Occup Environ Med 2001 Chen 31 7

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Exposure to silica and silicosis among tin minersin China: exposure-response analyses and riskassessment

W Chen, Z Zhuang, M D Attfield, B T Chen, Pi Gao, J C Harrison, C Fu, J-Q Chen,W E Wallace

Abstract

ObjectivesTo investigate the risk of

silicosis among tin miners and to investi-

gate the relation between silicosis and

cumulative exposure to dust (Chinese

total dust and respirable crystalline silica

dust).

MethodsA cohort study of 3010 miners

exposed to silica dust and employed for at

least 1 year during 19605 in any of four

Chinese tin mines was conducted. Histori-

cal total dust data from China were used

to create a job exposure matrix for facility,job title, and calendar year. The total dust

exposure data from China were converted

to estimates of exposure to respirable

crystalline silica for comparison with

findings from other epidemiological stud-

ies of silicosis. Each workers work history

was abstracted from the complete em-

ployment records in mine files. Diagnoses

of silicosis were based on 1986 Chinese

pneumoconiosis Roentgen diagnostic cri-

teria, which classified silicosis as stages

I-IIIsimilar to an International Labour

Organisation (ILO) classification of 1/1 or

greater.

ResultsThere were 1015 (33.7%) minersidentified with silicosis, who had a mean

age of 48.3 years, with a mean of 21.3 years

after first exposure (equivalent to 11.0 net

years in a dusty job). Among those who

had silicosis, 684 miners (67.4%) devel-

oped silicosis after exposure ended (a

mean of 3.7 years after). The risk of silico-

sis was strongly related to cumulative

exposure to silica dust and was well fitted

by the Weibull distribution, with the risk

of silicosis less than 0.1% when the

Chinese measure of cumulative exposure

to total dust (CTD) was under 10 mg/m3-

years (or 0.36 mg/m3-years of respirable

crystalline silica), increasing to 68.7%when CTD exposure was 150 mg/m3-years

(or 5.4 mg/m3-years of respirable crystal-

line silica). Latency period was not corre-

lated to the risk of silicosis or cumulative

dose of exposure. This study predicts

about a 36% cumulative risk of silicosis for

a 45 year lifetime exposure to these tin

mine dusts at the CTD exposure standard

of 2 mg/m3, and a 55% risk at 45 years

exposure to the current United States

Occupational Safety and Health Adminis-

tration and Mine Safety and Health Ad-

ministration standards of 0.1 mg/m3 100%

respirable crystalline silica dust.

ConclusionsA clear exposure-response

relation was detected for silicosis in

Chinese tin miners. The study results

were similar to most, but not all, findings

from other large scale exposure-response

studies.(Occup Environ Med2001;58:3137)

Keywords: silicosis; exposure to silica; dose-responserelation

Silicosis remains a common occupationalrespiratory disease of workers exposed to dustaround the world. Millions of workers areexposed to crystalline silica, whether they areemployed in developed or developing coun-tries. According to results from statisticalsurveys, about 1.9 million workers in theUnited States and more than 10 million work-ers in China are potentially exposed to silicadust.1 2 Cases of silicosis were first reported inthe early 1950s in China, and the number ofcases increased rapidly with the growth ofindustry and the absence of methods to controldustfor example, wet drilling and

ventilationbefore 1960.2

In 1992, 5779 newcases were registered, rising to 9871 in 1995.3

At the same time, because of the administrativeand engineering controls instituted in work-places where airborne silica dust occurs, silico-sis had become less of a problem in developedcountries such as the United States, Sweden,and Japan.1 4

In Chinese metal mines, silicosis is socommon and severe that it is thought to be themain occupational disease. A greater than30-fold excess of mortality from pneumoconio-sis was found in a cohort study of 68 285 metalminers and pottery workers in China.5 In asubcohort of tin mine workers from that study,58 deaths from silicosis were observed against1.3 expected from national rates (standardisedmortality ratio=4351).

