Disease clusters in occupational medicine: a protocol for their investigation in the workplace

A survey of laboratory and statistical issues related to farmworker exposure studies

Developing internally valid, and perhaps generalizable, farmworker exposure studies is a complex process that involves many statistical and laboratory considerations. Statistics are an integral component of each study beginning with the design stage and continuing to the final data analysis and interpretation. Similarly, data quality plays a significant role in the overall value of the study. Data quality can be derived from several experimental parameters including statistical design of the study and quality of environmental and biological analytical measurements. We discuss statistical and analytic issues that should be addressed in every farmworker study. These issues include study design and sample size determination, analytical methods and quality control and assurance, treatment of missing data or data below the method's limits of detection, and post-hoc analyses of data from multiple studies. Key words: analytical methodology, biomarkers, laboratory, limit of detection, omics, quality control, sample size, statistics.

TCE Meta-Analyses: Wartenberg et al.'s Response

Comments on Trichloroethylene and Cancer: Epidemiologic Evidence by Daniel Wartenberg, Daniel Reyner, and Cheryl Siegel Scott. Environ Health Perspect 108(suppl 2):161-176 (2000).

Epidemiological response to a suspected excess of cancer among a group of workers exposed to multiple radiological and chemical hazards

OBJECTIVE

An excess of cancer was suspected by workers of the metallurgy department at the French Atomic Energy Commissariat (CEA) after several deaths from cancer were reported in 1983 and 1984. After a descriptive study performed by the CEA in 1985 the results of which were not conclusive enough to put an end to the controversy, the present cohort study was undertaken in 1989.

METHODS

As no specific exposure, or a precise cancer site was suspected, it was decided to include all subjects who had worked at the metallurgy department for at least 1 year between 1950 and 1968. The cohort was followed up to 31 December 1990. Individual occupational exposures were determined retrospectively for each year from 1950 to 1990, both qualitatively (annual job, and hazard records, and assistance from former workers) and quantitatively (for external radiation). On the basis of these exposures, three types of occupational tasks were identified: handling of chemicals, radionuclides, and external radiation. Standardised mortality ratios (SMRs) were calculated to estimate the risk of death, and the existence of an association between risk of cancer and each of the three tasks was tested.

RESULTS

The cohort included 356 workers, followed up for an average of 30 years (total of 10,820 person-years). The number of deaths from all causes and from all cancer sites were respectively 44 and 21. No excess of cancer deaths was found for the study period (SMR 0.77), nor was there a peak in 1983-4. The risk of death from all cancer sites increased with the duration of exposure to chemicals.

CONCLUSION

The results do not justify the workers' impression of an excess of cancer. They suggest, however, that the duration of work at some tasks that involved handling chemicals may be an indicator of risk of cancer. Communication to the workers during the study played an important part in reducing their concern, contributing to their better understanding of the results.

A method for the follow-up of clusters of adverse reproductive outcomes

Clusters of adverse reproductive outcomes are reported with increasing frequency to public health authorities. Most are random events and only a very small proportion is likely due to identifiable environmental agents. Often, a preliminary study confirms the existence of a spatial and temporal excess but no biologically plausible cause is found. These cases require a follow-up of incidence to identify any continuing excess. A conceptually simple 'fixed window' technique of follow up is proposed. The power of the test is mainly influenced by the magnitude of the increase in rate, the number of adverse outcomes selected for observation and the acceptable false alarm rate. This technique has several advantages compared with other currently used methods. From a public health point of view, two important factors are to be considered in setting the parameters of the test: the delay in providing an answer for the community and the availability of resources for conducting environmental investigations.

Investigation of clusters of adverse reproductive outcomes, an overview

Clusters of adverse reproductive outcomes are reported with increasing frequency to public health services, but it is likely that only a minority of clusters are caused by a common environmental teratogen or mutagen. Many guidelines and protocols have been developed for the investigation and the management of chronic disease clusters and can be applied to the study of adverse reproductive outcomes. Cluster investigation normally follows four successive phases: (i) the generation of one or more etiologic hypotheses, (ii) the confirmation of the hypotheses, (iii) an intervention aimed at reducing any dangerous exposure, and (iv) the evaluation of the effect of the intervention. However, each situation is unique and there is no standard recipe for conducting the epidemiologic investigation at each successive phase, for choosing the appropriate statistical technique and for communicating with interested parties. In order to minimize both the risk of pursuing a worthless investigation and the risk of ignoring a real excess of cases caused by an adverse environmental factor, it is imperative to rely on a team of specialists with expertise in epidemiology, statistics, toxicology, embryology, and communication.

Environmental health response clinics. A survey of program options

Environmental Health Response Clinics are established in response to concerns about community exposures to hazardous situations (chemical, biological, radiological). They are developed in response to a demand for "clinical services" and operate outside the usual health care financing and delivery mechanisms. Prompted by their experience in California, the authors formed a focus group to identify possible goals and services. A mail survey of occupational-environmental health professionals was then conducted to evaluate the feasibility and priority of representative goals. The analysis suggests that services should focus on the specific hazard of concern and that communication and education are essential components. The tendency to "do a general physical examination" should be eschewed. Ratings for priority and feasibility were disparate for several possible goals. In some instances, a "hands-on examination" may not be the best use of resources. Environmental health professionals may serve by direct clinical service or by advising community-based practitioners. Providing routine clinical services alone cannot meet the expectations for an environmental health response clinic.

Assessment of cancer clusters using limited cohort data with spreadsheets: application to a leukemia cluster among rubber workers

This paper presents methods for initial investigation of occupational cancer clusters using limited data. Phase 1 of the methods developed uses basic data from the cluster of cases, but with cohort data limited to the size of the workforce at the plant. Phase 2 of the methods requires knowledge of the number of workers entering and leaving the workforce in each year. In the absence of data concerning age, the spreadsheet programs explore a variety of worker age distributions in synthetic cohort analyses. The methods were used to assess a cluster of six cases of leukemia with an average duration of work of 11 years in a tire manufacturing plant. It was concluded that the relative risk for the age range 30-50 was at least 7, providing further evidence for the association of leukemia with work in tire manufacture. The use of spreadsheet programs can provide a valuable first step assessment of apparent workplace cancer clusters.

Solving the cluster puzzle: clues to follow and pitfalls to avoid

Dozens of methods have been proposed for the identification of disease clusters, although only a few are used routinely in published investigations. New methods, although designed to exploit some particular aspect of the data or use some specific statistical tool, are rarely compared thoroughly in terms of power or performance. Users, when confronted with the multitude of methods available, often select methods arbitrarily, basing choices on software availability, ease of implementation, or use experience rather than considerations of statistical power, possible alternative hypotheses (that is, cluster structure) and likely confounding. In this review, we extend our typology of disease clustering methods and apply it to many of the extant methods identifying strengths, weaknesses and unique features of the methods. We conclude with recommendations for which methods should be applied to which types of situations.