoking information and silicosis diagnosis report were collected.All X-ray films were re-read from 2012 to 2015 by expert with pneumoconiosis diagnostic qualification,and judged for silicosis stage according to the 2009 Diagnostic Criteria for Pneumoconiosis.Using the research results in the first part,the iron mine historical respirable dust concentration were converted into the silica dust concentration.A job-silica dust exposure matrix was established,and the individual cumulative silica dust exposure was estimated combined with the workers‘ previous work history.2.Statistical analysis: The cumulative morbidity risk of silicosis between different positions was compared by using the life table method,and calculated by groups according to smoking status.The Cox proportional hazard model was used to analyze the effects of silica dust exposure,position type and smoking on the morbidity risk of silicosis among workers.The Poisson regression model was used to analyze the effects of silica dust exposure and smoking onto the morbidity risk of silicosis among workers,and to predict the morbidity risk due to various exposure concentrations and exposure durations.Based on the relationship between cumulative silica dust exposure and person-year incidence of silicosis,the life table method was used to predict future new cases of silicosis.Results 1.A total of 332 workers were diagnosed among 3658 silica-dust-exposed workers in the iron mine,including 112,178 and 42 in underground pit,locomotive transportation and ore-separation workshop,and 231,69 and 32 of Stage I,Stage II and Stage III cases,respectively.The person-year incidence and cumulative incidence of silicosis were 218.90/105 and 10.36%,respectively.A exposure-response relationship was found between the cumulative silica dust exposure and the morbidity of silicosis.For each 1 mg/m3-y increase of cumulative silica dust exposure,the morbidity risk of silicosis correspondingly increased by 37.5%.2.There were 271 and 61 silicotic patients with smoking and non-smoking status,and the morbidity risk of smoking workers was 1.637 times that of non-smoking workers,with statistical significance(P < 0.05).For each 1 mg/m3-y increase of cumulative silica dust exposure,the morbidity risk of silicosis in smoking and non-smoking workers correspondingly increased by 39.1% and 35.0%.3.The proportion of delayed silicosis,silicosis with tuberculosis and progression from Stage I silicosis was 51.20%,23.80% and 23.26%,respectively.The incidence rate of tuberculosis in patients with stage I to III silicosis was 16.88%,33.33%,and 53.13%,respectively.The overall average progression time for silicotic patients was 6.71 ± 5.45 years.Patients of underground pit,diagnosed before 35 years old and accompanied with tuberculosis were prone to progression,with average progression time of 7.95 ± 6.06 year,14.08 ± 6.47 year and 7.02 ± 5.40 year.4.The morbidity risk of silicosis would increase as the silica dust exposure concentration increased and the exposure duration extended.When the exposure level and the exposure duration were approximate,the morbidity risk of smoking workers was as 1.20~1.35 times as that of non-smoking workers.According to the current silica dust exposure level,there would be 218 new cases after 2012 when the follow-up ended.Conclusion There is a significant exposure-response relationship between the cumulative silica dust exposure and the morbidity risk of silicosis in the iron mine.The morbidity risk of silicosis increases correspondingly with the increase of the cumulative silica dust exposure.Smoking significantly increases the morbidity risk of silicosis.Part ? Characteristics of death and analysis of causes of death among iron workersObjective In order to evaluate the health effects of cigarette smoking and silica dust esposure on workers,in this part,we explored the relationship between silica dust exposure,cigarette smoking and the causes of death in various diseases,and analyzed the possible joint effects of silica dust exposure and cigarette smoking on workers‘ mortality.Our aim is to reduce the mortality risk of workers due to various diseases and to improve their life quality.Methods 1.Research subjects: An entire cohort of totally 7665 workers were included and their life status was followed up until the end of 2012.Among them,the retrospective follow-up was performed before 1986,and afterwards the prospective follow-up was conducted.The members‘ basic data,work history,death cause information and smoking information were collected.Individual cumulative silica dust exposure calculation was the same as the second part.Through questionnaires,we collected the average daily cigarettes smoked and the smoking duration from the research subjects,and the workers‘ pack-year was estimated.The underlying cause of death information was classified according to International Classification of Disease 10 th Revision(ICD-10),while 84.5% was from staff medical records,15.3% was from mine death certificates,and 0.2% was from oral description.2.Statistical analysis: Based on the national mortality rate of urban