Since the 1950s, the Chinese governmenthas enforced systematic dust sampling regula-tions which require mines and companies inthe dusty trades to measure the total dust con-centration monthly in dusty work areas wher-ever exposure to silica was expected, tomeasure crystalline silica content in bulk dust ifdeemed necessary, and to report the results tohigher administrative authorities quarterly.Over four decades of dust monitoring data areavailable for investigations. However, earlier

Occup Environ Med2001;58:3137 31

Tongji MedicalUniversity, School ofPublic Health,

Department of LaborHealth andOccupationalDiseases, 13 Hang

Kong Lu, Wuhan,Hubei, PeoplesRepublic of China

W ChenJ-Q ChenC Fu

National Institute for

Occupational Safety

and Health (NIOSH),1095 Willowdale Road,Morgantown, WV

26505, USAZ ZhuangM D AttfieldB T ChenP Gao

J C HarrisonW E Wallace

Correspondence to:Dr W [email protected]

Accepted 30 August 2000

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investigations were limited to studies of lungcancer and mortality among pottery workersand iron, copper, tin, and tungsten miners.Also, few studies have been conducted toestablish quantitative exposure-response rela-tions among workers exposed to silica. Thus,the purposes of this study were to investigatethe risk of silicosis in Chinese tin mines and tomeasure the relation between silicosis andexposure to cumulative dust (Chinese total

dust and respirable crystalline silica dust) withimplications for identifying protective exposurelimits.

Methods

STUDY POPULATION

This study was designed as a retrospectivecohort study. The cohort included 3010miners exposed to dust (2795 men and 215women) who worked for at least 1 year between1 January 1960 and 31 December 1965 in anyof four Chinese tin mines. The mean age was26.8 years when they entered the cohort.Exposure and medical data were obtained frompersonnel and medical examination records for

each employee. Each cohort member wasfollowed up for a diagnosis of silicosis from thefirst employment to the end of 1994. The dateof first exposure for some miners was after1965, because they previously worked on non-exposed jobs. As it usually takes at least 1 yearfor a new worker to develop silicosis, the crite-rion of at least 1 year of employment was usedto exclude miners with less than 1 year ofexposure. It is also presumed that an exposure-response relation for acute accelerated silicosiswould be diVerent from that for chronic silico-sis, in that acute accelerated silicosis needsshort, intense exposures, whereas chronicsilicosis requires long term, lower exposures.The 1 year criterion selected out those who

rapidly developed silicosis and focused thestudy on the exposure-response relation forchronic silicosis. Compared with cohorts thatcould have been defined over earlier or laterperiods, this is the earliest cohort with nearlycomplete dust monitoring data, work history,and medical records. Most workers everemployed in these four tin mines wererecruited during the 1950s and 1960s.

All four tin mines selected in this study arelocated in Guangxi province, China, and areunderground mining operations. All mineshave kept complete records of occupationalhistory, dust monitoring data, and medicalexamination records since the early 1950s.

Exposures before 1950 were estimated. Thus,it is possible to follow up miners for theirworking lifetime or to the end of 1994.

OCCUPATIONAL EXPOSURE DATA

Chinese industrial hygienists have been meas-uring and recording workers environmentalexposures to airborne dust and silica since the1950s. Historically, concentrations of silicadust in exposed jobs have been high. Concen-trations of Chinese total dust were dramaticallyreduced after wet dust suppression and ventila-tion systems were installed in all four tin minesin 1958.

The Chinese dust monitoring scheme isbased on a gravimetric method for assessmentof airborne concentration, a particle countmethod for size distribution, and a settled dustanalysis for silica content. The Chinese air-borne dust samplers are battery operated andcollect total dust directly onto an exposed pre-weighed filter. Samplers were typically runduring active working periods at a flow rate of25 l/min, for about 1520 minutes.6 After sam-

pling, the filters were weighed to measure theChinese total dust concentration.

