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    Methodological considerations in developing local-scalehealth impact assessments: balancing national,regional, and local data

    Bryan J. Hubbell & Neal Fann & Jonathan I. Levy

    Received: 2 October 2008 /Accepted: 12 March 2009 /Published online: 31 March 2009# The Author(s) 2009. This article is published with open access at

    Abstract National-scale health impact assessments (HIAs)have been conducted for many years and have becomereasonably systematized. Recently, there has been growinginterest in utilizing HIA methods at local scales, in thecontext of Environmental Public Health Tracking and inother settings. This paper investigates the data andanalytical challenges to estimating the incidence of healtheffects associated with changes in air pollution concen-trations at the local scale, focusing on ozone and fineparticulate matter. Although it could be argued that thelocal-scale HIA is simply a more geographically discreteversion of the national-scale assessment and, therefore, hassimilar challenges, in practice, many key inputs in national-scale assessments are assumed to be spatially uniform orvary only at coarse geographic resolution. For a national-scale assessment, this assumption may not contributeappreciable bias, but the bias could be significant for anyindividual location. Thus, local-scale assessments requiremore geographically resolved air quality data, concentrationresponse (C-R) functions, and baseline incidence rates thanare often used. However, comprehensive local data may notbe available, may be incomplete, or may be time-intensiveand resource-intensive to develop, especially for C-Rfunctions for which small-scale epidemiological studieswill often be underpowered. Given this context, this paper

    considers how best to develop credible local-scale HIAs,identifying factors that contribute to variability acrossgeographic areas, study designs, and time periods. Thispaper also describes which key sources of analyticaluncertainty change as the scope shifts from the nationalto the local scale. These challenges notwithstanding, thepaper concludes that a well-designed local-scale HIA,following key principles and recommendations, can beboth informative and defensible.

    Keywords Health impact assessment . Baseline incidencerate . BenMAP. Concentrationresponse function .

    Health impact function . Air quality modeling


    In the United States (U.S.), regulatory agencies such as theU.S. Environmental Protection Agency (EPA) have anextensive history of performing national-scale healthbenefits assessments of major air pollution regulations. Inone such analysis, the U.S. EPA estimated that implemen-tation of the Clean Air Act amendments would result inapproximately 23,000 premature deaths avoided in 2010,relative to a baseline of Clean Air Act implementationwithout the amendments (U.S. EPA 1999). More recently,EPA has systematized this approach by developing theEnvironmental Benefits Mapping and Analysis Program(BenMAP) for estimating national-scale health benefits ofpolicies and regulations to reduce air pollution. BenMAPwas used in recent Regulatory Impact Analyses (RIAs)of the Clean Air Interstate Rule and Nonroad Diesel Rule(U.S. EPA 2004, 2005), finding that, when fully imple-mented, those two rules combined would result in close to30,000 premature deaths from air pollution avoided annually.

    Air Qual Atmos Health (2009) 2:99110DOI 10.1007/s11869-009-0037-z

    B. J. Hubbell :N. Fann (*)Office of Air Quality Planning and Standards,U.S. Environmental Protection Agency,Research Triangle Park, NC 27711, USAe-mail:

    J. I. LevyDepartment of Environmental Health,Harvard School of Public Health,Boston, MA, USA

  • The general approach utilized within these analyses has beenfound to be reasonable and informative for policy decision-making in spite of inherent uncertainties (NRC 2002).

    In recent years, interest has grown among both regula-tory agencies and researchers in developing similar types ofassessments at subnational or local scales, focusing oneither the health impacts of pollutants or the health benefitsof control strategies. For example, while the RIA of theClean Air Interstate Rule (U.S. EPA 2005) estimated airquality changes at 36 km resolution and provided healthbenefits estimates only at the national scale, other studieshave used atmospheric models with census tract resolutionand provided health benefits estimates for individual tractsor as a function of distance from individual sources (Levyand Spengler 2002; Levy et al. 2002). In addition, BenMAPanalyses have been conducted to address a variety of riskmanagement questions in Philadelphia, Detroit, California,Georgia, and for the LakeMichigan Air Directors Consortium(Chang et al. 2007; Cohan et al. 2007; Clean Air Council2004; Deck 2006; U.S. EPA 2007a).

