MORTALITY AMONG WORKERS IN THE DIATOMACEOUS-EARTH INDUSTRY

A cohort mortality study was conducted among workers from two plants i n the diatomaceous earth mining and processing industry in California. Diatomaceous earth consists of the skeletal remains of diatoms. Expos ure to amorphous (non-crystalline) and crystalline silica in the form of quartz results from open pit mining and exposure to crystalline sil ica (principally cristobalite) occurs in the processing of the materia l. Lung cancer and non-malignant respiratory diseases have been the he alth outcomes of greatest concern. The main study cohort included 2570 white men (533 Hispanic and 2017 non-Hispanic workers) who were emplo yed for at least 12 months cumulative service in the industry and who had worked for at least one day during the follow up period, 1942-87. Vital status was ascertained for 91% of the cohort and death certifica te information was retrieved for 591 of 628 (94%) identified deaths. T he all causes combined standardised mortality ratio (SMR) was slightly increased (SMR = 1.12; 628 observed) compared with rates among US whi te males. The principal contributors to this excess were increased ris ks from lung cancer (SMR = 1.43; 59 observed) and nonmalignant respira tory disease (NMRD) excluding infectious diseases and pneumonia (SMR = 2.59; 56 observed). The excess of lung cancer persisted when local co unty rates were used for comparison (SMR = 1.59). Internal rate compar isons by Poisson regression analysis were conducted to assess potentia l dose-response relations for lung cancer and NMRDs. Mortality trends were examined in relation to duration of employment in dust exposed jo bs and with respect to an index of cumulative exposure to crystalline silica. The crystalline silica index was a semiquantitative measure th at combined information on duration of exposure, differences in exposu re intensity between jobs and calendar periods, the crystalline conten t of the various product mixes, and the use of respiratory protection devices. Increasing gradients of risk were detected for lung cancer an d NMRD with both exposure indices. The relative risk trends for lung c ancer and NMRD with crystalline silica exposure lagged 15 years were r espectively: 1.00, 1.19, 1.37, and 2.74, and 1.00, 1.13, 1.58, and 2.7 1. Based on a review of available but limited data on cigarette smokin g in the cohort and from application of indirect methods for assessing confounding variables, it seems unlikely that smoking habits could ac count for all of the association between exposure to dust and lung can cer. The intense and poorly controlled dust exposures encountered befo re the 1950s were probably the most aetiologically significant contrib utors to risks from lung cancer and NMRDs. The absence of an excess of lung cancer among workers hired since 1960, and the finding of no dea ths attributed to pneumoconiosis as an underlying cause of death among workers hired since 1950 indicate that exposure reductions in the ind ustry during the past 40 years have been successful in reducing excess risks to workers. Further mortality follow up of the cohort and the a nalysis of radiographic data will be needed to determine conclusively the long term patterns of disease risks in this industry.