To develop retrospective exposure estimates,a job title dictionary was first made for majoractivities in the mines (underground mining,open cast mining, ore separation processing,and other activities). Then, job titles in the dic-tionary were classified into jobs with and with-out exposure to dust. All available industrialhygiene data were used to create a facility, jobtitle, calendar year exposure matrix. From1950 to 1986, Chinese total dust concentra-tions and exposure duration (number of hours)per shift were summarised for each facility/jobtitle in 3 year intervals. Concentrations of

exposure to Chinese total dust and duration ofexposure per shift were measured every yearafter 1986. The concentrations of exposurebefore 1950 were estimated from the monitor-ing data for 1950. About 50% of the facility, jobtitle, calendar year combinations of the jobexposure matrix were estimated based ondirect monitoring data. The others wereestimated with monitoring data for similarjobs, or data for the same job at diVerent times,with adjustment for other historical exposureinformation and task description of the jobtitle. To do this, consensus estimates weremade by industrial hygiene experts, publichealth doctors, safety engineers, samplers, andlocal supervisors, based on the history of

control measures, major changes in technicalprocesses in the mines, and comparisons withthe other jobs. The job exposure matrix is amodified version of that originally formulatedby Dosemeci et al for a nested case-controlstudy of lung cancer among pottery and mineworkers exposed to dust containing silica.7

Each persons work history was abstractedfrom their complete employment records inmine files. These records include job titles andcalendar work years for miners full duration ofemployment. Chinese data on exposure toCTD were calculated for each person with thefollowing equation:

where CTD=Chinese data on cumulativeexposure to total dust (mg/m3-years),n=number of job combinations held by thesubject in the work history, Cj=8 hour timeweighted mean concentration of total dust(mg/m3) for the jth job and employment period(from the job-exposure matrix), andTj=duration of employment (years) in job j.

For people without silicosis, exposure toCTD was calculated from the start of mining tothe end of employment or to the end of 1994.

32 Chen, Zhuang, Attfield, et al

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For people with silicosis, exposure to CTD wascalculated to the time of first diagnosis byradiography. For subjects that were lost duringthe follow up period, their exposures to CTDup to the time they were lost were also used tocalculate the cumulative risk.

The mean dust concentration for each studysubject was calculated by dividing CTD by netyears in a dusty job. One net year is defined as8 hours a day and 270 days a year. This is done

because these workers often had much shorterwork shifts than 8 hours. Net years in a dustyjob was calculated with the following equation.

where n=number of job combinations heldby the subject in their work history,Tj=duration of employment (years) in job j,and Hj=average duration of exposure (hours)per shift in job j.

To apply the Chinese data in judging theappropriateness of current standards for respir-able crystalline silica in the United States, it isnecessary to express the exposure variable interms of concentrations of respirable crystal-line silica. For comparison with results fromother studies, the same exposure variable con-version is needed. Thus, the Chinese estimatesof exposure to CTD were converted toexposure to respirable crystalline silica. Theconcentration of respirable crystalline silicawas estimated to be 3.6% (SD 2.5%) of theChinese total dust (mixed mine dust) concen-tration in tin mines.6 This estimate was basedon side by side airborne dust samples collectedwith American nylon cyclones and traditionalChinese samplers in the tin mines in a specialsampling survey during 19889. The conver-

sion factor between Chinese traditional dustsamples and American measurements of re-spirable crystalline silica was the ratio of theconcentration of respirable crystalline silicaobtained from each cyclone sample to thepaired total dust concentration measured bythe Chinese airborne dust sampler during thespecial 19889 sampling survey. The concen-tration of respirable crystalline silica convertedfrom the Chinese total dust concentration wasequivalent to a full shift, time weighted averageconcentration for the actual duration of thework shift on that day. The conversion factor ateach facility was obtained by averaging theratios of the concentrations of respirablecrystalline silica to that of the Chinese totaldust over sampling sites within the facility, andthe industry wide conversion factor wasobtained by taking the same average over allsampling sites for facilities within the industry.The conversion factor of 3.6% was applied tothe Chinese data on exposure to total dust toestablish a dose-response relation which can becompared with those from other studies.6

SILICOSIS DATA

Since 1963, workplaces with exposure to silica inChina, have been required by a national law tokeep registries of employees with silicosis. Yearly

radiographs have been taken of workers exposedto dust since then, and diagnoses of silicosis areincluded in the silicosis registry. Diagnoses ofsilicosis made before the silicosis registry lawbecame eVective in 1963 were also included inthe silicosis registry. All cohort members contin-ued to be examined and radiographed every 23years even after exposure ended.