    A local-scale health impact assessment (HIA; consideredhenceforth to be urban-scale) applies essentially identicalmethods as a national-scale HIA, but is distinguished by theheightened importance of spatially refined input data. Inprinciple, a national-scale assessment would rely on thesame spatially varying input data as used in a local-scaleassessment. However, in practice, many key inputs areassumed to be spatially uniform or to vary only acrossregions of the country. For a national-scale assessmentfocused on total national impacts, this may not contributeappreciable bias, but the bias may be larger for individualurban areas.

    As a result, there are multiple tensions and tradeoffs thatanalysts must weigh in attempting to construct defensiblelocal-scale HIAs. In general, as the spatial scale decreases,national or generic data may become less representative.At the same time, local data may not be available or may bemore uncertain given smaller sample sizes. In addition,there is a tension between developing assessments that arelocally meaningful and using methods that are consistentwith other HIAs, so that results can be compared acrosssettings. Finally, if HIAs are to be conducted on a regularbasis, the HIA methodology should provide a mechanismfor collection and use of regularly updated data andconcentrationresponse (C-R) functions.

    This paper explores each of these issues in the context ofozone and fine particulate matter (PM2.5) HIAs in the U.S.Historically, these are the two pollutants that dominate thehealth benefits of air pollution control strategies. As aresult, ozone and PM2.5 were selected within the Environ-mental Public Health Tracking (EPHT) program frameworkas key pollutants to evaluate within a local-scale HIAcontext (CDC 2007). In the Overview of health impact

    assessment methods section, we provide a brief conceptualoverview of HIA methods. In the Analytical challenges toperforming a local-scale HIA section, we discuss criticaldata issues that must be addressed when conducting a local-scale HIA. In the Consideration of time-varying factors inconducting local-scale health impact assessments overtime section, we discuss the need to consider the influenceof time-varying factors in conducting local-scale HIAs overtime. In the Characterizing uncertainty in local-scale healthimpact assessments section, we discuss characterization ofuncertainty, including aspects that are amplified at the localscale. Finally, in the Conclusions and recommendationssection, we conclude with some recommendations for theconduct of local-scale HIAs.

    Overview of health impact assessment methods

    The key elements of health impact assessments (HIAs) areillustrated in Fig. 1, and include:

    1. Estimate a change in or increment of ambient airquality, using monitoring data (from ground-based orsatellite measurements), modeled air quality, or acombination of the two. These air quality changesmay be actual or hypothetical.

    2. Combine air quality changes with population informa-tion to determine changes in population exposure in aform that is relevant given the epidemiological evi-dence (e.g., the appropriate averaging time).

    3. Combine changes in population exposure to ambient airpollution with impact functions1 to generate distribu-tions of changes in the incidence of health effects. Theimpact functions are constructed using population data,baseline health effect incidence and prevalence rates,and C-R functions.

    4. Characterize the results of the HIA, through the use ofsummary statistics (e.g., mean, 95% confidence interval),graphs (e.g., cumulative distribution functions and box-plots), and maps.

    Health impact functions estimate the change in a healthendpoint of interest, such as hospital admissions, for a

    1 The term impact function as used here refers to the combination of(a) a C-R function obtained from the epidemiological literature, (b) thebaseline incidence for the health effect of interest in the modeledpopulation, and (c) the size of that modeled population. The impactfunction is distinct from the C-R function, which strictly refers to theestimated equation from the epidemiological study relating the relativerisk of the health effect and ambient pollution. We refer to the specificvalue of the relative risk or estimated coefficients in the epidemiolog-ical study as the C-R function. In referencing the functions used togenerate changes in incidence of health effects for this paper, we usethe term impact function.

    100 Air Qual Atmos Health (2009) 2:99110

  • given change in air pollution concentrations (here, consid-ered to be either ambient ozone or PM2.5). A typical healthimpact function might look like:

    y y0 ebx 1

    where y0 is the baseline incidence (the product of thebaseline incidence rate times the potentially affectedpopulation), is the C-R function, and x is the estimatedchange in ambient concentrations. There are other func-tional forms, but the basic ele


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