Occupational Health Risk Assessment Of Silicosis Caused By Silica Dust Exposure In Non-ferrous Metal Mines

Productive silica dust is an occupational hazard factor that seriously damages mining worker’s health.Long-term inhalation of high concentration of silica dust,especially with high content of free silica,could cause silicosis.In recent years,China has got great achievements in the silicosis prevention and control,but the high incidence of silicosis in China and the burden of disease caused by silicosis are still an important factor hindering Chinese occupational health improvement.The latest research result shows that the death hazard ratio(HR)of silicosis among non-ferrous metal mine workers exposed to dust is still higher than that of pottery workers even at the same concentration of silica dust.At present,domestic studies on the dose-response relationship between the cumulative silica dust exposure and the cumulative incidence of silicosis in non-ferrous metal mines are still insufficient,and there is also a lack of large scale sample study on occupational health risk assessment of silicosis caused by silica dust exposure in non-ferrous metal mining enterprises.In this study,we selected typical non-ferrous metal mining enterprises,and collected enterprises basic information and silica dust exposure data of key positions by using on-site hygiene investigation and detecting methods.At the same time,a retrospective cohort study was conducted in non-ferrous metal mines with relatively complete silica dust exposure data and occupational health monitoring data,and the life-table method was used to establish the dose-response relationship between the cumulative exposure to silica dust and the cumulative incidence of silicosis.Finally,combining the International Mining and Metal Commission(ICMM)risk rating table method and occupational hazard risk index method(INDEX),as well as the above dose-response formula between the cumulative exposure of silica dust and the cumulative incidence of silicosis,we made comprehensive assessment of the occupational health risk level of silicosis caused by silica dust exposure in non-ferrous metal mines.Part One Dose-response relationship between cumulative exposure to silica dust and cumulative incidence density of silicosisObjective:To explore the dose-response relationship between cumulative exposure to silica dust and cumulative incidence density of silicosis in non-ferrous metal mines.Methods:Using retrospective cohort study,three non-ferrous metal mining enterprises were selected to collect silica dust exposure monitoring data of different types of work over the years,as well as occupational health files of workers exposed to silica dust,and we established the dose-response relationship between cumulative exposure to silica dust and cumulative incidence density of silicosis in non-ferrous metal mines.The life-table method was used to calculate silicosis cumulative incidence density according to total cohort,cohort with/without smoking history.Finally,the linear regression equations between the cumulative exposure to silica dust and the cumulative incidence density of silicosis were calculate,and we also test the similar formula made by previous researcher.Results:Results of the retrospective cohort study showed that:(1)We collected one large and two medium-sized non-ferrous metal mining enterprises from January 2008 to October 2020.The concentrations of total dust and respirable dust,and content of free silica ranged from 0.06 to 41.94 mg/m3,0.05 to 12.54 mg/m3,and 16.8%to 17.3%,respectively;(2)The silica dust exposure rate of three non-ferrous metal mining enterprises ranged from 28.14%to 65.38%.Finally,2551 workers were included in this study:male workers were 94.98%,on-the-job were 59.27%,retired workers were 40.73%,workers with smoking history were 47.47%,and their initial dust exposure year ranged from 1940 to 2020.Furthermore,included subjects were followed up for 57480 person-years,and the number of silicosis patients was 247 with 89.47%patients diagnosed with stage Ⅰ;(3)Both cumulative exposure to total silica dust(Dt)and cumulative exposure to respiratory silica dust(Dr)had an approximate linear relationship between cumulative incidence density of silicosis(Pn).Their formulas were logitPn=1.76lgDt-5.59(F=486.84,P<0.001)and logitPn=1.55lgDr-4.53(F=545.44,P<0.001),respectively,and both their coefficients of determination(R2)were above 0.97.Compared with previous researcher’s related formula,the self-built dose-response formula was more suitable;(4)When Dt and Dr were less than 13.53 mg/m3-years and 3.47 mg/m3-years,respectively,the Pn of the cohort with smoking history was higher than that of the non-smoking cohort.Whereas when Dt and Dr were more than 13.53 mg/m3-years and 3.47 mg/m3-years,respectively,except for 20.91 mg/m3-years(Dt)and 6.55 mg/m3-years(Dr),the Pn of the cohort with smoking history was lower than that of the cohort without smoking history.Conclusions:There was a dose-response relationship between cumulative exposure to silica dust and cumulative incidence density of silicosis in non-ferrous metal mines.Moreover,previous smoking may only increase the cumulative incidence density of silicosis within the range of low cumulative silica