Occupational mortality studies. Principles of validity

Low-dose ionizing radiation and cancer mortality among enlisted men stationed on nuclear-powered submarines in the United States Navy

BACKGROUND

Men stationed on nuclear-powered submarines are occupationally exposed to external ionizing radiation at very low levels and radiation dose for each individual is closely monitored. Little is known about ionizing radiation (IR) risks of cancer mortality for populations with levels of cumulative ionizing radiation exposure this low.

MATERIALS AND METHODS

This historical cohort study followed 85,033 enlisted men who had served on a nuclear-powered submarine in the U.S. Navy between 1969 and 1982 to determine patterns of cancer mortality. Occupational radiation doses were measured by badge dosimeters for each individual for all periods of Navy service potentially involving radiation exposure. Deaths were ascertained through 1995 by searches of multiple national mortality databases. Within-cohort dose-response relationships for cancer mortality were estimated using linear Poisson regression models. Individual level smoking status was not available so cancer risks were estimated separately for cancers with and without previously published evidence of consistently moderate or strong associations with smoking.

RESULTS

A total of 584 cancer deaths occurred during a follow-up period of up to 27 years. The mean and median cumulative occupational radiation doses received while in the Navy were 5.7 and 1.1 milliSieverts (mSv) respectively, range 0-242 mSv. Mortality Excess Relative Risks (ERRs) per 10 mSv and 95% confidence intervals (CI) were 0.053 (CI -0.03, 0.17) for all cancers, 0.052 (CI -0.03, 0.18) for all solid cancers, and 0.003 (CI -0.29, 0.30) for leukemias excluding chronic lymphocytic leukemia. The ERRs per 10 mSv were 0.052 (CI -0.07, 0.17) for cancers previously associated with smoking and 0.012 (CI -0.10, 0.12) for cancers that were not.

CONCLUSIONS

The ERR point estimates for solid cancers and leukemia were statistically compatible with those reported in previous published studies of other ionizing radiation-exposed and monitored cohorts, albeit with wide confidence intervals. This study, with high quality measurements of in-Navy occupational external IR doses, high follow-up proportion, and detailed IR dose-response analyses, is consistent with the premise of a small excess cancer risk from low-dose IR.

Commercial Fishing as an Occupational Determinant of Opioid Overdoses and Deaths of Despair in Two Massachusetts Fishing Ports, 2000-2014

The opioid epidemic has had disproportionate effects across various sectors of the population, differentially impacting various occupations. Commercial fishing has among the highest rates of occupational fatalities in the United States. This study used death certificate data from two Massachusetts fishing ports to calculate proportionate mortality ratios of fatal opioid overdose as a cause of death in commercial fishing. Statistically significant proportionate mortality ratios revealed that commercial fishermen were greater than four times more likely to die from opioid poisoning than nonfishermen living in the same fishing ports. These important quantitative findings suggest opioid overdoses, and deaths to diseases of despair in general, deserve further study in prevention, particularly among those employed in commercial fishing.

Bayesian Approach to "Healthy Worker Hire Effect" in Standardized Mortality Ratio Analysis

OBJECTIVES

In this study, we address the healthy worker hire effect that arises when people with greater than average health are recruited to work in industrial jobs.

METHODS

Epidemiologists have used both general and working population reference rates to gauge influence of healthy worker hire effect on the standardized mortality ratio. We propose a Bayesian procedure that uses information derived from general and working population reference rates to calculate standardized mortality ratio.

RESULTS

The procedure is illustrated in the context of heart disease and lung cancer mortality analyses of a cohort of workers from a fluoropolymer production facility.

CONCLUSIONS

Application of our method should allow for fuller discussions of the healthy worker effect when one of its components, the healthy worker hire effect, is evaluated quantitatively. Our method can be utilized to improve risk estimates for a cohort with occupational exposure.

Estimation of potential gain in quality of life from early detection of cervical cancer

OBJECTIVE

To estimate the lifetime gain in the health-related quality of life (HRQOL) from early detection of cervical cancer.

