Retrospective Cohort Mortality Study Research Articles

Mortality of aircraft manufacturing workers in Southern California

A retrospective cohort mortality study was conducted among men and women employed for four or more years, between 1958 and 1982, at an aircraft manufacturing company in San Diego County. Specific causes of death under investigation included cancer of the brain and nervous system, malignant melanoma, and cancer of the testicle, which previous reports have suggested to be associated with work in aircraft manufacturing. Follow-up of the cohort of 14,067 subjects for a mean duration of 15.8 yr from the date of first employment resulted in successful tracing of 95% of the cohort and found 1,804 deaths through 1982. Standardized mortality ratios (SMRs) were calculated based on U. S. national mortality rates and separately based on San Diego County mortality rates. Mortality due to all causes was significantly low (SMR = 75), as was mortality due to all cancer (SMR = 84). There was no significant excess of cancer of the brain, malignant melanoma, cancer of the testicle, any other cancer site, or any other category of death. Additional analyses of cancer sites for which at least ten deaths were found and for which the SMR was at least 110 showed no increase in risk with increasing duration of work or in any specific calendar period. Although this study found no significant excesses in cause-specific mortality, excess risks cannot be ruled out for those diseases that have latency periods in excess of 20 to 30 yr, or for exposures that might be restricted to a small proportion of the cohort.

A retrospective cohort mortality study of workers exposed to formaldehyde in the garment industry.

In order to assess the possible human carcinogenicity of formaldehyde we conducted a retrospective cohort mortality study of workers exposed for at least three months to formaldehyde in three garment facilities which produced permanent press garments. A total of 11,030 workers contributing 188,025 person-years were included in the study. Vital status was successfully ascertained through 1982 for over 96% of the cohort. The average (TWA) formaldehyde exposure at the three plants monitored in 1981 and 1984 by NIOSH was 0.15 ppm but past exposures may have been substantially higher. In general, mortality from nonmalignant causes was less than expected. A statistically significant excess in mortality from cancers of the buccal cavity (SMR = 343) and connective tissue (SMR = 364) was observed. Statistically nonsignificant excesses in mortality were observed for cancers of the trachea, bronchus and lung (SMR = 114), pharynx (SMR = 112), bladder (SMR = 145), leukemia and aleukemia (SMR = 113), and other lymphopoietic neoplasms (SMR = 170). Mortality from cancers of the trachea, bronchus and lung was inversely related to duration of exposure and latency. In contrast, mortality from cancers of the buccal cavity, leukemias, and other lymphopoietic neoplasms increased with duration of formaldehyde exposure and/or latency. These neoplasms also were found to be highest among workers first exposed during a time period of high potential formaldehyde exposures in this industry (1955-1962). However, it should be recognized that these findings are based on relatively small numbers and that confounding by other factors may still exist. The results from this investigation, although far from conclusive, do provide evidence of a possible relationship between formaldehyde exposure and the development of upper respiratory cancers (buccal), leukemias, and other lymphopoietic neoplasms in humans.

A retrospective cohort mortality study of males mining and milling attapulgite clay.

To evaluate the possible health effects of occupational exposure to a nonasbestos mineral fiber, a cohort of 2,302 males employed for at least 1 month between 1940 and 1975 at an attapulgite (clay fiber) mining and milling facility was followed through 1975. A significant deficit of mortality (SMR = 43, 90% CI 23-76) from nonmalignant respiratory disease (NMRD) was observed for the cohort based on age-, calendar year-, and race-specific rates for U.S. males. A marked deficit of NMRD was seen regardless of presumed dust exposure level, induction-latency period, or duration employed. A statistically significant excess of mortality from lung cancer was observed among whites (SMR = 193, 90% CI 121-293), but a deficit occurred among nonwhites (SMR = 53, 90% CI 21-112). Lung cancer risk in either race was not altered substantially with presumed dust exposure level, induction-latency period, or duration employed with one exception-those employed for at least 5 years in high-exposure-level jobs.

