9

Use of the Framework to Assess Both Health and Welfare Effects

Integrated Science Assessments (ISAs) have consistently applied a common causal determination framework to assess both health and welfare effects. While there are aspects of welfare effects that are distinct from health effects (see Chapter 4), a common framework has been employed in ISAs for the 11 National Ambient Air Quality Standards (NAAQS) reviews conducted prior to the publication of this report, including reviews for all six criteria pollutants that have been completed since 2007. Over that period, there have been several reviews of the primary standards for nitrogen dioxide (NO₂), sulfur dioxide (SO₂), particulate matter (PM) and ozone, and a combined review for the welfare effects of oxides of nitrogen and sulfur and PM, although only one of those reviews has led to secondary standards that are different from the primary standard. This might lead some to question whether the causal determination framework is appropriate for both the health and welfare reviews, and is, in fact, a question posed in the committee’s Statement of Task (see Box 1.1).

Potential differences between causal considerations for health and welfare effects are considered in multiple places in the NAAQS review process (e.g., in the construct of the Integrated Review Plan [IRP], the ISA, the Risk and Exposure Assessment [REA], and Policy Assessment [PA]). The utility and scientific foundation of the Preamble’s causal determination framework for both health and welfare are examined by considering differences and similarities in the linkages between pollutants and health and welfare outcomes, and how they are treated in the framework. Given that the secondary (i.e., welfare) NAAQS have typically been set equal to the primary (i.e., health) standards, the committee primarily focuses on potential issues that may be related to welfare causal determinations, with comparisons made to health causal determinations.

ACCOUNTING FOR DIFFERENT CHARACTERISTICS OF CAUSAL CHAINS

There are important differences in the nature of the causal chains of occurrences that lead to effects in humans (related to the primary NAAQS) and on welfare (related to the secondary NAAQS). Those differences arise from the form of exposure (e.g., air concentration versus deposited material), the timing and extent of exposure (e.g., acute, chronic, influence of past exposures),

differences in the receptor for effects (e.g., inhalation to human respiratory tract, deposition to soil, freshwater, and coastal waters), air (impacts on visibility, radiative forcings), and differences in the biological mechanisms through which effects are manifested, among others. Those differences are addressed to some degree in the current framework, but further consideration of those differences is critical to inform whether a distinct causal determination framework is needed to consider health versus welfare endpoints. Such differences are examined in the following sections.

Mechanisms of Exposure Resulting in Health and Welfare Effects

The term “exposure” (see Box 9.1) includes many aspects of the pollutants examined in the causal determination process. Exposures may include, for example, contact at the interface of the human respiratory zone, atmospheric deposition to soil or plant surfaces, or can include the atmosphere as the receptor. The receptor receives the pollutant and is the interface where pollutant exposure initiates the effects for which causal determinations are made. Each exposure is relevant to specific types of effects in a specific receptor in a specific environment. Processes such as atmospheric chemistry, long-term transport, and the cumulative response to a causal agent result in exposure history. Factors that evolve over long timescales, such as climate change, likewise can influence those processes and therefore need to be considered to properly define exposure in each causal context.

Atmospheric concentrations, for example, in the breathing zone or adjacent to stomata of plants, can be practical and relevant exposure measures for causal determinations related to human and some welfare effects on plants and animals that are subjected to gaseous or particulate pollutants. Welfare effects can also directly and indirectly result from atmospheric deposition, such as acidification due to atmospheric sulfur and nitrogen deposition or eutrophication due to nitrogen deposition to ecosystems. Deposition and subsequent ingestion can also result in human exposures to lead. Assessing effects of nitrogen deposition are further complicated because total nitrogen deposition is comprised of both oxides of nitrogen that are regulated through the NO₂ NAAQS and reduced nitrogen from emissions of ammonia, which is not a criteria pollutant. Effects of atmospheric deposition are a function of deposition loads by which a pollutant in the atmosphere is transferred to the Earth surface, and further mediated by soil and surface water chemistry. Long-term cumulative loads can modify current and future impacts. Other kinds of welfare effects, such as effects of PM and ozone on climate and crops, effects of ozone on ultraviolet radiation, and effects of PM and NO₂ on visibility, are caused by the presence of these pollutants in the ambient air, rather than the direct exposure of an affected organism to a pollutant, consistent with the definition