METHODS

A consecutive, cross-sectional sample of 421 patients with cervical cancer was administered the World Health Organization Quality of Life-brief version questionnaires. A nationwide sample of 22,543 patients with invasive cervical cancer (ICC) was collected from the national cancer registry for estimation of lifetime survival function from 1998 to 2007, which was further multiplied by the ratio of HRQOL score functions for patients with ICC and patients with carcinoma in situ (CIS), and summed up over lifetime to obtain expected relative-quality-adjusted survival. The difference between lifetime survival and the expected relative-quality-adjusted survival gives the expected total dissatisfied time during the life course.

RESULTS

In comparison with patients with CIS postconization, patients with ICC showed consistently lower scores in the physical and psychological domains and that of sexual life after adjustment for other risk factors. The expected years of life lost for an invasive cancer was 6.48 years using the general population as the reference cohort, while the durations of equivalent to living with a very dissatisfied HRQOL were 1.71 and 0.25 for the physical and psychological domains, respectively, and 1.47 years for sexual life. Validation of the extrapolation method based on a subcohort followed from the 6th to the 13th year shows a relative bias of 0.4%. Sensitivity analysis with 37,000 CIS cases as the reference cohort yields a similar result.

CONCLUSIONS

Early detection of cervical cancer not only avoids premature mortality but also prevents long-term living under lower HRQOL scores, including sexual life.

Population Attributable Risk for Occupational Gancer in Italy

Population studies estimating the proportion of cancer attributable to occupation (PAR) in different geographical areas in Italy are reviewed. Studies using lists of industrial activities and occupations which are known or suspected to entail exposure to lung carcinogens gave lung cancer PARs between 5% and 36%. Those using job-exposure matrices estimated PARs of 3%-53%, with most of the values ranging between 17 and 33%. For bladder cancer, PARs ranged between 4% and 24%. The uses and limitations of calculating population attribultable risk are discussed.

Mortality among dentists in Taiwan, 1985-2009

BACKGROUND/PURPOSE

Controversy exists in the literature regarding whether dentists with multiple occupational exposures suffer from premature mortality. A cohort mortality study was conducted to evaluate the survival outcome and determine if potential exposure to harmful agents leads to premature mortality among dentists.

METHODS

Using the Life Table Analysis System, we calculated standardized mortality ratios (SMRs) for a cohort of 11,700 dentists affiliated with the Taiwan Dental Association. These dentists were followed from 1985-2009. Reference rates were derived from cause-, gender-, and age-specific mortality rates of the general population of Taiwan and 18,664 Taiwanese internists, who were considered to be more socioeconomically proximal to dentists. A Cox proportional hazard model was also constructed to determine multiple risk factors associated with mortality.

RESULTS

Compared with the general population, dentists in Taiwan consistently demonstrated reduced from all-cause mortality. However, compared with internists, significant and excess mortality were observed in dentists for overall mortality (SMR=1.13; 95% confidence interval [CI]=1.00-1.26), drowning (SMR=6.62; 95% CI=2.15-15.45), and heart diseases (SMR=1.66; 95% CI=1.22-2.21). After adjusting for other risk factors, the Cox model showed an increased hazard ratio of 1.17 (95% CI=1.01-1.37) for dentists.

CONCLUSION

Taiwanese dentists demonstrated significant elevated SMRs for overall causes, drowning, and heart diseases. Careful precaution should be taken to reduce these trends. Future studies are also needed for in-depth exploration of the mechanisms regarding how professional stress and exposure contribute to the increased risk of mortality in Taiwanese dentists.

Disparities in mortality among doctors in Taiwan: a 17-year follow-up study of 37 545 doctors

OBJECTIVES

The authors used cohort data from the registry of all doctors in Taiwan to determine if the effect of health disparities exists after control of potential confounding by different occupational exposures in different specialties.

DESIGN

Retrospective cohort study, 1990-2006.

SETTINGS

The Taiwan Medical Association.

PARTICIPANTS

A total of 37 545 doctors from the registry of the doctor file maintained by the Taiwan Medical Association. The registry has been required by the governmental regulation for verification of credentials of all practicing doctors.