Mortality of workers exposed to polychlorinated biphenyls--an update.

A retrospective cohort mortality study of workers exposed to polychlorinated biphenyls (PCBs) in two plants manufacturing electrical capacitors was reported in 1981. The study was conducted primarily to examine the risk of cancer mortality associated with exposure to PCBs. Based on animal data, liver cancer was the disease of most interest. Due to the small number of deaths and a relatively short observation period, the study was considered inconclusive. This study has been updated by adding 7 yr of observation. The number of deaths in the study cohort has increased from 163 to 295. Mortality from all causes was found to be lower than expected (295 observed vs. 318 expected deaths) as well as mortality from all cancers (62 observed vs. 80 expected deaths). A statistically significant excess in deaths was observed in the disease category that includes cancer of the liver (primary and unspecified), gall bladder, and biliary tract (5 observed vs. 1.9 expected; p less than .05). Most of this excess was observed in women employed in one plant. Due to the small number of deaths and the variability of specific cause of death within this category, it remains difficult to interpret these findings in regard to PCB exposure.

Formaldehyde Levels in Seven Industries

Abstract Historical exposure estimates to formaldehyde were made in a recent retrospective cohort mortality study on workers exposed to formaldehyde in ten different plants. To supplement the historical information, personal air monitoring was conducted in each of the plants using the DuPont Pro-Tek[rgrave] badge. Three full-shift samples were taken in the summer and three in the winter. Some of the levels reported are unexpectedly high, possibly resulting from off-gassing of formaldehyde from the formaldehyde-containing dust, and may not reflect actual ambient air concentrations. The geometric means of the sampling results for formaldehyde in two manufacturers of formaldehyde ranged for the two seasons from 0.03 to 1.88 ppm and from 0.49 to 0.81 ppm. In the companies manufacturing formaldehyde resins, geometric seasonal means ranged from 0.11 to 30.45 ppm. Workers in the plants producing formaldehyde molding compounds were exposed to levels which were generally higher than those in resin manufacturing (0.04–60.77 ppm). Employees in the two plants producing molded plastic parts had seasonal mean exposures from none detected to 26.49 ppm. Mean concentrations for the two different seasons in the plant manufacturing decorative laminates were generally below 0.60 ppm, with only two jobs having mean exposures to formaldehyde greater than this level. In the plant manufacturing plywood paneling, mean exposures were below 0.40 ppm for all but one job. The majority of the jobs in the two plants which manufactured photographic film had mean exposures below 0.50 ppm. Although the results cited in this study may not reflect actual air levels, they may indicate that a significant source of formaldehyde exposure may occur from resin and molding compound dust.

An Industry-Wide Study of Respiratory Cancer in Chemical Workers Exposed to Chloromethyl Ethers<xref ref-type="fn" rid="FN2">2</xref>

An industry-wide retrospective cohort mortality study was conducted on 6,152 chemical workers (2,460 exposed and 3,692 nonexposed) engaged in chloromethyl ether manufacture at 7 major U.S. companies between 1948 and 1980. A previous study at 6 companies from 1948 through 1972 reported excess respiratory cancer (RC) mortality and significant exposure-response relationships in exposed workers at 1 company (company 2). The present study, which extended follow-up of an additional 7 years for companies 1-6 and included company 7 for follow-up from 1953 through 1980, found excess RC mortality in exposed workers at company 2 [observed (Obs) = 32, standardized mortality ratio (SMR) = 430] and company 7 (Obs = 9, SMR = 603). External comparisons of RC mortality at both companies showed significant exposure-response relationships with respect to cumulative time-weighted exposure. At company 2, where the greatest number of RC deaths occurred, external comparisons showed that RC risk remained constant in relation to age at first exposure and decreased with increasing time since last exposure. With the use of Mantel-Haenszel and relative risk (RR) regression methods, internal comparisons at company 2 demonstrated significant findings of increasing RR with cumulative duration of exposure and cumulative time-weighted exposure and with decreasing time since last exposure. No association was found between RR and age at first exposure. An interesting finding was a significant negative interaction between cumulative time-weighted exposure and age at risk. The best-fitting logistic regression model for the exposed group predicted RR at 2.79 (95% confidence interval = 1.66-4.69) for workers with the mean cumulative exposure score of the 32 RC deaths (lagged by 6 yr) compared with those with negligible exposure (assuming mean age at risk of the RC deaths, 51 years old, and time since last exposure held constant). Qualitative assessment of the results suggests that chloromethyl ether exposure affects both an early as well as a late stage of a putative multistage respiratory malignant process.

