Exposure to ozone (often referred to as "smog"), regardless of where that ozone originates, has been linked to negative human health effects, including respiratory ailments and premature death. Children, the elderly, and persons with respiratory illnesses are particularly susceptible to adverse health impacts from ozone exposure. EPA estimates that ozone exposure was responsible for more than 15,000 premature deaths in the United States in 2007 based on 2006-2008 average ambient ozone concentrations. Ozone has also been linked to plant damage and decreases in crop yield. Concentrations of ozone at the ground level, originally considered a local issue, is increasingly recognized as a global challenge. Ozone is not emitted directly but is formed in the atmosphere from chemical reactions of nitrogen oxides (NO x ) with volatile organic compounds (VOCs, a type of hydrocarbon) in the presence of sunlight. NO x and VOCs are known as "precursor" emissions, and their relative contributions to the formation of ozone depends on a number of factors, including weather conditions and concentrations of other pollutants. The lifetime of ozone in the atmosphere ranges from hours to weeks, providing time—under the right conditions—for pollution emitted in one location to affect the health and welfare of populations far downwind (see Figure 2 ). While local emissions of ozone precursors are still the dominant source of ozone in many areas, state and local air quality agencies face ozone pollution arising from sources outside of their jurisdictional control ("background ozone"). As will be described and discussed in this report, potential long-range transport of air pollutants presents a challenge to downwind communities. It can also be an opportunity for cooperation among localities, states, and countries. The Clean Air Act (CAA) directs the U.S. Environmental Protection Agency (EPA) to establish National Ambient Air Quality Standards (NAAQS) to protect public health (primary standards) and welfare (secondary standards). The law directs that "the attainment and maintenance of [primary standards] are requisite to protect the public health." While the standards are set to limit adverse impacts, EPA acknowledges that these standards do not suggest that concentrations below these levels present zero risk. The NAAQS set limits for the concentrations in ambient air of six common "criteria" pollutants: lead, nitrogen oxides, sulfur dioxide, carbon monoxide, particulate matter, and ozone. There is no evidence of a safe level of ozone exposure below which no adverse health effects occur. However, uncertainty between exposure and health response increases at very low ozone concentrations—that is, below 20 parts per billion (ppb). Air quality monitoring stations in the United States measure concentrations of the six criteria pollutants, and these measurements are used to determine whether locations meet the NAAQS. If a monitor measures concentrations above the standard for any of those six pollutants for an averaging time specified in the NAAQS, the area around that monitor may face a "nonattainment" designation for that pollutant. Once designated nonattainment, the state containing that area must propose a plan to bring the area into attainment of the NAAQS. These State Implementation Plans (SIPs) require approval by EPA. State air quality regulators develop SIPs to attain NAAQS for ambient air in their states, and they have jurisdiction only over the sources of emissions within their borders. The levels of pollution flowing into nonattainment regions, generally referred to as "background pollution," may be making it more difficult for some areas of the United States to meet attainment. Congress recognized this challenge when it enacted the original CAA, adding the "good neighbor provision," which addressed interstate transport of human-caused air pollution that contributes to nonattainment. For detailed information about domestic air transport, see CRS Report R45299, The Clean Air Act's Good Neighbor Provision: Overview of Interstate Air Pollution Control , by Kate C. Shouse. The CAA also mandates EPA to review the NAAQS every five years and revise them as appropriate. EPA completed its most recent review of the ozone standard in 2015, when it lowered the standard from 75 ppb to 70 ppb. EPA reported that it has begun the next ozone NAAQS review and that it intends to complete it by 2020. The procedure for reviewing and setting the NAAQS explicitly does not consider what sources contribute to total ozone, including background sources. In 2018, EPA announced plans to streamline the NAAQS review process and obtain Clean Air Scientific Advisory Committee advice regarding background pollution and potential adverse effects from NAAQS compliance strategies. EPA's "Back-to-Basics" memorandum described concerns that background levels of pollution pose a challenge to meeting NAAQS standards. The memorandum noted a call from certain state regulators for advice on how to treat background ozone, stating that "state environmental agencies have sought this advice, citing the 'absolute need for a valid source of information about background concentrations.'" Additionally, EPA created a task force to develop "additional flexibilities for states to comply with the ozone standard." Much of the focus of ozone transport and control has historically been on upwind domestic sources. Members of Congress may have an interest in better understanding background ozone from natural and international sources, particularly as EPA reviews the 2015 ozone standard. Contributions from sources of background ozone may become important as states with nonattainment areas develop SIPs that attempt to quantify these contributions and consider ways to address them. To assist Congress in understanding these issues, this report defines background ozone , focusing on natural and international sources, and describes what is currently known about these sources. The report then goes on to discuss the limitations in the scientific community's understanding and options for deepening that understanding. This report will discuss background air pollution primarily in the context of ground-level ozone. As of 2018, with the 2015 ozone standard set at 70 ppb, there are 52 areas in the United States designated "nonattainment" for ozone (see Figure 1 ). Current research suggests that natural sources and sources outside the United States may contribute to total ozone in those areas at certain times. Many of the issues discussed here are not unique to ozone, however, and may apply to other pollutants covered by NAAQS, and any potential actions taken to understand or reduce background