MAIN OUTCOME MEASURES

Cause-specific standardised mortality ratios for surgeons and anaesthesiologists were compared with those of the internists. The Cox proportional hazard model was constructed to explore multiple risk factors for mortality, including specialties, age, gender, geographic region of practices, regional health resources, ages of beginning practices and years of beginning practice.

RESULTS

The all-cause-specific standardised mortality ratios for surgeons and anaesthesiologists were marginally elevated at 1.15 (95% CI 0.98 to 1.34) and 1.62 (95% CI 0.93 to 2.64), respectively. The Cox regression model showed that the anaesthesiologists had the highest HR of 1.97, seconded by surgeons at 1.23. Localities with the doctor-to-population ratio lower than 1:500 were associated with an increased HR of doctor mortality.

CONCLUSIONS

The doctor-to-population ratio and the region of practice may influence doctor's mortality. Increasing number of doctors and/or improving the practice environment may be helpful in reducing the health disparities in regions with poor resources.

Occupational demands and health effects for bricklayers and construction supervisors: A systematic review

BACKGROUND

knowledge was gathered on occupational demands and health effects of two occupations in the construction industry, bricklayers and supervisors, in order to design a job-specific workers' health surveillance (WHS) for construction workers.

METHODS

we systematically searched MEDLINE, EMBASE, PsycINFO, HSELINE, NIOSHTIC-2, and Picarta up to December 2008.

RESULTS

a total of 60 articles were included. Evidence was found for the following demands for bricklayers: energetic load (exceeding 25% heart rate reserve), load on the lower back (exceeding the NIOSH-threshold value of 3.4 kN), repetitive force exertions of the upper extremities, frequent bending with trunk flexion exceeding 60° and working with the arms more than 60° elevated. Environmental demands include: dust and quartz exposure (exceeding the limit values of 3.0 and 0.05 mg/m(3), respectively), vibration and noise (exceeding the limit value of 80 dBA). Bricklayers are at increased risk of lung cancer, low back pain, complaints of arms and legs and getting injuries. Among construction supervisors are walking and standing common physically demanding activities. Psychosocial demands with evidence for supervisors were mental demands, workload, time pressure, working long hours, and social-organizational factors. Supervisors are at increased risk of lung cancer and injuries.

CONCLUSIONS

for bricklayers evidence was found for physical demands and risk on low back pain and complaints of arms and legs, for construction supervisors on psychosocial demands. Both occupations are at increased risk of lung cancer and injuries. Job-specific demands and health effects should be incorporated in WHS for construction workers.

Increased mortality risk for workers with a compensated, permanent occupational disability of the upper or lower extremities: a 21-year follow-up study

This 1986-2006 study sought to determine whether specific causes led to increased mortality risks for Taiwanese workers with an approved compensation claim for permanent occupational disability (ACCPOD) of the upper or lower extremities. All cases of ACCPOD between 1986 and 2006 were collected from the database of compensation claims at the Bureau of Labor Insurance. Standardized mortality ratios and 95% confidence intervals were calculated for different causes of death among workers with an ACCPOD of the upper or lower extremities. A total of 800,047 person-years were accrued for 71,001 workers with a single type of disability. Standardized mortality ratios were significantly increased for all causes, including liver cirrhosis, injuries, and intentional self-harm. Standardized mortality ratios for workers with amputations of the lower extremities increased to 7.66 (95% confidence interval (CI): 5.36, 10.61), 2.40 (95% CI: 1.44, 3.75), 2.07 (95% CI: 1.03, 3.70), and 5.09 (95% CI: 2.20, 10.03) for those with diabetes mellitus, cerebrovascular disease, liver cirrhosis, and chronic renal failure, respectively. The authors concluded that workers with occupational disabilities involving an upper or lower extremity should be assisted to prevent further injuries or intentional self-harm, whereas those with lower limb amputations should be provided care related to proactive control of diabetes mellitus and associated complications during rehabilitation.