dust exposure.Part Two Occupational health risk assessment of silicosis caused by silica dust exposure in non-ferrous metal minesObjective:To explore the occupational health risk level of silicosis caused by silica dust exposure in typical non-ferrous metal mining enterprises.Methods:Using typical sampling method,44 non-ferrous metal mining enterprises were selected in seven provinces.Enterprises basic information and silica dust exposure data of key positions were collected by using on-site hygiene investigation and detecting methods.On-site hygiene investigation was conducted according to "Specifications of air sampling for hazardous substances monitoring in the workplace"(GBZ 159-2004),"Determination of dust in the air of workplace-Part 1:Total dust concentration"(GBZ/T 192.1-2007),and"Determination of dust in the air of workplace-Part 2:Respirable dust concentration"(GBZ/T 192.2-2007),and area sampling with short time(15 minutes)was used in this study.The determination of free silica content was conducted according to "Determination of dust in the air of workplace-Part 4:Content of free silica in dust"(GBZ/T 192.4-2007).Based on different production processes,equipment layouts,workplaces,production tasks,work content,post production systems,and labor quotas,dust-exposed positions were classified into different types of work according to the nature of the work characteristic.Finally,the International Mining and Metals Commission’s risk rating table method and occupational hazard risk index method,as well as the above self-built formula for the relationship between cumulative exposure to silica dust and cumulative incidence density of silicosis,were used to analyze the occupational health risk level of silicosis caused by silica dust exposure in non-ferrous metal mines from total and respirable dust views,respectively.Meanwhile,the square weighted Kappa test was performed to analyze the consistency between two risk assessment methods.Results:The On-site hygiene investigation and detecting results showed that:(1)This study finally included 37 non-ferrous metal mining enterprises,and most of them were underground enterprises(97.30%).A total of 8635 workers were employed in the included enterprises,and there were 4358 workers(50.47%)exposed to silica dust.M(Q1,Q3)of silica dust exposure rate was 60.71%(43.09%,73.41%),and medium enterprises,small enterprises,silver mining enterprises and antimony mining enterprises had relatively higher silica dust exposure rates(64.69%,60.71%,84.69%and 72.22%,respectively).All daily medians of different production scales,mining methods or product types were eight hours,and the daily silica dust exposure time median ranged from 4.00 to 6.00 hours.The personal dust protective equipment wearing rate medians ranged from 65.00%to 95.00%;(2)This study collected 501,456 and 498 samples for the determination of free silica content,total dust concentration and respirable dust concentration.M(Q1,Q3)of free silica content was 21.18%(17.03%,30.47%),1.60(0.86,2.46)(mg/m3)for total dust concentration median,294(64.47%)total dust concentration samples exceeding Chinese permissible concentration-time weighted average(PC-TWA),0.36(0.13,0.68)for total silica dust concentration median,0.68(0.30,1.18)for respiratory dust concentration median,249(50.00%)and 495(99.40%)respirable dust concentration samples exceeding Chinese PC-TWA and American Conference for Governmental Industrial Hygienists(ACGIH)Recommended Exposure Limit(REL),respectively,0.17(0.06,0.31)for respirable silica dust median;(3)Medium-sized enterprises had the highest level of silica dust exposure:26.68%(14.82%,32.28%)for free silica content median,1.72(0.97,2.92)for total dust concentration median,68.42%total dust samples exceeding Chinese PC-TWA,0.46(0.15,0.81)of total silica dust concentration median,0.76(0.59,1.25)for the respirable dust concentration median,53.49%respirable dust samples exceeding Chinese PC-TWA,0.19(0.10,0.42)for respirable silica dust concentration median,followed by small-sized enterprises;(4)Rock drillers and crushers were exposed to higher levels of silica dust:24.73%(17.26%,35.50%)and 23.50%(17.05%,26.78%)for their silica content medians,2.25(1.39,3.11)and 2.23(1.29,3.01)for their total dust concentration medians,84.00%and 76.47%for their total dust samples exceeding Chinese PC-TWA,0.47(0.37,0.88)and 0.51(0.22,1.02)for their total silica dust concentration medians,1.28(0.87,1.59)and 0.94(0.47,1.40)for their respiratory dust concentration medians,80.00%and 66.67%for their respirable dust samples exceeding Chinese PC-TWA,0.30(0.24,0.53)and 0.18(0.08,0.64)for their respiratory dust concentration medians.Occupational health risk assessment and dose-response relationship prediction results showed that:(1)The overall occupational health risk level of silicosis caused by silica dust exposure in non-ferrous metal mining enterprises was medium:①In term of total dust,ICMM risk rating table method and INDEX method indicated high and medium risks,respectively(Sweighted were 3.52 and 2.79);②In term of respirable dust,both ICMM risk rating table method and INDEX method indicated medium risks(Sweighted were 2.78 and 2.35);③Medians of silicosis cumulative incidence density in term of total dust after 10,20 and 30 