The following paragraphs are taken from the published article that reported the data in crist.txt.  References are not given.
 

Silicosis and associated restrictive lung disease are well established consequences of prolonged intense occupational exposures to crystalline silica (1).  Whether crystalline silica is a human lung carcinogen remains a controversial issue.  Consistently elevated lung cancer risks have been observed among cohorts composed of silicosis cases from numerous industries (2), although questions have been raised regarding confounding from cigarette smoking and selection biases in some of these cohorts (3).  Potential confounding by co-occurring exposures to other known and suspected lung carcinogens (e.g. radon in he underground mines) has clouded the interpretation of the epidemiologic literature (4).  Lung cancer excesses have been reported from epidemiologic studies of workers in settings where confounding exposures to known lung carcinogens are absent or minimal, including the quarry and stone (5,6) refractory brick (2,8) and diatomaceous earth (9,10) industries.  Overall cohort relative risks in these studies have not been large (most less than 2.0), although some provide indication of dose-response relations, evidenced by greatest excesses among silicotics (8) or among workers with the largest cumulative exposures (5,7,9).  Dose-response estimation for silicosis, lung cancer, and other diseases has been impeded by a lack of historical quantitative dust exposure data in many studies that have relied on employment duration as dose surrogates. 

We report here on an extended follow-up and quantitative dose-response analysis of mortality risks among a cohort of workers in the diatomaceous earth mining and processing industry.  Diatomaceous earth (DE) is derived from the skeletal remains of diatoms that are deposited on ocean and lake beds.  After extraction of the mineral, which exists as amorphous (non crystalline) silica, from open pit mines, DE is crushed and calcined in kilns to produce the final product consisting of 10-60 percent crystalline silica, principally in the form of cristobalite.  DE is marketed commercially as a filtration medium for water, foods and beverages, as a filler in paints and construction materials, and as a carrier of agricultural chemicals.

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Cohort enumeration and follow-up:
 

The two DE plants are located in Lompoc, California, approximately 50 miles north of Santa Barbara.  DE mining and milling at the larger plant, on which the analyses reported here are based, have operated continuously since 1902. The study cohort and procedures have been described previously (9).  Briefly, the original study cohort was defined as workers employed for at least 12 months cumulative service at either plant, and employed for at least 1 day between January 1, 1942 and December 31, 1987.  The main study cohort from the earlier analysis was composed of 2037 non-Hispanic and 533 Hispanic white males.  The cohort for whom data are presented differs in two respects from the previously defined main study cohort:  1) in this analysis, workers (317) employed at the smaller plant were excluded because their exposure data were judged to be inadequate for quantitative estimation; and 2) 89 workers previously excluded from the main study cohort because of job assignments in DE mixing operations involving asbestos exposure were included in the present analysis because quantitation of the asbestos exposures was possible.  The data presented are thus for 2342 white males. 

Vital status was determined previously for the years 1942-87 using several data sources, including the National Death Index (NDI), state drivers' license bureaus, and a commercial credit bureau (9).  Vital status information for the  years 1988-94, inclusive, was determined from an updated NDI search.  Workers known to be alive in 1979 and who did not match the NDI files were assumed to be alive as of the end of follow-up; this assumption is supported by the previously noted high sensitivity of the NDI for death identification (13).  Copies of death certificates, obtained from state vital statistics offices, were coded by a trained nosologist according to the International Classification of Disease codes in effect at the times of death (5th-9th Revisions). 
 

Exposure assessment

Detailed job history data were updated through 1994 for workers still employed as of the end of 1987.  The methods for estimating quantitative dust exposures are described in detail elsewhere (14), and will be summarized  briefly here.  Quantitative air monitoring data, including 1709 measurements made during 1962-88 were obtained from the company.  These data were supplemented with earlier occupational hygiene data for the years 1948-62 (686 measurements) that were discovered subsequently from he company's archives.  All of the pre-1962 data were based on particle counts, whereas nearly half of the post-1962 data were measured as gravimetric quantities of total dust (17%) or respirable dust concentrations (32%).  Conversion of the data from the older units of million particles per cubic foot (mppcf) to modern gravimetric units in milligrams per cubic meter (mg/m3) was performed by linear regression modeling on companion sampling data for both measurement methods.  Job-specific exposures for years before measurement data were available (pre-1948) were estimated by regression modeling extrapolation based on observed temporal changes and knowledge of dates when dust exposure reduction interventions occurred.  Arithmetic mean plant-wide exposure estimates, across 135 jobs, ranged from 3.55 mg/m3 before 1948 to 0.29 mg/m3 for 1974 and later.  Job- and time-specific estimates were then made for respirable dust and respirable crystalline silica.  The respirable crystalline silica estimates incorporated data on the percentages of crystalline silica in the various product mixes and estimated job-specific fractions of exposure times ot these products (9).  The estimates of crystalline silica content were:  1 percent for uncalcined DE (quartz content of the ore); 19 percent for calcined DE; and 20 percent for flux-calcined DE.  The latter two percentages mainly represent cristobalite.  The crystalline silica percentages were provided by knowledgeable industry personnel, and were based on measured concentrations in bulk product samples, rather than on data from measurements of airborne dust, which were not available.  Cumulative exposures (mg/m3-yrs) were computed as the summed products of job-specific exposure intensities and associated durations in days. 

Chrysotile asbestos had been used in two small operations in the plant at various times from the 1920s to 1977.  Gibbs and Christensen (11) derived quantitative estimates of asbestos exposures (fibers/milliliter [f/mL]) for all cohort members.  These estimates were based on historical exposure monitoring data for jobs where asbestos was handled directly, production records, and recorded quantities of asbestos included in various mixed products.  Additionally, asbestos exposure levels (of unknown fiber type) were estimated for maintenance workers whose exposures were episodic, such as kiln re-lining.  The assessment provided asbestos exposure estimates for the years since 1930 (11).  In order to complete the asbestos exposure characterization for all relevant periods of employment, we extrapolated the 1930 job-specific intensity estimates  to earlier years of plant operation.  Cumulative exposures to asbestos (f/mL-yrs) were derived in similar manner as for the silica exposure indices.
 

Last update:  5/02/01