Abnormal predicted diffusion capacities in healthy Asians: an inequality with a solution

BACKGROUND

Asian lung volumes are 10-15% less than those of Caucasians.

OBJECTIVES

To test the hypothesis that healthy Asians might be labeled as abnormal using three commonly used Caucasian-derived prediction equation estimates (PEE) of DLCO currently used. In addition, a Chinese-derived PEE of DLCO was tested to determine its validity in non-Chinese Asians.

METHODS

Forty-one healthy Asians underwent DLCO testing. Controls consisted of the PEE and 12 healthy Caucasians. Measured DLCO was compared with the Miller, Knudson, Crapo and one Chinese PEE. Abnormal was defined as a DLCO <80% predicted. Gas dilution and plethysmography estimated alveolar volume. Proportions in parentheses in the results below are DLCO adjusted for alveolar volume.

RESULTS

The average Asian DLCO was 25.75 +/- 5.55 ml/min/mm Hg, no different than the predicted DLCO of 25.29 +/- 5.53 seen with Chinese PEE. This was different (p < 0.01) than the predicted DLCO of 27.82 +/- 5.09, 33.66 +/- 6.29, and 31.64 +/- 5.33 for the Miller, Knudson, and Crapo equations, respectively. This resulted in 4/41 (0/41), 27/39 (2/39), 21/41 (3/41) and 1/41 (0/41) DLCO measurements being defined as abnormal using Miller, Knudson, Crapo and Chinese PEE, respectively. In Caucasians, the measured DLCO was similar to the Miller but significantly lower than the Knudson and Crapo PEE. Measured lung volumes were significantly smaller compared to predicted for the three Caucasian PEE in Asians, with no difference in Caucasians. There was no difference in measured lung volumes and Chinese PEE.

CONCLUSIONS

Current Caucasian PEE for DLCO when used in healthy Asians result in an abnormal reading that is incorrect from 10 to 50% of the time. This PEE failure is related to a reduction in lung volume not accounted for. The Chinese PEE for DLCO works for non-Chinese Asians and should replace Caucasian PEE in the US in all Asians.

The reuse of subjects in clinical studies: is a bias introduced? Is there a threat to study validity?

The reuse of subjects in clinical studies to answer specific research questions is a common practice. This reuse can take many forms and is at least partly related to study efficiency. It is much easier to reuse known subjects who know the system(s) or are easy to find than to recruit new ones. A hypothetical question is: Are there instances when such practices may not be scientifically valid? This question will be addressed from the perspective of the case-control and experimental study design.

Monte Carlo estimation of extrapolation of quality-adjusted survival for follow-up studies

The expected quality-adjusted survival (QAS) for an index population with a specific disease can be estimated by summing the product of the survival function and the mean quality of life function of the population. In many follow-up studies with heavy censoring, the expected QAS may not be well estimated due to the lack of data beyond the close of follow-up. In this paper, we first created a reference population from the life tables of the general population according to the Monte Carlo method. Secondly, we fitted a simple linear regression line to the logit of the ratio of quality-adjusted survival functions for the index and reference populations up to the end of follow-up. Finally, combining information on the reference population with the fitted line, we predicted the expected quality-adjusted survival curve beyond the follow-up period for the index population. Simulation studies have shown that the simple Monte Carlo estimation procedure is a potential approach for estimating expected QAS and the survival function beyond the follow-up with a certain degree of accuracy.

Mortality from injuries and other causes in a cohort of 21,800 Brazilian steel workers

OBJECTIVES

Injury is the leading cause of death in the male working population of Brazil. An important fraction of these deaths are work related. Very few cohort studies of steel workers, and none from developing countries, have reported on mortality from injuries. This paper analyses mortality from work and non-work related injuries among Brazilian steel workers.

METHODS

Deaths during employment from 1 January 1977 to 30 November 1992 were analysed in a cohort of 21,816 male steel workers. Mortality rates specific for age and calendar year among the workers were compared with those of the male population of the state where the plant is located. Work related injuries were analysed by comparing the mortality rates for different subgroups of the cohort.