Chest radiographs for each cohort memberwere kept by hygienists in all four tin mines. In

1989, 12 Chinese professional radiologistsfrom diVerent institutes in south central Chinawere divided into four groups to read all chestradiographs. The diagnoses of silicosis weredefined by at least two of three radiologists whoused the 1986 Chinese pneumoconiosis Roent-gen diagnostic criteria, which classified silicosisas suspected or at stage I, II, or III.8 Only caseswith a diagnosis of stages IIII were consideredto have silicosis in this study. A panel of threeChinese radiologists at a hospital aYliated toTongji Medical University read chest radio-graphs obtained after 1989 with the same 1986Chinese pneumoconiosis Roentgen diagnosticcriteria.

In 1991, a comparison was made betweenthe results of classification of 150 chestradiographs of 75 Chinese and 75 Americanworkers according to the standard Chineseclassification system (1986) and the Inter-national Labour Organisation (ILO) criteria(1980).810 The Chinese suspected case,stage I, II, and III were found to agree closelywith ILO profusion category 0/1 for smallopacities, category 1, category 2, and category3, respectively. There was a valid correspond-ence (agreement on the major category of67%) between the Chinese classification sys-tem and American interpretation that used theILO criteria (1980).10 Of the 150 radiographs,51 were found to be normal (without silicosis)

by both systems. The agreement between theChinese classification system and ILO classifi-cation was 89.3% in detecting the presence orabsence of silicosis.10

ANALYTICAL METHODS

The statistical analysis system (SAS) proce-dure, PROC LIFETEST, was used to fitsurvival distributions. After assessment of themean, scale, shape variables, and survival func-tion estimates, the Weibull model seemed tobest fit the survival distributions.11 The cumu-lative risk can be estimated by the acceleratedfailure exposure as follows.

where CR(x)=cumulative risk, x=Chinesedata on CTD exposure from the at the end offollow up or occurrence of silicosis, and arethe Weibull scale and shape variables, respec-tively, estimated by the SAS PROC LIFEREGprocedure. Chinese data on CTD exposurewere defined previously in equation 1. Cumu-lative risk is defined as the percentage of apopulation, exposed to a given value of theChinese data on CTD exposure, who developsilicosis.

Exposure to silica and silicosis among tin miners in China 33

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Results

Cohort status is summarised in table 1. At theend of 1994, 400 (13%) miners were stillworking in the tin mines, 184 had left for other

jobs, 1593 had retired, and 833 had died. Only16 miners (50 5 (0.5) 11 (0.6)

Year of first diagnosis:19569 78 (7.7)19604 163 (16.1)19659 136 (13.4)19704 86 (8.5)19759 110 (10.8)19804 380 (37.4)19859 53 (5.2)19904 9 (0.9)

Age at first diagnosis:209 F27 (2.7)309 188 (18.5)409 364 (35.9)509 322 (31.7)609 87 (8.6)709 27 (2.7)

*Values in parentheses indicate the percentage of total observa-tions in each category.


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Chinese total dust was converted to respirablecrystalline silica dust as noted in the methodssection.6 The relation between cumulative riskand cumulative respirable crystalline silica dustis shown in figure 2, together with that fromcomparable studies. With the estimates ofrespirable crystalline silica dust being 3.6% ofthe Chinese total dust, to calculate thecumulative dose of respirable crystalline silica,

the range of exposure in the tin mines wasabout 0.26 mg/m3-years. The risk of silicosis ispredicted to be 55% with 4.5 mg/m3-years ofcumulative exposure to respirable crystallinesilica.