Mortality among workers at a uranium processing facility, the Linde Air Products Company Ceramics Plant, 1943-1949.

A retrospective cohort mortality study of 995 white males employed more than 30 d at a uranium processing facility in upstate New York between 1943 and 1949 investigated the association between excess observed deaths and long-term occupational exposure via inhalation to uranium compounds. Two comparison groups were used, the white male population of the United States and the white male population in the New York counties of Erie and Niagara. The vital status determination was 94.3% complete through 31 December 1979. With the use of the national comparison group, statistically significantly increased standardized mortality ratio (SMR) values were observed for all causes (SMR 118), laryngeal cancer (SMR 447), all circulatory diseases (SMR 118), arteriosclerotic heart disease (SMR 119), all respiratory diseases (SMR 152), and pneumonia (SMR 217). Site-specific outcomes of special interest with a statistically increased number of deaths above expected were laryngeal cancer (observed 5) and pneumonia (observed 17). No association was found with length of employment or work in the most hazardous areas of the plant. The comparison with regional rates gave similar results.

Notifying workers at risk: the politics of the right-to-know.

Holding that workers exposed to toxic substances in the workplace have a right to know about the extent to which they may have been put at risk, Bayer describes the history of the political controversy over whether the National Institute of Occupational Safety and Health (NIOSH) ought to provide individual notification to workers found through its retrospective cohort mortality studies to be at increased risk for disease. He comments that both advocates and opponents of notification have focused their arguments on the potential clinical consequences, even though the issue actually entails questions about the distribution of the burdens of uncertainty among powerful interests.

An exposure matrix for the NIOSH dioxin registry

A systematic procedure has been developed in order to assess the extent of potential exposure to dioxin for use in the retrospective cohort mortality study of U.S. production workers. Using informaton such as process descriptions; job descriptions; analytical data on dioxin content in substances from the various processes; and industrial hygiene, safety and medical data, estimates of potential exposure to dioxin will be developed. Presented is the rationale used to develop these estimates and an example of the estimation procedure.

Mortality of a municipal worker cohort: II. Females

Women have become an increasingly important segment of the total work force, yet there are very few published occupational mortality studies of female workers. This paper reports the findings of a retrospective cohort mortality study of 1,371 full-time female municipal employees of the City of Buffalo, New York, who were employed at least 1 day between January 1, 1950, and October 1, 1979, and who worked a minimum of 5 years. Vital status was ascertained for 88% of the female cohort, resulting in the identification of 214 observed deaths. This predominantly white-collar, service-oriented female cohort demonstrated significantly lower mortality than that expected based on U.S. mortality rates for white females. This strong "healthy-worker effect" was consistent across the time period of the study, across cause-specific mortality especially for all malignant neoplasms and all diseases of the circulatory system, and across different workers groups. Findings are discussed in light of the methodological issues involved in occupational studies of female workers.

Radiation, work experience, and cause specific mortality among workers at an energy research laboratory.