RESULTS

The number of deaths (391) was less than half that expected based on death rates of the general population. Over 60% (242) of deaths were due to injuries. Mortality from most causes was substantially below that in the general population, but that from unintentional injury, was 50% above that of the general population. Standardised mortality ratios (SMRs) were highest for the youngest and the oldest employees and for labourers and clerical workers. Mortality from motor vehicle injury was twice that expected from population rates (SMR = 209, 95% confidence interval (95% CI) 176-244). There was a 67% fall in the age adjusted mortality from occupational injuries in the study period.

CONCLUSION

The healthy worker effect in this cohort was greater than that commonly found in studies of occupational groups in developed countries, probably because of a greater socioeconomic gap between employed and unemployed populations in Brazil, and unequal distribution of health care resources. Mortality was especially high for motor vehicle injuries. The fall in mortality from occupational injuries during the study period was probably due to improvement in safety standards, increased automation, and better medical care. There is a need to investigate risk factors for unintentional injuries among steel workers, especially those due to motor vehicle injuries. Prevention of occupational and nonoccupational injuries should be a main priority in Brazil.

[Occupation and hypertension]

Scientific reports on Arterial Blood Hypertension for the period from 1970 to 1989 are reviewed, with special reference to its epidemiological focusing among workers. The knowledge gained and the theoretical and methodological advances associated with it are assessed.

From conjecture and refutation to the documentation of occupational diseases in Taiwan

The documentation of occupational diseases in a developing country like Taiwan is a challenge to an occupational physician. For lack of a system including material safety data sheet (MSDS), workers are usually not aware of what chemicals they are exposed to. Since many occupational diseases have long latencies and do not show any specific symptoms and signs, recognition and identification of the occupational origin are often very difficult. Using databases which provide a relatively complete list of industrial chemicals and a set of specific signs and/or symptoms, combined with the epidemiologic approach of conjectures and refutations--i.e., considering and ruling out all possible alternative explanations--we have documented eight kinds of occupational diseases and an outbreak of botulism. We recommend that a similar approach be applied to any other country with a similar situation, and that a system involving an identification sheet (e.g., MSDS) for each chemical be advocated and implemented in such countries as one means to enable prompt recognition and prevention of occupational diseases.

The sick person effect

Very often criteria by which subjects are selected for epidemiological studies are associated in some manner with their health. The Healthy Worker Effect (HWE) or Healthy Person Effect (HPE) is well known. Little has been said about the converse case in which selection is associated with decreased health status, the Sick Person Effect (SPE). The SPE may introduce a bias for some cohort, most clinical follow-up, and some case-control studies when risks are standardized against an inappropriate referent. We demonstrate the existence of the SPE in two studies. Study 1 compares the incidence of a number of different diseases among individuals who were selected as children for medical treatment with that among their siblings. Study 2 computes the Standardized Morbidity Ratios (SmRs) for various acute and chronic diseases for individuals who have reported particular chronic symptoms. The SPE is clearly apparent for all instances where the general population is taken as the referent. The HPE and SPE may present serious problems for the validity of conclusions with respect to risk levels.

Mortality pattern of silicotic subjects in the Latium region, Italy

A mortality study was carried out on 595 workers who were compensated for silicosis in the Latium region, Italy, during the period 1946-84 who died between 1 January 1969 and 31 December 1984. Respiratory disorders, tuberculosis, lung cancer, bone cancer, and cirrhosis of the liver showed significantly increased risk ratios (4.1, 3.7, 1.5, 4.1, and 1.9 respectively); excesses of brain cancer and leukaemia did not reach statistical significance. Lung cancer mortality was further analysed by age, period of compensation, final degree of disability, and occupational activity. The possible confounding role of smoking was assessed by comparing the lifetime smoking habits of a sample of silicotic subjects with those of the general male population as estimated by a national health survey; the prevalence of ever smokers among silicotic subjects (70.7%) was similar to that estimated for the general population (68.5%). The present study indicates that silicosis is associated with lung cancer even though it does not clarify the respective roles of exposure to silica and silicosis.