DiscussionIn this study, 1015 tin miners in a cohort of3010 were diagnosed to have silicosis. Expo-sures in the cohort started as early as the1920s, with most starting before 1960. Themean age at onset of silicosis was 48 years, aftera mean of 21 years from first starting exposureand a meanof 11.0net years ina dusty job. The

cumulative risk of silicosis increased with CTDexposure. When this exceeded 20 mg/m3-yearsas measured in these mines, the increase incumulative risk of silicosis was accelerated.Risk of silicosis estimated by the Weibull distri-bution was greater than 68% when CTD expo-sure reached 150 mg/m3-years.

Three recent studies of the relation between

exposure to silica and incidence of silicosishave used similar methods to the ones used inthis study.1214 The results from those studiesand from this study are presented in figure 2. Itcan be seen that our results are very similar tothose of the other three studies.

On the other hand, there are three otherpublished studies in which the estimatedcumulative risks of silicosis were much lowerthan those seen in figure 2.1517 Table 6 summa-rises the findings from all published studies,with the exception of one, which could not beincluded because the units of measurement ofsilica were diVerent.18 The cumulative risks intable 6 are for a risk of silicosis of category 1/1or above, after 45 years of exposure at 0.1mg/m3 respirable crystalline silica.

The key point to be taken from the table isthat the estimated cumulative risk was heavilydependent on the extent of follow up afterleaving work. In the studies by Muir et al15 andRosenman et al16 there was virtually no followup after leaving work, and their predictionswere much lower than in those studies with fol-low up after employment. It is known from ourwork and that of others that silicosis commonlydevelops after leaving employment. The clearimplication of the results in table 6 is that stud-ies based on data collected only on employed

Table 4 Characteristics of workers exposed to dust with and without silicosis

CharacteristicAll miners(mean (SD))

No silicosis(mean (SD))

Silicosis(mean (SD))

Subjects (n) 3010 1995 1015Year of birth 1931 (9.2) 1933 (8.2) 1926 (8.2)Age at first exposure 25.7 (7.7) 25.1 (7.7) 27.0 (7.5)Year of first exposure 1956 (6.1) 1958 (6.0) 1953 (5.0)Chinese data on CTD exposure (mg/m3- y) 6 4. 7 (5 8. 3) 4 6. 9 (4 3. 4) 9 9. 7 ( 67 .3 )Chinese total dust concentration* (mg/m3) 7.5 (6.1) 6.1 (5.6) 10.3 (6.0)Net years in dust 9.6 (6.2) 8.9 (6.3) 11.0 (5.6)Latency period 21.3 (8.6)

Age at onset of silicosis 48.3 (10.1)Year at onset of silicosis 1975 (9.1)Age at last exposure 41.2 (10.9) 40.4 (11.4) 42.9 (9.6)Age of leaving work 53.5 (7.4) 53.3 (7.2) 54.0 (7.9)

*Mean concentration was calculated by dividing Chinese data on cumulative exposure to totaldust (CTD) by net number of years in dust.Latency period is the time from first exposure to the onset of silicosis.

Table 5 Cumulative risk of silicosis by Chinese data on exposure to CTD

Exposure toCTD (mg/m3-y) Cases (n)

Workers enteringcategory (n)

Cumulativerisk*

Mean (SD) netexposure (y)

Mean (SD)latency period (y)

150 155 221 0.917 15.7 (5.8) 25.4 (9.1)

*Cumulative risk=1exp(sum of (hazardsinterval width)), where the hazards for each categoryof cumulative exposure are (n cases/(width(n entering category0.5n cases0.5n withdraw-als))).Mean net years in dust for each category was calculated for both cases and non-cases for whomexposure stopped at that categorythat is, excluding subjects for whom exposure passed throughthe category)mean latency period was calculated similarly for cases only.

Figure 1 Relation between cumulative risk of silicosis andChinese data on cumulative exposure to total dust.