of exposure in Box 9.1. Changes in some welfare effects, like visibility, respond instantaneously to changes in ambient pollutant concentrations, while other welfare effects such as those resulting from long-term cumulative deposition are expected to have substantial lag or hysteresis effects and may, in some cases, be irreversible.

Relevant Pollutant Averaging Times in Health and Welfare Causal Determinations

Many welfare effects occur over timescales or averaging times that differ from those typically used for health studies supporting causal determinations. For example, the effects of ozone on crops and trees are associated with cumulative seasonal exposure and are influenced by environmental conditions such as soil moisture. Ozone effects on vegetation are typically quantified by a weighted cumulative aggregation of ambient ozone concentrations over the daylight hours for a 90-day growing season (termed W126; Capps et al., 2016; Lefohn et al., 1988, 1997). This averaging time differs substantially from the level, averaging time, and form of the ozone health standard, based on the fourth highest daily 8-hour maximum. Visibility effects (predominantly caused by PM_2.5) are experienced over short (e.g., almost immediate to multihour) periods, while the primary and secondary PM NAAQS are set at annual and daily averaging times. Ecological effects of sulfur and nitrogen deposition result from decades of cumulative deposition of these pollutants, but current primary and secondary standards for these pollutants range from 1 to 3 hours for SO₂ and 1 hour to annual for NO₂. Acute effects to larval fish and other aquatic organisms of atmospheric acidic deposition that occur during hydrologic events such as snowmelt are themselves the result of chronic accumulation of cumulative deposition in soils and watersheds. Lead is an interesting example of a health-relevant exposure driven by deposition where ISA causal determinations recognize that the exposures are due, in part, to long-term deposition. The framework for assessing causality allows consideration of alternative averaging times for exposure metrics to ensure they are scientifically relevant to the endpoints at issue. However, the framework could be improved to encourage discussion of how the most relevant exposure metrics relate to the existing averaging time and form of the standards, to support consideration of whether changes are needed.

Cumulative Effects

Cumulative effects of pollutants are linked to health and welfare endpoints. Lead, for example, accumulates in soils and in the body over prolonged exposures. The resulting primary NAAQS for lead is based on a 3-month average atmospheric concentration. Chronic exposure to PM is typically found to be the exposure leading to the greatest health damage. The primary NAAQS for PM_2.5 include both an annual and 24-hour averaging times.

While it may be argued that all air pollutants have long-term cumulative effects associated with health and welfare endpoints, cumulative effects can be particularly persistent for ecosystem impacts of atmospheric deposition. There are two elements of cumulative effects of atmospheric deposition on ecosystems. The first is associated with the fate of the contaminant. For example, a fraction of the atmospheric sulfur, nitrogen, lead, mercury, and other contaminants deposited is retained in the ecosystem (in soil or biomass). This retained material will accumulate and cycle through the ecosystem resulting in ongoing effects; the ecosystem will eventually reach steady-state with respect to the input of the contaminant (Galloway et al., 1983). With decreases in deposition, these legacy contaminants will either be exported from the ecosystem associated with drainage or gaseous losses, or become permanently sequestered (Rice et al., 2014). However, the legacy contaminants can persist for decades following their original deposition. The second aspect of cumulative effects is the perturbation to the ecosystem caused by the deposited contaminant. For example, acidic atmospheric deposition to acid-sensitive forest ecosystems acts to acidify soils and

deplete available forms of nutrient cations from soil (Driscoll et al., 2001). A second example is biological changes to ecosystems that occur with elevated atmospheric nitrogen deposition, given that nitrogen is typically the growth-limiting nutrient in terrestrial and marine ecosystems. In the presence of elevated nitrogen inputs, organisms that are effective at assimilating low levels of nitrogen and cycling nitrogen conservatively are displaced by less diverse communities that proliferate under eutrophic conditions, reducing biodiversity (Seabloom et al., 2021). Eutrophication causes a shift in the structure and function of ecosystems. It is not clear if ecosystems can recover to their previous oligotrophic condition following decreases in nitrogen inputs, and, if so, the extent and timescale of recovery are highly uncertain (Gilliam et al., 2019; Milchunas and Lauenroth, 1995).