A retrospective cohort mortality study on workers of two thermoelectric power plants: fourteen-year follow-up results

Although many studies have been performed to evaluate the environmental impact of coal energy production, few studies are available on the health risk for the people working in coal power plants. A retrospective cohort study was performed on the workers of two power plants near Venice (which use coal since 1968) in order to test the association between exposure to coal dust and ashes and mortality for all causes, all cancers, and respiratory and digestive cancer. One thousand three hundred seven male workers were followed up from 1968 to 1984. During this period 41 workers died; causes of death were collected from the local Mortality Registers and/or from the Hospital Records. The observed mortality of the study cohort was compared with the mortality expected from the Italian death rates in the same period. No Standardized Mortality Ratio (SMR) was found in excess in the working cohort with respect to the standard population for any of the investigated effects.

Influence of design characteristics on the outcome of retrospective cohort studies

Retrospective cohort studies are increasingly being applied in occupational health. To describe and investigate further this type of study 179 retrospective cohort studies published in six scientific journals between 1975 and 1985 inclusive were reviewed. A description of the 179 reviewed articles was made and relations between investigator orientated variables, design characteristics, and the outcome of the study were investigated. Retrospective cohort studies focusing on exposures in the chemical industry appeared to yield most negative findings, which is partly explained by the relation between the affiliation of the investigator and the outcome of the study. Studies requiring a minimal latency period, an occupational reference group, and a low percentage of lost to follow up tended to have a higher chance of a positive finding. Study size, however, did not appear to be related to the outcome.

Epidemiology in protection and prevention against environmental mutagens/carcinogens. Examples from occupational medicine

Subjects occupationally exposed to potential mutagens/carcinogens represent the most suitable groups for epidemiological studies aimed at assessing the risk for the individual or the offspring. Several cancer risks to humans have been detected by epidemiological studies performed in occupational settings. Cancer epidemiology studies have been able (a) to identify specific occupations or agents associated with the risk; (b) to verify the results of experimental studies; (c) to test the effectiveness of changes in production or preventive measures in decreasing risks. Reproductive epidemiology has suggested a risk of spontaneous abortions or of malformation in the offspring of workers exposed to some chemicals or occupations, but data are often conflicting due to methodological problems. With the aim of early assessment of risk in mind, the epidemiological use of indicators of exposure or of the early effect of exposure to genotoxic agents is increasingly applied to occupational groups. Cytological monitoring of subjects at risk of occupational cancer of lung or bladder is carried out mainly to diagnose precancerous lesions of target tissues. Cytogenetic methods (chromosome aberrations, SCE, micronuclei) in somatic cells provide a means for detecting early effects of occupational exposure to known or potential mutagens/carcinogens in selected groups of individuals, but their significance is widely debated. Monitoring of urinary mutagenicity, as applied in nurses handling cytostatic drugs, is an example of how an indicator of exposure to genotoxins can be used to evaluate the impact of preventive measures. Among the perspectives, biochemical epidemiology seems to be promising in detecting individuals genetically susceptible to cancer.

A case-referent study of lung tumors in non-asbestos textile workers

A case series in the Province of Florence showed an increased risk of mesothelioma in textile workers (nonasbestos) and a survey of working conditions confirmed potential exposure to asbestos. In order to investigate the risk in textile workers, including some specific job titles, a case-referent study on lung tumors was carried out. The lung cancer cases included 441 males with histologically confirmed primary lung cancer during the period 1980-1983. Referents included 1,075 males selected from two hospitals and matched for age, sex, and smoking habits. Those who had "ever worked" in the textile industry showed an adjusted odds ratio of 1.52 (95% C.I. 1-2.25) compared with other "industrial workers." This moderately increased risk is maintained in selected jobs in the textile industry. An analysis of the modifying effect of time factors showed an increased risk in the period of 15-35 years from the date of first employment in the industry. The results support the hypothesis that a probable risk of lung cancer in textile workers in the Prato area was related to asbestos exposure.