1.0

0.8

0.6

0.4

0.2

0.0 160

Cumulative exposure to total dust

(mg/m3-y)

C

umulativeriskofsilicosis

0 14012010080604020

Figure 2 Cumulative risk of silicosis versus cumulativeexposure to respirable crystalline silica.

1.0

Steenland and Brown12

Hnizdo and Sluis-Cremer13

This study

Kreiss and Zhen14

0.8

0.6

0.4

0.2

0.05.0

Cumulative exposure torespirable crystalline silica (mg/m

3-y)

Cumulativeriskofsilicosis

0.0 4.03.02.01.0

Table 6 Lifetime risk of silicosis (at least 1/1) byconcentration of exposure in various studies

StudyFollow up afteremployment

Lifetime risk atOSHA standard(0.10 mg/m3) (%)

Muiret al15 No 2

Rosenmanet al16

No 3Ng and Chan17 Some 1520Steenland and Brown12 Yes 70Hnizdo and Sluis-Cremer13 Yes 77Kreiss and Zhen14 Yes 92This study Yes 55


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workers may grossly underestimate the inci-dence of silicosis.

There are other reasons for diVerent findingsbetween studies, but they are likely to be lessimportant than the duration of follow up. Dif-ferent diagnostic methods for detection of sili-cosis were used in diVerent studies. Thefraction of respirable crystalline silica in totaldust and its estimation methods were diVerentamong diVerent studies. The silica in diVerent

studies is likely to have had diVerent surfaceproperties, which may influence toxicity.19

Also, variations in modelling approach, varia-tions in methods of estimating exposure, andthe uncertainties inherent in estimating theconcentration and duration (number of hours)of exposure per shift, may have resulted in dif-ferences between studies.

Despite the diVerences among the studies,there is consistent evidence that exposure at thecurrent United States Occupational Safety andHealth Administration (OSHA) and MineSafety and Health Administration (MSHA)permissible exposure limits of 0.1 mg/m3 for100% respirable crystalline silica dust is not

protective against silicosis. When the OSHAand MSHA standards were used to estimatethe lifetime risk of silicosis for tin minersexposed to respirable crystalline silica dust for45 years (4.5 mg/m3-years of cumulativerespirable crystalline silica), the cumulative riskof silicosis was 55% (fig 2). The same is true forthe Chinese situation. When free silica isgreater than 10%, the Chinese total duststandard is 2 mg/m3. This concentration isassociated with a cumulative risk of 36% for tinminers with 45 year lifetime exposure (fig 1). Ifthe lifetime risk of silicosis is to be under 1 in1000 (a criterion used by OSHA) for a lifetimeexposure of 45 years, then the mean Chinesetotal dust concentration must be lower than

0.14 mg/m3 (or lower than 0.005 mg/m3 respir-able crystalline silica). A workplace thatconsistently meets a standard has a lower meandust concentration than the standard, so a duststandard higher than 0.005 mg/m3 respirablecrystalline silica might be protective.20 Greaveshas recently summarised the literature andsuggests that 0.01 mg/m3 silica would be a rea-sonable standard to prevent disease.21

There were four limitations worth mention-ing in this study. Firstly, the actual concentra-tions of Chinese total dust before 1950 are notknown. Instead, industrial hygienists, publichealth doctors, and safety engineers who werefamiliar with the mines estimated the expo-

sures based on the data of 1950. However, only5% (101 workers with silicosis and 57 without)of cohort members were first exposed to silicadust before 1950. The potential bias associatedwith cumulative exposure estimates for theseminers may not be significant. It should benoted that the Chinese total dust exposureestimates for the period 195094 were basedon thousands of dust samples; this suggestsgood precision for these values for that period.

Secondly, the Chinese silicosis registry lawbecame eVective in 1963 and the yearly radio-graphy began around that time. It is possiblethat there was a delay in detecting some early

cases. The cumulative exposures for these earlycases may have been overestimated. However,only 8% and 16% of 1015 cases were firstdiagnosed during 19569 and 19604, respec-tively. The number of cases with overestimatesof exposures is thought to be very few. Also, theChinese diagnostic system classifies suspectedcases before stage I. The suspected cases areusually monitored closely and many cases haverecords documenting the progress from sus-

pected to stages I, II, and III. Workers wereusually moved from exposure when they hadstage I silicosis.