This complexity implies that causal statements for welfare effects that result from studies of long-term cumulative pollutant impacts are often unclear regarding the temporality of the causal association—and temporality is a key component of the Bradford Hill aspects of association (Hill, 1965). For example, the 2020 ISA on oxides of nitrogen, oxides of sulfur, and PM ecological criteria (EPA, 2020b) identifies a causal association between nitrogen deposition and the alteration of soil biogeochemistry in terrestrial ecosystems, which had previously been identified in the 2008 secondary NO_x-SO_x-PM ISA (EPA, 2008c). Sulfur and nitrogen deposition have been altering soil chemistry for more than a century in many parts of the United States (Lehmann et al., 2008; Warby et al., 2009), with maximum deposition rates peaking roughly 50 years ago and declining substantially since that time. Chemical recovery in soils can lag decades behind emissions changes (Lehmann et al., 2008; Rice et al., 2014), and biological recovery, in locations where it has been observed, can lag considerably behind chemical recovery, and may not return to pre-acidification conditions (Driscoll et al., 2001; EPA, 2020b). With nitrogen deposition in exceedance of vegetation critical loads across much of the United States (Pardo et al., 2011), there is potential for a cumulative loss of ecosystem biodiversity, leading to irreversible changes in ecosystem function and ecosystem services (Gilliam et al., 2019; Simkin et al., 2016). In some cases, current inputs may be sufficient to assure that some effects may continue indefinitely, while in other cases they may serve to slow the rates of chemical and biological recovery. Again, while the framework allows for consideration of such long-term processes, it could be expanded to guide consideration of the dynamic nature of causal relationships for welfare endpoints.

Climate Effects

Climate change and its effects alter how air quality and atmospheric deposition influence health and welfare (von Schneidemesser et al., 2020) by modifying exposures and exposure-response relationships. Examples of exposures modified by climate change include the enhanced production of ozone under increases in air temperature (Jacob and Winner, 2009; Shen et al., 2019) and production of fine particulate matter associated with wildfire (von Schneidemesser et al., 2020) or the enhanced suspension of soil to the atmosphere (dust) due to desertification (Hudson et al., in press). Climate change also alters how health and welfare are affected by exposures to given levels of air pollution. With respect to welfare, the changing climate is causing fundamental changes to the structure and function of ecosystems, and these changes co-occur with effects of air pollution. Climate effects, including changes in temperature and modifications of the hydrologic cycle, can either exacerbate or mitigate air pollution impacts on ecosystems, and can vary regionally and temporally. Air pollution, including criteria pollutants such as ozone and PM, also affects the feedbacks from altered ecosystems to the radiative forcings of the Earth system, as considered in recent ISAs for PM (EPA, 2019c) and ozone (EPA, 2020a). Example feedbacks that can be altered by air pollution include increases in susceptibility of ecosystems to wildfire, changes in carbon sequestration

in soil and biomass, and changes in land-atmosphere exchanges of methane and nitrous oxides. Although climate change poses long-term threats to both health and environment, the relatively long timescales of climate effects are temporally relevant to long-term ecological effects, such as those resulting from cumulative multidecadal deposition of nitrogen and sulfur compounds. Those pollutants are interacting with climate change to alter the structure and function of ecosystems in fundamental ways.