residents from 1970 to 2012,the expected death number of various disease causes among iron mine workers was calculated,and then the standardized mortality ratio(SMR)was estimated.The survival analysis was used to compare the survival time and cumulative survival rate between different positions,silica dust exposure and smoking subgroups.The life table method was used to compare the cumulative mortality risk of different causes of death.The Cox proportional hazard model was used to analyze the impact of cumulative silica dust exposure and smoking pack-years on the mortality risks of various diseases.The relative excess risk due to interaction(RERI),attributable proportion due to interaction(AP),synergy index due to interaction(SI),multiplicative interaction term,and population attributable risk(PAR)percentage were used to explore the possible joint effects of silica dust exposure and cigarette smoking on workers' mortality risk of various diseases.Results 1.A total of 2814 workers were deceased in iron mine.The person-year mortality and mortality proportion were 891.15/105 and 36.71%,respectively.The standardized mortality ratio was 1.49.The average age of death was 64.10 ± 12.66 years old.The underlying causes of death were orderly malignant neoplasm,cerebrovascular disease,cardiovascular disease and respiratory disease.2.The person-year mortality of silica-dust-exposed workers and non-silica-dust-exposed workers were 1042.03/105 and 751.60/105,respectively.The total mortality risk of silica-dust-exposed workers was 1.301 times that of non-silica-dust-exposed workers,and the mortality risks of malignant neoplasm,lung cancer,cardiovascular disease,ischemic heart disease,respiratory disease and chronic bronchitis among silica-dust-exposed workers were 1.349,1.682,1.438,1.571,3.129 and 2.038 times that among non-silica-dust-exposed workers,respectively,with statistical significance(P < 0.05).For each 1 mg/m3-y increase of cumulative silica dust exposure,the total mortality risk correspondingly increased by 11.8%,and the mortality risk of cardiovascular disease,pulmonary heart disease,respiratory disease,silicosis and chronic bronchitis increased by 17.7%,21.4%,35.1%,40.9%,38.8% and 25.8%,respectively.A exposure-response relationship was found between cumulative silica dust exposure and the total mortality,lung cancer mortality,cardiovascular disease mortality,pulmonary heart disease mortality,respiratory disease mortality,silicosis mortality,and chronic bronchitis mortality.3.The person-year mortality of smoking workers and non-smoking workers were 922.99/105 and 829.95/105,respectively.The mortality risks of malignant neoplasm,lung cancer,pulmonary heart disease,respiratory disease and silicosis among smoking workers were 1.675,4.208,1.977,1.502 and 2.445 times that among non-smoking workers,respectively,with statistical significance(P < 0.05).For each 1 pack-year increase of smoking pack-years,the total mortaility risk correspondingly increased by 0.1%,and the mortality risk of malignant neoplasm,lung cancer,respiratory disease and chronic bronchitis increased by 0.7%,2.3%,0.6% and 0.9%,respectively.The smoking pack-year had a exposure-response relationship with malignant neoplasm death,lung cancer death,respiratory disease death,and chronic bronchitis death.4.The total mortality risk of silica-dust-exposed and smoking workers was 1.344 times that of non-silica-dust-esposed and non-smoking workers.The mortality risks of malignant neoplasm,lung cancer,cardiovascular disease,ischemic heart disease,pulmonary heart disease,respiratory disease,silicosis and chronic bronchitis among silica-dust-exposed and smoking workers were 2.196,6.586,1.709,1.690,3.931,5.052,8.658 and 3.725 times that among non-silica-dust-exposed and non-smoking workers.They were all with statistical significance(P < 0.05).5.Silica dust exposure and cigarette smoking had a joint effect on total mortality and lung cancer mortality which significantly exceeded the additive model.Of the total mortality,12.56% were attributed to simply silica dust exposure,5.50% were attributed to simply cigarette smoking,21.20% were attributed to both silica dust exposure and cigarette smoking.Of the malignant neoplasm,lung cancer,cardiovascular disease and respiratory disease mortality,14.28%,24.56%,17.29% and 50.40% were attributed to simply silica dust exposure,respectively,30.87%,67.97%,10.96% and 24.93% were attributed to simply cigarette smoking,respectively,and 48.34%,81.38%,35.68% and 76.02% were attributed to both silica dust exposure and cigarette smoking,respectively.Conclusion There is a significant exposure-response relationship between silica dust exposure and the workers‘ total mortality and mortality of cardiovascular disease and respiratory disease,as well as between cigarette smoking and the workers‘ mortality of malignant neoplasm and respiratory disease.There is a joint effect of silica dust exposure and cigarette smoking on the total mortality risk and mortality risk of lung cancer,which exceeds the simply additive effect. |
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