Thirdly, conversion of Chinese total dustmeasurements into exposure to respirable crys-talline silica values used a single factor of 3.6%.This factor was developed by a few comparativeworkplace measurements in 19889. Theseused same shift sampling by the Chinese totaldust and United States respirable dust methods,and a comparison of the Chinese phosphoricacid gravimetry and the United Statesx ray dif-fractometry methods for silica analysis.6

Fourthly, most miners came into the cohortperiod of recruitment (1 January 1960 to 31

December 1965) with a significant miningwork history (table 3). If miners had either diedor moved to other occupations due to ill healthcaused by exposure to silica before this period,then they would have removed themselves fromobservation and never entered the study. Theimpact of this exclusion could be that minersmore sensitive to the eVects of silica wereexcluded from recruitment into the study, andthis potentially caused a biasing of quantitativemeasures of association. Those early workerswho survived to 1960 might be expected tohave a relatively low risk of silicosis. Thus, therisk may have been underestimated. To investi-gate this problem, we excluded from the cohort158 miners with exposure before 1950, and

recalculated risk. Similar results for the cumu-lative risk of silicosis were obtained. Those withCTD exposure of 90 mg/m3-years had a risk ofsilicosis of 39% compared with the originalestimate of 36%. The predicted risk of silicosisincreases slightly from 55% to 60% with 4.5mg/m3-years of cumulative exposure to respir-able crystalline silica.

In summary, the results from this study,which are based on over 1000 cases of silicosisand many thousands of dust measurements,indicate a clear exposure-response relationbetween cumulative exposure to silica and sili-cosis. This study predicts about a 36%cumulative risk of silicosis for a 45 year lifetime

exposure to these tin mine dusts at the Chinesetotal dust exposure standard of 2 mg/m3. TheChinese total dust exposure data converted torespirable crystalline exposure to silica esti-mates (which introduces additional uncer-tainty into the exposure estimates) showed thatthe results were similar to most, but not all,findings from other large scale exposure-response epidemiological studies of silicosis.When the OSHA and MSHA permissibleexposure limits of 0.1 mg/m3 was used to esti-mate the lifetime risk of silicosis for tin minersexposed to respirable crystalline silica dust for45 years (4.5 mg/m3-years of exposure to


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cumulative respirable crystalline silica), thecumulative risk of silicosis was 55%.

We thank Dr Kyle Steenland for providing comments and someof the data for table 6, Dr Sidney Soderholm, Patricia SchleiV,Rebecca Stanevich, Dr Kathleen Kreiss, Dr Dan Sharp, and DrRichard Monson (Harvard School of Public Health, Boston,MA) for reviewing the manuscript and providing many valuablecomments during the internal review process required by theNational Institute for Occupational Safety and Health. We alsothank Dr Chen Rongan at Tongji Medical University and lead-ers and industrial hygienists in the four tin mines for participat-ing in this survey.

1 National Institute for Occupational Safety and Health(NIOSH). Work-related lung diseases surveillance report.Cincinnati,Ohio: Department of Health and Human Serv-ices (NIOSH) 1991:91113.

2 Ministry of Health, Peoples Republic of China. Nationalpneumoconiosis epidemiology investigation from 1949 to 1986,edition 1. Beijing: Beijing Medical University and UnionMedical University Press, 1992:9.

3 Chinese National Statistical Bureau.Annual statistical report(ASR) of 1995. Beijing China: CNSB, 1996.

4 Chiyotani K. Current status of pneumoconiosis. AsianMedical Journal1987;30:367.

5 Chen J-Q, McLaughlin JK, Zhang J-Y, et al . Mortalityamong dust-exposed Chinese mine and pottery workers.JOccup Med1992;34:31116.