The role of climate change in influencing effects of air pollution on both health and welfare is expected to grow (IPCC, 2021; USGCRP, 2018b; von Schneidemesser et al., 2020). Climate change will have health and welfare consequences beyond air quality and other effects from combinations of climate and air quality that can vary over the multidecadal timescales identified above. Some of the combined long-term environmental effects also relate to complex, large-scale ecological processes, such as the environmental cycling of carbon, nitrogen, phosphorus, and other nutrients (Campbell et al., 2009; Matyssek et al., 2017; Zaehle, 2013). The framework does not address how the current causal determinations would capture the ways changing climate likely will impact causal linkages between criteria pollutants and long-term ecological effects. The current framework as described in the Preamble contains numerous references to climate effects, but these are limited to considering effects of the criteria pollutants on climate forcings (i.e., processes affecting the Earth’s climate through a number of forcing factors, including greenhouse gas absorption of outgoing radiation).

Welfare Effects Involve Multiple Species and Ecosystem Elements

Pollutant effects on human health are varied and complex but are limited to a single species: humans. Ecosystems are comprised of thousands of plant, animal, and microbial species and assemblages that are potentially impacted by air pollutants, affecting the structure and function of ecosystems and associated ecosystem services. Organisms can be affected by air pollutants to varying degrees (due to differences in lifetimes, life histories, interactions among species, as in mycorrhizal associations between plants and fungi, and mechanisms of exposure or perturbation [Averill et al., 2018]), but their cumulative impact on carbon cycling, resistance to disturbance, and provision of services such as wood products (described as ecosystem services) can result from interactions of air pollutants with the entire soil and plant system. Abiotic endpoints (soil, sediment concentrations) can be important in assessing effects of air pollution. Pardo et al. (2016) estimated empirical critical loads of nitrogen for a variety of endpoints across the United States, finding large differences in the critical loads of nitrogen for different endpoints and large spatial variation in the values of critical loads for a given endpoint. Horn et al. (2018), Simkin et al. (2016), and Geiser et al. (2021) determined critical loads for tree species, herbaceous plant species, and lichens, respectively, across the conterminous United States, finding adverse effects for these organisms at different levels of atmospheric sulfur and nitrogen deposition. Baron et al. (2011) found similar patterns for freshwaters across the United States dependent on current levels of atmospheric nitrogen deposition, antecedent levels of atmospheric deposition, and biophysical characteristics of surrounding watersheds. At present, the causal determination framework as described in the Preamble (EPA, 2015a) includes no references to terms such as biodiversity, sustainability, irreversibility, or ecological cascades—terms that could both help aggregate multiple ecological endpoints and also convey a sense of the magnitude and scale of the adversity of ecological effects due to air pollution. Integration of responses across a range of endpoints might provide effective, cohesive evidence that could support secondary NAAQS to protect welfare.

Welfare and Health Effects Involving Sources of Biophysical Variability and Susceptibility

Welfare and health effects both are impacted by variability in vulnerability and susceptibility. The Preamble devotes considerable discussion (nearly three pages) to identification of populations or life stages considered to be most “at risk” (sensitive, vulnerable) to health effects of air pollution, but provides only a single paragraph of guidance for addressing this topic as relevant to welfare. One of the challenges associated with assessing causality of welfare effects is addressing the complexity and uncertainty that arise from the inherently large biophysical variability. That variability leads to differences in susceptibility and resilience in ecosystem elements and processes that collectively influence the response of organisms and ecosystems to stressors. For terrestrial and freshwater ecosystems, these biophysical characteristics include bedrock geology, topography, elevation, climate, and soil moisture and development. Freshwater environments additionally require consideration of hydrologic connectivity with the adjacent terrestrial environment, gradient, hydrologic residence time, and depth. For the many biological components of these ecosystems, such as microbes, plants, insects, and animals, there are many common biochemical, physiological, and behavioral processes that contribute to variability and susceptibility. Building on the science assessment and causal determinations in the framework, U.S. Environmental Protection Agency (EPA) staff introduced an approach for addressing spatial variability in ecosystem sensitivity to acidification in the review completed in 2012 for secondary standards for sulfur and nitrogen oxides.¹ The country was divided into 84 “ecoregions” based on grouping a variety of vegetation, geologic, and hydrologic attributes that were directly relevant to sensitivity of fresh waters to aquatic acidification. This approach allowed consideration of a uniform national standard that was spatially modified by the inherent sensitivity of regional ecosystems (EPA, 2011b). Similar consideration could be applied to spatial variations in sensitivity to nitrogen deposition, ozone effects on sensitive plant or crop species, or effects of PM on visibility. For example, building on past studies showing that perception of adverse effects on visibility is dependent on the scene being viewed. Malm et al. (2019) explored scene-dependent visibility metrics to better represent people’s preferences.

