Clearing the Air Asthma and Indoor Air Exposures (2000) / Chapter Skim
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2 Methodological Considerations in Evaluating the Evidence
2 Methodological Considerations in Evaluating the Evidence
Pages 39-66
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From page 39... ...
The committee was asked to evaluate the strength of the scientific evidence associating exposure to indoor pollutants with asthma, to discuss what was known about how and in what way~s) various pollutants influence asthma, and to examine the risk for development or exacerbation of asthma associated with indoor exposures.
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From page 40... ...
reports characterizing scientific evidence regarding vaccine safety (IOM, 1991, 1993) and the health effects of herbicides used in Vietnam (IOM, 1994, 1996, 1999)
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From page 41... ...
Some clinical research for example, that addressing challenge tests and animal studies were considered where appropriate. Engineering, architecture, and physical sciences literature informed the discussions of building characteristics, exposure assessment and characterization, indoor dampness, pollutant transport, and related topics; public health and behavioral sciences research was consulted for data on the effectiveness of interventions to limit exposure to problematic indoor agents.
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From page 42... ...
Statistical significance is a quantitative measure of the extent to which chance that is, sampling variation might be responsible for the observed exposure-adverse event association. The magnitude of the probability value or the width of the confidence interval associated with an effect measure such as the relative risk or risk difference is generally used to estimate the role of chance in producing the observed association.
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From page 43... ...
In the example of socioeconomic status, a simple comparison of asthma rates among the exposed and unexposed would exaggerate an apparent difference in asthma rates, since socioeconomic status is also thought to influence asthma incidence. If exposed individuals were of higher socioeconomic status, the simple comparison would tend to mask any true association between exposure and asthma by spuriously elevating the risk of disease in the unexposed group.
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From page 44... ...
The committee did not in general consider the risk of publication bias to be high among studies of indoor air exposures and asthma because 1. there were numerous published studies showing no positive association; 2.
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From page 45... ...
This third form of causality is what is meant when scientists say that cigarette smoking causes lung cancer. Not everyone who smokes will develop lung cancer and not everyone who develops Jung cancer smokes.
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From page 46... ...
For each indoor air exposure for which evidence indicated the presence of an association with asthma, the committee assessed the applicability of each of five general considerations, based on these criteria: 1. Strength of Association: Strength of association is usually expressed in epidemiologic studies as the magnitude of the measure of effect, for example, relative risk or odds ratio.
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From page 47... ...
The existence of a possible mechanism increases the likelihood that the exposure-disease association in a particular study reflects a true association. In addition, the committee considered factors such as evidence in humans of an association between the exposure in question and diseases known to have causal mechanisms similar to asthma and evidence that asthma outcomes are associated with occupational exposure levels.
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From page 48... ...
SUMMARIZING CONCLUSIONS REGARDING THE EVIDENCE Categories of Association The committee summarized its conclusions using a common format, described below, categorizing the strength of the scientific evidence in two areas: 1. health effects: the association between exposure to an indoor agent and asthma development or exacerbation; and
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From page 49... ...
Sufficient Evidence of a Causal Relationship Evidence is sufficient to conclude that a causal relationship exists between the action or agent and the outcome. That is, the TABLE 2-1 Examples of Actions and Outcomes Usecl in Categories of Eviclence Category Action Outcome Exposure reduction strategies Health effects Exposu re to an indoor agent Implementation of a strategy to avoid or reduce exposure to an indoor agent Asthma development or exacerbation Actual reduction of exposure or reduction of asthma incidence
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From page 50... ...
: Is increased exposure to the possible cause associated with increased response? For consideration of exposure reduction strategies, does more effective implementation of the remediation strategy result in a greater reduction in exposure or in asthma incidence?
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From page 51... ...
In addition, the possibility of a very small elevation in risk at the levels of exposure studied can never be excluded. ASSESSING EXPOSURES TO AGENTS IN INDOOR AIR Assessments of exposure to environmental agents in indoor air play a central role in epidemiology studies seeking to characterize population risks, in screening studies aimed at identifying individuals at risk, and in interventions designed to reduce risk.