A retrospective cohort mortality study was conducted among 8375 white male employees who had worked at the Oak Ridge National Laboratory for at least one month between 1943 and 1972. This plant has been the site of energy related research, including uranium and plutonium reactor technology and radioisotope production. Radiation doses, primarily from gamma rays, were generally low; the median cumulative exposure for workers was 0.16 rems. Historical follow up was conducted for the years 1943-77 and ascertainment of vital status was achieved for 92.3% of the cohort. Standardised mortality ratios (SMRs) were computed to contrast the subjects' cause specific mortality experience with that of the United States white male population. The observed number of 966 deaths from all causes was 73% of the number expected. Mortality deficits were also seen for arteriosclerotic heart disease (SMR = 0.75; 344 observed) and all cancers (SMR = 0.78; 194 observed). These results are indicative of the healthy worker effect and the favourable influence on health of the cohort's relatively high socioeconomic status. Non-statistically significant raised SMRs were seen for all leukaemias (SMR = 1.49, 16 observed), cancer of the prostate (SMR = 1.16, 14 observed), and Hodgkin's disease (SMR = 1.10, 5 observed). Internal comparisons of mortality (standardised rate ratios, SRRs) were made between subgroups of the cohort according to radiation dose level and duration of employment in various job categories. No consistent gradients of cause specific mortality were detected for radiation exposure. Leukaemia mortality was highest among workers with greater than or equal to 10 years employment in engineering (SRR = 2.40) and maintenance (SRR = 3.12) jobs. The association of leukaemia with employment in engineering was unexpected; maintenance jobs entail potential exposures to radiation and to a wide range of organic chemicals; metals, and other substances.

Mortality among retired fur workers. Dyers, dressers (tanners) and service workers.

A retrospective cohort mortality study was conducted on 807 fur dyers, fur dressers (tanners), and fur service workers who were pensioned between 1952 and 1977 by the Fur, Leather and Machine Workers Union of New York City. Workplace exposures of fur workers varied with job category. Dyers were exposed to oxidative dyes used in commercial hair dyes; dressers and service workers were exposed to tanning chemicals. In a comparison with the New York City population, no significant increases in mortality were observed among the fur dyers. Among fur dressers, mortality from all malignant neoplasms [standardized mortality ratio (SMR) 151] and lung cancer (SMR 232) was significantly elevated, as was mortality from cardiovascular disease (SMR 126) among fur service workers. When examined by ethnic origin, the elevated SMR values and directly age-adjusted rate ratios suggested that foreign-born fur dressers and eastern European-born fur workers experienced the highest risks for lung and colorectal cancers, respectively. These data support previous findings of increased mortality from colorectal cancer in the foreign-born population of the United States and suggest a possible occupational etiology for the observed lung cancer excess.

A retrospective cohort mortality study of a phosphate fertilizer production facility.

A retrospective cohort mortality study of phosphate fertilizer production workers was undertaken to determine whether this group is at increased risk of dying from any cause, particularly from lung cancer. A total of 3,199 workers who had ever been employed at one facility were included in this investigation. These workers were followed for vital status ascertainment from their first date of employment up to December 1, 1977, or the date of death, whichever occurred first. Overall, no statistically significant elevations in cause-specific mortality were observed for the entire study population. However, when the analysis was stratified by duration of employment, and length of follow-up, a statistically significant (P less than .05) excess in lung cancer mortality was observed among workers with more than 10 yr of employment and follow-up (standardized mortality ratio = 411). Because of the small number of deaths involved, and because we had prior knowledge of a lung cancer cluster at this plant, we believe that these findings should be viewed as suggestive, and that other investigations in plants with similar exposures are needed to clarify whether an occupationally related lung cancer excess truly exists.

Mortality of a municipal worker cohort: I. Males.