The Preamble states that consideration will be given to public welfare impact endpoints or services that are “differentially affected” but lacks specifics on how this will be done. There is substantial spatial variability in the inherent sensitivity to some welfare effects—including ozone effects on forest plants and agricultural crops, ecological effects from sulfur or nitrogen deposition, and perceived effects on visibility. There are also threatened or endangered species and critical habitats or ecosystems, including those at the global scale, that warrant heightened attention. All of these lend themselves to development of indices, surrogate indicators, and map presentations that can describe this spatial variability.

Recent ISAs are missing assessments at the largest scale, where effects to individual ecosystems and their processes are influencing, or at least contributing to, global loss of biodiversity and net ecosystem production. There is no guidance in the Preamble related to these global scale cumulative implications of air quality to ecosystems. For example, while the NO_x-SO_x-PM ISA (EPA, 2020b) includes language on the effects on ecosystem processes (e.g., net ecosystem production [NEP]), the evaluation does not go beyond identifying causality for individual ecosystems. Language is devoted to addressing effects of atmospheric nitrogen deposition on biodiversity, but not to the cumulative impacts of atmospheric nitrogen deposition effects on biodiversity and NEP even though there is a growing literature on the topic and consideration of references to these effects in the ISA.

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¹ See https://www.govinfo.gov/content/pkg/FR-2012-04-03/pdf/2012-7679.pdf (accessed May 24, 2022).

ADEQUACY AND EFFECTIVENESS OF THE CURRENT EPA CAUSAL DETERMINATION FRAMEWORK FOR BOTH HEALTH AND WELFARE STANDARDS

Previous chapters describe the current causal determination framework, its components, and its use in the NAAQS review process for health and welfare standards. As described in sections above, there are differences in the nature of health and welfare effects, how these effects relate to changes in ambient air concentrations, and how the framework is applied in the process for reviewing primary and secondary NAAQS. The next sections include discussion on the adequacy and effectiveness of the current methods and framework used in the causal determinations.

Adequacy

Despite fundamental differences in the characteristics of health and welfare effects evaluated in recent ISAs, evidence indicates that the current framework functions to support both the health-based and welfare-based reviews. Multiple scientifically sound and comprehensive assessments have been produced for both health and welfare effects, as evidenced and supported by CASAC reviews, citations of the ISAs in the scientific literature, and the extensive reference to and reliance on the ISAs in air quality health assessments conducted by other countries and international health organizations (e.g., Health Canada, 2013a,b; WHO, 2021).

A primary purpose of the framework and each ISA is to support the downstream EPA REAs and PAs by identifying relevant health and welfare endpoints, typically those for which causal or likely causal associations were found. The REAs and PAs are used by a separate EPA office to recommend ranges in primary and secondary standards for the administrator to consider in deciding whether to retain or revise the NAAQS. Over the past decade, ISAs developed under the current framework have informed these downstream stages of review for both health and welfare effects. Typically, the REAs and PAs include analyses for effects that have been determined to be “causal” or “likely causal,” although further analyses have been conducted for effects determined to be “suggestive” (EPA, 2013b).