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From page 52... ...
the practical and available exposure surrogate that correlates to a greater or lessor extent with the ERR. The ERR will differ depending on the health outcome under study (e.g., asthma development versus asthma exacerbation)
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From page 53... ...
Figure 2-1 summarizes the principal approaches that are available for developing exposure surrogates (adapted from NRC, 1991~. Direct exposure surrogates include personal monitoring involving the measurement of agent concentrations using monitors carried by individual subjects, and biological markers involving the measurement of the agent or its metabolite in biological samples such as urine and blood.
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From page 54... ...
Unfortunately, the space and time continuity assumptions are rarely met in practice, severely limiting the utility of the microenvironmental modeling approach for estimating personal exposures. This is also likely to be the case for many of the indoor agents with plausible roles in asthma, such as particulate allergens.
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From page 55... ...
Internal dose is defined as the amount of an agent that is absorbed into the body over a given time whereas biologically effective dose is the amount of an agent or its metabolites that has interacted with a target site over a given period of time. For inhaled gases, the primary determinant of the lung deposition fraction (the proportion of inhaled mass that is deposited in the lungs)
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From page 56... ...
From the viewpoint of asthma development or exacerbation, the relevant sites and nature of interactions between inhaled agents and human body remain uncertain, limiting our ability to define biologically effective dose in this context. In the case of allergens, however, the measurements of serum IgE and allergen skin test reactivity represent surrogates for biologically effective dose.
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From page 57... ...
allergen CR counts by trapping CR IgE CR allergen in bedroom dust CR skin test CR allergen in kitchen dust Animal (dog, cat, etc.) allergen Self-reported animal Pet allergen in dust Pet allergen in air Pet-specific IgE Pet-specific skin test Fungal allergen Mold odor Fungal-specific IgE Moisture problems Fungal-specific skin test Visual evidence Culturable fungi Spore counts Pollens and plant Pollen counts in air Pollen-specific IgE allergen Allergen concentration in air Pollen-specific skin test Environmental tobacco Self-reported household smoking Cotinine in urine, blood, smoke (ETS)
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From page 58... ...
Like HDM, cockroach allergens are thought to be associated primarily with larger particles that become airborne during and immediately after active disturbance of dust reservoirs. Thus, the same measurement issues apply here as for HDM.
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From page 59... ...
Exposure surrogates based on questionnaire or inspection such as water damage and visible fungal growth also have very uncertain relationships with exposure to airborne fungal allergens. Although it is clear that individuals can be allergic to fungi, measurements of fungal allergen concentrations are very rarely included in epidemiological studies.
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From page 60... ...
Little data exist on the temporal variability of indoor ETS concentrations and it is not known what averaging time is adequate for characterizing long-term airborne ETS exposure of building occupants. Personal sampling represents an attractive approach in terms of sampling location; however, to be valid, the averaging time must be sufficiently long to estimate the long-term average exposure.
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From page 61... ...
The sampling methods inherently account for these factors, as well has being specific for NO2. However, as with most sampling methods, area and personal sampling characterizes only a snapshot in time, which may or may not be a good surrogate for long-term average indoor NO2 levels.
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From page 62... ...
that there was sufficient evidence to generate confident quantitative estimates of the asthma risk associated with indoor air exposures. It is not possible to make general statements about the relative risk of various exposures because this is highly dependent on the characteristics of a particular environment and its occupants.
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From page 63... ...
Publication bias and public health policy on environmental tobacco smoke. Journal of the American Medical Association 272~2~:133-136.
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1994. Toxic volatile organic compounds in environmental tobacco smoke: Emission factors for modeling exposures of California populations.
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From page 65... ...
1991. Human Exposure Assessment for Airborne Pollutants.
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From page 66... ...
1996. A Modified Diffusion Sampler for Measuring 24-Hour VOC Concentrations in Personal, Indoor and Community Air, in Proceedings of the 7th International Conference on Indoor Air Quality and Climate, Vol.
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