Mortality patterns of New York State Vietnam Veterans

Mortality odds ratios (MORs) comparing veterans with Vietnam service who died in New York State to veterans of the Vietnam era with no Vietnam service were estimated (N = 1,496). The most elevated MORs and their confidence intervals were non-motor vehicular injuries of transport (MOR = 2.18, (1.19, 3.96)), other accidents and burns (MOR = 1.37, (0.95, 1.98)), and homicide (MOR = 1.59, (0.86, 2.94).

Mortality studies of dockyard workers (longshoremen) in Italy

A polycentric mortality study was performed in five Italian harbours: Savona, Marina di Carrara, Livorno, Civitavecchia, and Ravenna. The cohort included active and retired workers (longshoremen), who had been employed between 1960 and 1981. Causes of death were ascertained from death certificates; expected numbers of deaths were generated from age, sex, calendar year, and cause-specific national mortality rates. A low overall mortality was found, which is in agreement with procedures of health selection, typical of physically demanding jobs; an excess of lung cancer was, however, found in four out of five harbours.

The "case-control" study: valid selection of subjects

For valid selection of subjects in "case-control" (case-referent) studies it is critical to understand that these studies do not represent an alternative to cohort studies but, rather, to census-ascertainment of the facts about the study base. Specifically, in these studies, the fact-finding scheme is to obtain a census of the study base with respect to outcome, and then a census of the cases together with a sample of the base to gather information on the determinant(s) as well as modifiers and confounders. If the base is defined a priori (primary base), then the challenge is to devise a scheme to obtain a census of the cases in it and a sample of the base itself ("control" or reference series) that is representative of it, conditional on the covariates that will be controlled in the analysis of the data. On the other hand, if the definition of the base is secondary, a corollary of the way the cases are selected, then the case series is best viewed as the totality of the cases in the base as a matter of definition. The corresponding secondary base is the population experience in which each potential case, had it occurred, would have been included in the case series. Representative sampling of a secondary base tends to call for the use of subjects coming to the source of cases because of other conditions--conditions whose occurrence is known to be unrelated to the determinant under study and whose diagnosis and referral to the source are known to have the same relation to the determinant as those of the illness under study. With both types of base, primary and secondary, the accuracy of the information on the determinant should be comparable between the case and reference series, and this requirement of comparability, just as that of representativeness, can have important implications for the selection of the study subjects.

Polychlorinated biphenyls: influence on birthweight and gestation

Fifty-one infants born to women employed at two capacitor manufacturing facilities with a history of high exposure to polychlorinated biphenyls (PCBs) had a mean birthweight of 153 grams less than that of 337 infants born to women who had worked in low-exposure areas (90 per cent confidence interval, -286 to -20 g); mean gestational age was 6.6 days shorter in the high-exposure infants (90 per cent CI, -10.3 to -2.9 days). After adjusting for gestational age, the difference in birthweight was markedly reduced, indicating that the observed reduction in birthweight was due mainly to shortening of gestational age in the high-exposure group.

Proportion of lung cancers due to occupational exposure

The proportions of male lung cancers due to occupational exposure and, respectively, to cigarette smoking in a highly industrialized area of Northern Italy were estimated in a population-based case-control study in 1976-9. Two hundred and four out of the 211 lung cancer cases and 351 controls sampled from the source population were questioned about their occupational and smoking histories. On the basis of the occupational history each subject was classified as probably exposed (+), possibly exposed (?), or unexposed (-) to one or more of the chemicals known to be carcinogenic for the human lung, namely asbestos, polycyclic aromatic hydrocarbons, arsenic, nickel and chromium compounds, BCME, CMME and vinyl chloride. Upon stratification by cigarette smoking, contrasting the occupationally exposed subjects, whether certainly or uncertainly defined, with the unexposed ones, the RR for lung cancer was 2.1 and the occupational etiologic fraction was 0.33 (95% confidence interval 0.19-0.47). The tobacco etiologic fraction was 0.81, while the two exposures together accounted for 89% of the total burden of incident cases. If 33% of all male lung cancers were of occupational aetiology, then this alone would represent 5% of all cancer deaths.