For example, applying what was essentially the current framework in the ozone NAAQS review completed in 2015 (EPA, 2015a), the 2013 ISA (EPA, 2013a) noted “causal” or “likely causal” effects of ozone on visible foliar injury, reduced vegetation growth, reduced productivity in terrestrial ecosystems, reduced carbon sequestration in terrestrial ecosystems, reduced yield and quality of agricultural crops, alteration of the water cycle of terrestrial ecosystems, alteration of belowground biogeochemical cycles, and alteration of terrestrial community composition (EPA, 2013a). The 2013 Ozone ISA supported the 2014 REA for welfare effects, which in turn supported EPA’s final Ozone PA (EPA, 2014c). This PA recommended establishing a new secondary ozone standard based on a weighted (W126) summation of daylight ozone concentrations aggregated over the 3-month “growing season” in the range of 7 to 17 ppm-hours, which was based on the scientific literature reviewed in the ISA and the associated causal determinations. While that standard was ultimately not adopted, the PA reflected the scientifically sound determination that vegetative effects of ozone are causally linked to cumulative exposure over the growing season.

Maintaining a consistent causal determination framework across multiple NAAQS reviews is also an effective way of conveying changes over time. For example, the 2020 Ozone ISA identified five new endpoints with causal or likely causal welfare effects and upgraded another effect from likely causal to causal, clearly presenting the new science relevant to a potential secondary standard. A second example demonstrates how EPA, building on the causal determinations in the ISA, structured a scientifically founded proposed form of the standard relevant to the combined deposition of oxides of nitrogen and sulfur (initial NO_x-SO_x Secondary, 2011) accounting for biophysical, spatial

variability in the environment. While the NAAQS are expected to be nationally uniform, EPA staff developed, in 2011, an innovative approach to address spatial variability in ecological sensitivity—which also accommodated regional variability in the ratios of deposition to ambient concentrations, nitrogen saturation in soils and reduced nitrogen (ammonium) deposition—as components of an Atmospheric Acidification Protection Index (AAPI), proposed in the 2011 PA (EPA, 2011a). The approach was not adopted but the framework supported its development, and the proposed standard was consistent with, and supported by, the causal determinations made in the ISA.

Effectiveness

The purpose of the ISAs is to compile, review, and clearly convey the scientific information on effects of air pollution that is relevant to NAAQS decision making under the Clean Air Act. While the current framework works for both health and welfare effects, there is potential for improving its effectiveness in conveying the state of scientific understanding. There are opportunities to improve the effectiveness of the framework for making causal determinations relevant to NAAQS for both welfare and health effects, including ensuring that causal determinations are adequately supported and explained and that the significance of, and differences in relevant exposure patterns for health and welfare endpoints are made clear.

ISAs have found causal associations between air pollutants and numerous adverse effects on public health and welfare, though the lack of separate and distinct welfare-oriented standards constrains the effectiveness of the causal determination framework in guiding the communication of the scientific understanding of links between air pollution and welfare effects as needed to inform the secondary NAAQS. For example, scientific studies find that the impact of ozone on plant growth is related to longer term (multimonth) exposures than an 8-hour primary NAAQS driven by the science of human health responses to elevated ozone, though the resulting secondary standard is set as the primary standard. The framework may need to be modified to ensure that ISAs specifically address relationships between exposure metrics that are scientifically relevant for welfare endpoints separate from health endpoints and relevant forms, indicators and averaging times are considered in the causal determinations for both primary and secondary standards. Additional considerations for improving the effectiveness of the framework for welfare endpoints are discussed below.

The above discussion leads to the conclusion that the current framework is scientifically supported for use in making causal determinations in the ISAs. The framework is subject to and open to significant internal and external review by both a group of independent scientific experts (the Clean Air Scientific Advisory Committee [CASAC] and CASAC panels) and other stakeholders, and EPA responds openly to the comments and criticisms as laid out by the framework.