A retrospective cohort mortality study was conducted on 11,499 full-time municipal employees of the City of Buffalo, New York, who were employed at least one day between January 1, 1950 and October 1, 1979 and worked a minimum of five years. This paper outlines the method of the study and presents the all cause and cause-specific mortality for the male cohort of 10,128. Statistically significant deficits in mortality are seen for infectious diseases, diseases of the circulatory system, diseases of the respiratory system, and all external causes. Statistically significant increased mortality is seen for both malignant and benign neoplasms. All cause mortality was significantly lower than expected for professional, manager, and clerical workers. White-collar workers exhibit a decreased risk of mortality from all diseases of the circulatory system, all diseases of the respiratory system, and all external causes of death. No statistically significant increased or decreased risk of mortality from specific cancer sites is seen for white-collar workers. Blue-collar workers show statistically significant deficits in mortality from infectious diseases, all diseases of the circulatory system, all respiratory diseases, and all external causes. Blue-collar workers exhibit statistically significant increases for benign and malignant neoplasms and in particular, malignant neoplasms of the esophagus, large intestine, and rectum. The meaning of these findings will be clarified through analyses of specific worker groups.

Sidak-type simultaneous prediction intervals for the mortality measures RSRR i about the corresponding S ePMR i for several competing risks of death in an epidemiologic study

Simultaneous prediction intervals are obtained involving the externally standardized proportional mortality measures S ePMR i which include the corresponding RSRR i with probability 0.95, or better, for several competing risks of death, under a reasonable assumption, namely, that the total number of deaths d.. due to all risks in the mortality study be large. The results are obtained under a very general situation when several risks compete for an individual's life in the study, irrespective of any covariance structure among the risks of death, known or unknown, positive or negative, or zero. As a particular case, if the risks of death may be assumed as independent, then the simultaneous prediction intervals reduce to those of the present authors (1985a). It is demonstrated that asymptotically (as d.. → ∞) the joint c.d.f. of the statistics Z 1, Z 2, ·, Z M , where Z i = RSRR i /S ePMR i , may be approximated by means of the joint c.d.f. of the multivariate ( M-variate) normal distribution. This is the basic result from which the prediction intervals are derived, using Sidak's (1967, 1968) multivariate normal probability inequalities. Our results are empirically evaluated using real data from a recent retrospective cohort mortality study of Enterline and Marsh (1983), concerning respiratory cancer among man-made mineral fiber workers. These prediction intervals do indeed quantify the observation that for each cause i of death, the measure S ePMR i is close in value to the corresponding RSRR i .

Long-Term Mortality Study of Oil Refinery Workers. IV. Exposure to the Lubricating-Dewaxing Process

A retrospective cohort mortality study of 1,008 male oil refinery workers who ever worked on the lubricating-dewaxing process of the lube oil department and who have been followed for a period of 43 years is presented. These workers were exposed to a number of solvents, primarily methyl ethyl ketone [(MEK) CAS: 78-93-3] and toluene (CAS: 108-88-3), but at levels far below the current Occupational Safety and Health Administration's standard. The standardized mortality ratio (SMR) for all causes (0.70) and the SMR for cancer (0.86) are much lower than unity when they are compared to the mortality experience of the U.S. population. Also observed in this study were 8 prostate cancer deaths (4.4 expected) with an SMR of 1.82, which was not statistically significant (P = .16). Seven of these 8 prostate cancer deaths occurred among nonwhite males, who showed an SMR of 2.47 (P = 0.53). However, only 1 prostate cancer death was seen among workers specifically assigned to the MEK units. The remaining deaths occurred among maintenance workers who had lube oil department-wide assignments. This cancer risk increased with increasing duration of employment in the lube oil department. A latency of 20 years or more was also observed for these prostate cancer deaths. In this study the processing of lubricating oils was found to be at least as important as the MEK solvents, and department-wide maintenance workers were as much at risk as the MEK unit workers. In view of this finding and findings obtained by others, it seems prudent to continue to study lubricating-dewaxing process workers, including the medical monitoring of all such workers for prostate cancer.

Retrospective mortality and medical surveillance studies of workers in benzene areas of refineries.