years were 9.98%(4.67%,16.47%),19.95%(9.34%,32.95%)and 29.93%(14.01%,49.42%),and medians of silicosis cumulative incidence density in term of respirable dust after 10,20 and 30 years were 3.27%(1.85%,4.78%)and 6.55%(3.70%,9.84%)and 9.82%(5.55%,14.33%);(2)Medium-sized enterprises risk level was higher than other two production scales enterprises:①In term of total dust,ICMM risk rating table method indicated high risks for large,medium and small production scale enterprises,respectively(Sweighted were 3.06,3.71 and 3.63),whereas the INDEX method indicated medium,high and medium risks for large,medium and small production scale enterprises,respectively(Sweighted were 2.53,3.16 and 2.60);②In term of respirable dust,both ICMM risk rating table method and INDEX method indicated medium risks for large,medium and small production scale enterprises,respectively(ICMM risk rating table Sweighted were 2.80,2.89 and 2.73,INDEX Sweighted were 2.17,2.85 and 2.16);③In term of total and respirable dust,the medium-sized enterprises medians of silicosis cumulative incidence density after 10 years,20 years and 30 years were greater than those of small or large enterprises;(3)Rock driller and crusher had higher risk levels than those of other types of work,and command work had the lowest risk:①In term of total dust,ICMM risk rating table method indicated extremely high risks for both rock driller and crusher(Sweighted were 4.32 and 4.11).and INDEX method indicated high risks for both rock driller and crusher(Sweighted were 3.12 and 3.44).ICMM risk rating table method and INDEX method indicated extremely low risks for command work(both Sweighted were 1.00);② In term of respirable dust,ICMM risk rating table method and INDEX method indicated extremely high and medium risks,respectively,for rock driller(Sweighted were 4.04 and 2.84),and both ICMM risk rating table method and INDEX method indicated high risks for crusher(Sweighted were 3.86 and 3.00).ICMM risk rating table method and INDEX method indicated extremely low and low risks,respectively,for command work(both Sweighted were 1.00);③In term of total and respirable dust,the rock driller and crusher medians of silicosis cumulative incidence density after 10 years,20 years and 30 years were greater than those of other work types.Furthermore,the lowest silicosis cumulative incidence density was screening work in term of total dust,whereas the lowest silicosis cumulative incidence density was tailings worker in term of respirable dust.ICMM risk rating table method and the INDEX method consistency analyses showed that these two risk assessment methods had strong consistencies in terms of total dust or respirable dust(Kappa were 0.644 and 0.600,and both P values<0.05),but further analyses according to production scale or work type indicated some changes:(1)Two risk assessment methods results based on large enterprises total dust and respirable dust exposure levels had strong consistencies(kappa values were 0.786(95%CI:0.698～0.874)and 0.627(95%CI:0.463～0.791),respectively,both P values were 0.000);results based on medium enterprises had general consistency(kappa value were 0.554(95%CI:0.374～733)and strong consistencies(kappa value was 0.606(95%CI:0.487～0.725),respectively;results based on small enterprises had strong consistency(kappa value was 0.615(95%CI:0.508～0.722))and general consistencies(kappa value was 0.582(95%CI:0.489～0.675),and all the above indexes had statistical significance(all P values were 0.000);(2)In term of total dust,all the assessment results of blasting,inspection and screening works had stronger consistency(all kappa values>0.80);crushing,mineral export and tailing works had strong consistency(all kappa values>0.60);drilling,driving,tunnelling,control engineering and grinding works had only general consistency(kappa values<0.60);in terms of respirable dust,tailing works assessment results had stronger consistency(kappa value was 0.800);blasting,crushing,grinding,screening and inspection works had strong consistency(all kappa values≥0.60);drilling,driving,tunnelling and mineral export works had only general consistency(kappa values<0.60),and control engineering works had poor consistency(kappa value was-0.216).Conclusions:(1)Non-ferrous metal mining enterprises silica dust exposure rate median in was 60.71%,and medians of free silica content,total silica dust concentration,and respirable silica dust concentration were 21.18%,0.36 mg/m3 and 0.17 mg/m3,and the proportions of total dust and respirable dust samples exceeding Chinese China permissible concentration-time weighted average were 64.47%and 50.00%,respectively;(2)Dust exposure levels of medium production-scale mining enterprises,rock driller and crusher were more higher;(3)The overall occupational health risk level of silicosis caused by silica dust exposure in non-ferrous metal mining enterprises was medium,and the risk levels of medium production-scale mining enterprises,rock driller and crusher were more higher(4)On the whole,the International Mining and Metals Commission’s risk rating table method and occupational hazard risk index method had strong consistency when conducting occupational health risk assessment of silicosis caused by non-ferrous metal mining enterprises silica dust exposure.