Considerations for Strengthening the Framework

The analysis presented in this section leads to a range of suggestions that could be the basis for strengthening the causal determination framework as currently implemented for both health and welfare. It would be useful for the framework to guide ISA discussions on how specific welfare and health effects relate to all four aspects of the current NAAQS (indicator, averaging time, level, and form). How exposure is characterized is important in making causal determinations, and thus to informing the form and averaging times of a standard based on those causal determinations The averaging times and forms of the current secondary standards for ozone, PM_2.5, SO₂, and NO₂ are not driven by the scientific studies used for addressing the most important welfare effects associated with these pollutants. SO₂ and NO₂ indicators, alone, are not responsible for total S and N deposition, which results from all gaseous and particulate forms of oxidized S and total (oxidized and reduced) N in the atmosphere. Acidification effects result from the combined influence of S and N deposition, but the current secondary standards consider each pollutant separately, despite reduced

N being an important, unregulated, pollutant. A PM “light extinction” indicator for visibility effects has been recommended in the last two PM NAAQS reviews, and the current annual averaging period for the secondary, visibility-related PM_2.5 (mass) standard also contrasts with the hourly or few-hour averaging time recommended in the past three NAAQS reviews. There are only a few references to the “level” (concentration) of the standards in the Preamble, and indicator, averaging time, and form are not currently mentioned. The causal determination framework and methods used as laid out in the Preamble could be updated to seek and emphasize new scientific information relevant to all aspects of the standards, which pertain to relationships between concentrations and endpoints and how exposures are characterized, and to clarify how aspects of these exposures should be considered up front in the ISAs.

The Preamble’s framework could provide guidance for clearer (or additional) causal determinations in the ISAs that better articulate the separate and combined influences of current and historical deposition and how they compare to the standard being reviewed. This information would address how long-time scales leading to current and future effects from air pollutants are best characterized in the causal determinations and thus how those determinations support the NAAQS review process.

The Preamble, in addressing language on the causal determinations process, includes no reference to terms like biodiversity, sustainability, irreversibility, or ecological cascades—terms that could both help aggregate multiple ecological endpoints and convey a sense of the magnitude and scale of the adversity of ecological effects due to air pollution. Consideration of these effects would benefit from more explicit guidance. Similarly, given the complexities involved in the biophysical variabilities involved in welfare responses to air pollution, further emphasis on how that variability in ecosystem sensitivity ise treated should be included as part of the Preamble’s causal determination framework.

The Preamble states that consideration will be given to public welfare impact endpoints or services that are “differentially affected,” but the Preamble lacks specifics on how this will be done. There is substantial spatial and species-specific variability in the inherent sensitivity of some welfare effects—including ozone effects on plants and agricultural crops, ecological effects from sulfur or nitrogen deposition, and perceived effects on visibility. There are also threatened or endangered species and critical habitats or ecosystems that warrant heightened attention. All these considerations lend themselves to development of indices, surrogate indicators and maps that can help describe spatially variable responses to air pollutants—guidance for discussion of which could be incorporated into the framework. This approach was implemented in the ISA for Oxides of Nitrogen, Oxides of Sulfur, and Particulate Matter—Ecological Criteria (EPA, 2020b). A standard directly using this type of scientifically motivated exposure metric was not adopted, but the framework supported its development. The causal determination framework could be modified to guide expanded coverage of information relevant to the spatial variability of welfare effects, with the aim of informing those setting secondary standards so that those standards protect against adverse effects in more sensitive or critical areas.

As discussed above, the influence of climate change on potential levels of effects of air pollution on both health and welfare is expected to become increasingly important (IPCC, 2021; USGCRP, 2018b; von Schneidemesser et al., 2020). Given the multidecadal time scales involved, there is potential for some environmental and health effects of air pollution to be evaluated in the context of projected future climate conditions. The current causal determinations do not capture how changing climate likely will impact causal linkages between criteria pollutants and long-term ecological effects. The current Preamble contains numerous references to climate effects, but these are limited to considering effects of the criteria pollutants on climatic forcings. The Preamble could be updated to seek and emphasize new information on the effects of climate change on air quality, as well as the expected long-term coeffects of changing air quality and climate on large-scale ecological processes and human vulnerability. This is an area that could benefit from research.

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