Gulf War and Health Updated Literature Review of Depleted Uranium (2008) / Chapter Skim
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4 Methodology
4 Methodology
Pages 73-92
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From page 73... ...
terms uranium and uranium compounds. Uranium-related studies identified by the committee that wrote Gulf War and Health, Volume 1: Depleted Uranium, Pyridostigmine Bromide, Sarin, Vaccines (IOM, 2000b; hereafter referred to as Volume 1)
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From page 74... ...
BOX 4-1 Open-Session Presentations, June 28, 2007 Depleted Uranium Exposure and Health Melissa McDiarmid, Department of Effects in Gulf War Veterans Veterans Affairs Depleted Uranium Follow-Up Program Outcomes of the UK Ministry of Nicholas Priest, Atomic Energy of Defence–Sponsored Depleted Uranium Canada Limited, Canada (formerly Research Programme with Middlesex University, UK) Depleted Uranium and Veterans' Dan Fahey, Board Member, Veter Health: A Flawed Testing Process and ans for Common Sense, and PhD an Undersized, Politicized Study Limit candidate, University of California, Evaluation of Exposures and Effects Berkeley Current NIOSH Research on Uranium- Mary Schubauer-Berigan, National Exposed Workers Institute for Occupational Safety and Health Toxic and Radiologic Effects of Fletcher Hahn, Lovelace Respira U ranium: Animal Studies tory Research Institute Department of Defense Health Kenneth Cox, Department of Databases Defense
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From page 75... ...
. Epidemiologic studies can establish statistical associations between exposures and health effects, and associations are generally expressed by using relative risks or odds ratios.
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Factors Influencing the Relevance and Quality of Studies The committee considered several important issues in its evaluation of the epidemiologic studies and assessment of evidence on uranium-processing workers and civilian and deployed populations exposed to natural and depleted uranium. The discussion builds on the topics covered in Volume 1 inasmuch as many of the methodologic issues are common to the old and new studies.
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From page 77... ...
Studies of occupational exposure on which the committee relied heavily to evaluate the effect of uranium on disease used several methods and models to assess exposure, including direct measurement of individual exposure through estimates of internal and external radiation dose, the use of work histories to estimate cumulative exposure, and classification of workers by maximum exposure.
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From page 78... ...
Using Work History to Model Cumulative Exposure Several researchers approximated individual exposure by modeling cumulative exposure on the basis of a worker's job history in the plant and the level of exposure in each worksite. They measured uranium exposure in various worksites in the processing plant, using measures of urinary uranium or uranium in ambient dust.
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From page 79... ...
Such a study must use the US population or the population of the region in which the plant is sited as the external control group. In that approach, the healthy-worker effect is more likely to distort estimates of the effect of exposure on health outcomes, generally biasing results toward lower risk among the exposed.
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From page 80... ...
Numerous studies evaluated by the committee incorporated biomarkers of effect to evaluate health outcomes related to uranium exposure, including biomarkers to evaluate cell toxicity and renal dysfunction. Adequate Followup Period To strengthen the evidence of a true cause-effect association (particularly for some health outcomes, such as most cancers)
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The types of cancer are lung cancer, leukemia, lymphoma, bone cancer, renal cancer, bladder cancer, brain and other central nervous system cancers, stomach cancer, prostatic cancer, and testicular cancer; the nonmalignant diseases or conditions include renal disease, respiratory disease, neurologic disease, and reproductive and developmental effects. With the exception of prostatic and testicular cancers, the health outcomes were selected by the committee because there are plausible mechanisms of action (for example, lung cancer and respiratory disease were selected because inhaled insoluble uranium oxides lodge in the lung)
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In general, small values of the test statistic reflect consistency with the null, and large values consistency with the alternative. The magnitude of the test statistic is compared with its expected size under the null hypothesis.
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In some studies, the lack of control was a result of the study design; for example, ecologic studies, such as the residential studies in which exposure is determined solely by geographic proximity to an exposure source, by design cannot take individual-level factors into account. Retrospective cohort studies can only be analyzed on the basis of the data available; often, information on other risk factors was not collected, either because they were not known risk factors at the time or because collection of such information was not routine.
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Epidemiologic-Study Designs The major types of epidemiologic studies evaluated by the committee are cohort, case-control, cross-sectional, ecologic studies, and case reports and case series. Cohort Studies A cohort, or longitudinal, study follows a defined group, or cohort, over time.
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From page 85... ...
cohort study differs from a prospective study in temporal direction; the investigator traces back in time to classify past exposures in the cohort and then tracks the cohort forward in time to ascertain the rate of the health effect. Retrospective cohort studies often focus on mortality because of the relative ease of determining the vital status of individuals and the availability of death certificates to determine the causes of deaths.
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From page 86... ...
Those problems might lead to unidentified confounding variables that differentially influence the selection of cases or control subjects or the detection of exposure. For the reasons discussed above, case-control studies are often the first approach to testing a hypothesis about whether factors contribute to a specific health effect, especially a rare one.
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From page 87... ...
Ecologic studies require aggregate data on disease and exposure. Data on disease occurrence are commonly derived from incidence and mortality data, and exposure information is often based on an overall index, for example, environmental data, such as air or water quality.
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It had to measure exposure to uranium separately from other exposures; studies that dealt with multiple exposures and did not specifically report uranium exposures were excluded. • It had to deal with long-term health outcomes.
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The uranium miners were exposed primarily to radon decay products that were attached to (adsorbed onto the surface of) other particulate matter in the mine air.
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Sufficient Evidence of a Causal Relationship Evidence is sufficient to conclude that a causal relationship exists between the exposure to uranium and a specific health outcome in humans. The evidence fulfills the criteria for sufficient evidence of an association (below)
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Limited/Suggestive Evidence of an Association Evidence is suggestive of an association between exposure to uranium and a specific health outcome, but the body of evidence is limited by insufficient avoidance of bias, insufficient control for confounding, or large sampling v ariability. Inadequate/Insufficient Evidence to Determine Whether an Association Exists Evidence is of insufficient quantity, quality, or consistency to permit a conclusion regarding the existence of an association between exposure to uranium and a specific health outcome in humans.
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From page 92... ...
2000b. Gulf War and health, volume 1: Depleted uranium, sarin, pyridostigmine bromide, vaccines.
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