6 Zhuang Z, Hearl FJ, Odencrantz J,et al. Estimating histori-cal respirable crystalline silica exposures for Chinesepottery workers and iron/copper, tin and tungsten miners.Ann Occup Hyg. (in press).

7 Dosemeci M, Chen J-Q, Hearl FJ,et al. Estimating historicalexposure to silica among mine and pottery workers in thePeoples Republic of China.Am J Ind Med1993;24:5566.

8 Ministry of Health, Peoples Republic of China. The 1986Chinese roentgenodiagnostic criteria for pneumoconioses.Beijing: Chinese Academy of Preventive Medicine, 1986.

9 International Labour OYce. Guidelines for the use of ILOinternational classification of radiographs of pneumoconioses(revised 1980). Geneva: ILO, 1980. (Occupational Safetyand Health Series No 22.)

10 Hodous TK, Chen R-A, Kinaley KB, et al. A comparison ofpneumoconiosis interpretation between Chinese andAmerican readers and classifications. Journal of TongjiMedical University 1991;11:2259.

11 Lee ET. Some well-known survival distributions and theirapplications. In: Lee ET, ed. Statistical method for survivaldata analysis. Belmont CA: Lifetime Learning Publication,1980:157.

12 Steenland K, Brown D. Silicosis among gold miners:exposure-response analyses and risk assessment. Am J Pub-lic Health1995;85:13727.

13 Hnizdo E, Sluis-Cremer GK. Risk of silicosis in a cohort ofwhite South African gold miners. Am J I nd Med 1993;24:44757.

14 Kreiss K, Zhen B. Risk of silicosis in a Colorado miningcommunity.Am J Ind Med1996;30:52939.

15 Muir DCF, Julian JA, Shannon HS, et al. Silica exposureand silicosis among Ontario hardrock miners: III. Analysisand risk estimates.Am J Ind Med1989;16:2943.

16 Rosenman KD, Reilly MJ, Rice C, et al. Silicosis amongfoundry workers.Am J Epidemiol1996;144:8909.

17 Ng T, Chan S. Quantitative relations between silicaexposure and development of radiological small opacitiesin granite workers.Ann Occup Hyg1994;38(suppl 1):85763.

18 Miller BG, Hagen S, Love RG, et al. Risks of silicosis in coalworkers exposed to unusual concentrations of respirablequartz. Occup Environ Med1998;55:528.

19 Wallace WE, Harrison J, Grayson RL,et al. Aluminosilicatesurface contamination of respirable quartz particles fromcoal mine dusts and from clay works dusts. Ann Occup Hyg1994;38(suppl 1):43945.

20 Hewett P. Interpretation and use of occupational exposurelimits for chronic disease agents. Occupational Medicine

State of the Art Reviews1996;11:5610.21 Greaves IA. Not-so-simple silicosis: a case for public health

action.Am J Ind Med2000;37:24551.

Open reviewing

Many journals, including the BMJ, have moved to a system of open reviewing, whereby authorsknow the names of reviewers of their papers. Research has shown that named reviews, althoughnot of better quality than anonymous reviews, are not of worse quality either. Therefore in theinterests of transparency, it seems fair to let authors know who has reviewed their paper. AtOccupational and Environmental Medicine we have considered the issue carefully. There are someconcerns that reviewers, especially those who are more junior, might feel intimidated and not

wish to make negative comments about papers submitted by senior people in the field. On theother hand, some reviewers might hide behind the cloak of anonymity to make unfair criticismsso as to reduce the chances of publication by rivals. We have decided to introduce initially a sys-tem of open reviewing if the reviewers agree explicitly. So when a reviewer is sent a paper, he orshe is asked to indicate whether we can disclose their name or not when sending the authorstheir comments. We will be monitoring this to see how many of our reviewers are happy to benamed. If it is most of them, we will move to a system of open reviewing as the norm, with apossible opt out clause for reviewers.


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W Chen, Z Zhuang, M D Attfield, et al.and risk assessmentminers in China: exposure-response analysesExposure to silica and silicosis among tin

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