The mortality and health experiences of refinery workers employed in benzene processes or operations are described. A retrospective cohort mortality study of benzene workers employed from 1952 to 1978 revealed no excess in overall general mortality or in cancer mortality compared either with the experience of the U.S. general population or with that of an internal control group. Ascertainment of vital status was accomplished for 99% of the cohort. Recent industrial hygiene data that included 1,394 personal samples indicated that 84% of all benzene exposures were less than 1 part per million (ppm), with a median exposure of 0.14 ppm for the refinery workers, and 0.53 ppm for those in the benzene-related units. Among these workers, no deaths from leukemia were observed. A medical surveillance program for benzene workers is also described, with special emphasis on the effectiveness of laboratory screening. Evaluation of data for a 21-year period showed no significant changes in the blood indices of the workers as a group. The limited value of establishing screening guidelines without the support of epidemiological studies is discussed.

Exposures and mortality among chrysotile asbestos workers. Part II: mortality.

A retrospective cohort mortality study was conducted among a cohort of 1,261 white males employed one or more months in chrysotile asbestos textile operations and followed between 1940 and 1975. Statistically significant excess mortality was observed for all causes combined (standardized mortality ratio [SMR] = 150), lung cancer (SMR = 135), diseases of the circulatory system (SMR = 125), nonmalignant respiratory diseases (SMR = 294), and accidents (SMR = 134). Using estimated fiber exposure levels in conjunction with detailed worker job histories, exposure-response relationships were investigated. Strong exposure-response relationships for lung cancer and asbestos related non-malignant respiratory diseases were observed. Compared with data for chrysotile miners and millers, chrysotile textile workers were found to experience significantly greater lung cancer mortality at lower lifetime cumulative exposure levels. Factors such as differences in airborne fiber characteristics may partially account for the large differences in exposure response between textile workers and miners and millers.

Mortality From Brain Tumor and Other Causes in a Cohort of Petrochemical Workers

To evaluate a suspected excess of deaths due to brain tumor (BT) among petrochemical workers, a retrospective cohort mortality study was conducted among 7,595 men ever employed at a plant in Texas City, Texas, between 1941 and 1977. Among hourly employees, overall mortality was lower than expected from U.S. national rates [standardized mortality ratio (SMR) = 79]. However, 19 BT deaths (International Classification of Diseases, seventh revision, codes 193, 223, and 237) among hourly employees were observed as against 9.6 expected (SMR = 198), and with extension of the analysis to include BT deaths occurring in 1978 and 1979, 22 deaths were observed versus 10.7 expected (SMR = 206). Fifteen years or more after being hired, 19 of these workers died from BT versus 7.2 expected (SMR = 263), and the standardized mortality ratios increased with duration of employment to 377 for hourly workers who had worked over 20 years. Although nonoccupational etiologies cannot be dismissed, these data suggest an occupational etiology for certain BT deaths in petrochemical workers.

Environmental epidemiologic investigation of the styrene-butadiene rubber industry. Mortality patterns with discussion of the hematopoietic and lymphatic malignancies.

A retrospective cohort mortality study and an industrial hygiene assessment were undertaken in two styrene-butadiene rubber producing facilities in eastern Texas. Occupational history records were available from 1943 at plant A and from 1950 at plant B to the study cut-off date of 31 March 1976. With a two-sided test statistic, no statistically significant excesses in total or cause-specific mortality were observed for the overall worker population of either plant. However, the plant A study group demonstrated a nonsignificant statistical excess [standardized mortality ratio (SMR) of 203] for the cause-specific category of leukemia and aleukemia. Additional analyses were performed on a subgroup consisting of all white males with at least six months of employment at plant A between the beginning of 1943 and the end of 1945, a time which coincided with process and operational changes. An SMR of 278, also not statistically significant, was demonstrated for the leukemia and aleukemia cause-specific category. Due to the relative modest study population sizes, the power of this study to detect statistically significant excesses in leukemias or other malignancies of the hematopoietic and lymphatic tissues is not very large unless one is interested in substantial excesses, such as those that would correspond to a fourfold increase in risk.