ozone may also reduce background concentrations of other pollutants. This report deals exclusively with ozone measured at ground level and its adverse health and material effects. Ozone Transport 101 and Figure 2 provide additional information about ozone in different layers of the atmosphere and how each layer may interact with, or contribute to, ozone at ground level. EPA defined natural background (NB) and U.S. background (USB) in the final 2015 ozone rule (the chemical notation of ozone is O 3 ): NB is defined as the O 3 that would exist in the absence of any manmade precursor emissions. USB is defined as that O 3 that would exist in the absence of any manmade emissions inside the U.S. This includes anthropogenic emissions outside the U.S. as well as naturally occurring ozone. A third term, North American background , is also defined in this report for added clarity. Each is explained in more detail below. Natural background ozone is what the average concentration of ground-level ozone would be without any human influence. Ozone forms naturally in the lower levels of the atmosphere due to natural emissions of precursors from sources including lightning, vegetation, wildfires, and methane. Transport of ozone vertically from the stratosphere to the atmospheric layer at ground level (called stratospheric intrusions) is a fifth major source. No air pollution monitors today are able measure true present day NB because human contributions to the formation of ozone are so widespread globally. The only way to estimate NB is with global scale atmospheric chemistry simulation models with inputs representing conditions without human influence. The remainder of this section summarizes the five major contributors to NB. A key point is that t he estimated contri butions presented are uncertain and are very dependent on both location and timing . The "Challenges in Estimating Background Ozone" section discusses specific challenges and uncertainties associated with the data and the modeling projects. 1. Lightning (NO x ) . Lightning flashes cause naturally occurring nitrogen and oxygen in the atmosphere to react and generate NO x molecules. Lightning and the resulting emissions occur primarily during the warmer months and are released in the free troposphere, which is above the well-mixed ground layer (see Figure 2 ). There is data to suggest that lightning contributes to daily, as well as seasonal average, ozone concentrations in high impact areas. 2. Vegetation (VOCs) . Trees release VOCs as a byproduct of photosynthesis. Biogenic VOCs from vegetation are the largest emissions source of VOCs in the United States, making up about 70% of the total inventory. Vegetation emissions are largest during the spring and summer, when plants are actively growing. For plants and trees that have leaves only seasonally, emissions decline as leaves drop and photosynthesis ends. 3. Wildfires (VOCs and NO x ) . Wildfires release both NOx and VOCs (as well as fine particles), but the amount and the reactivity of the polluting emissions depend on the type of fuel that is burning and how quickly and how hot the fire burns. Controlled/prescribed fires tend to burn cooler and release fewer pollutants. Current research suggests that active fires contribute to daily as well as seasonal average ozone concentrations. Research to improve emissions inventories from fire events is ongoing. 4. Stratospheric intrusions . Ozone occurs naturally in the stratosphere at very high concentrations and can occasionally be transported down to lower atmospheric levels during certain weather events (see text box "Ozone Transport 101" for more information about atmospheric layers and vertical transport of ozone). Stratospheric intrusions are more likely to affect ground-level concentrations at high elevation sites in the western United States, simply because these areas are closer to the stratosphere. These events are more common in winter and spring months because the large storms that cause them are more likely to occur in late winter and spring. 5. Methane . Methane has not been traditionally considered an ozone precursor because it does not react quickly to produce ozone. Nonetheless, it will over time react and contribute to background ozone. Methane's atmospheric lifetime is on the order of a decade, compared to a timescale of months to days for other VOCs, and so it is considered well-mixed globally by the time it contributes to the formation of ozone. It is accumulating in the atmosphere as well, raising background ozone concentrations. This ozone contribution is considered approximately spatially uniform and is increasing as methane concentrations increase. The major source contributors to global methane are natural production by bacteria in anaerobic (oxygen-free) conditions in natural wetlands or in agriculture, fossil fuel emissions leaking or venting either naturally or from energy development, and incomplete combustion of biomass or carbon. North American background (NAB) is the estimated concentration of ozone that excludes the effects of all human-caused emissions in North America. NAB includes all NB sources as well as human-caused sources of emissions from countries outside of North America. Air quality monitors located at sites on the western coast of the United States are not consistently reliable measures of NAB for two reasons: (1) Air circulation can bring continental air over the Pacific Ocean and then back into North America. (2) If meteorological conditions are favorable, ozone pollution formed from emissions in North America can have a long enough lifetime to travel all the way around the globe and re-enter North America from the west. Therefore, like NB, modeling is the best way to estimate NAB. Human-caused sources outside of North America are currently dominated by Asian emissions. Estimates of Asian contribution to background ozone in the United States are highly time- and location-specific. Asian precursor emissions, and resulting ozone, travel across the Pacific Ocean in the free troposphere (see Figure 2 ). However, because this pollution is traveling at higher elevations, it is more likely to impact cities and locations in the western United States located at higher elevations. These upper-level air flows from areas in Asia are also more likely to affect the United States in the late winter, spring, and early summer due to seasonal variability in hemisphere-scale circulation patterns. Asian emissions begin to taper off in late winter, and a July/August monsoon season in eastern China reduces formation of ozone in late summer. These features suggest that the maximum impact from Asian pollution would likely occur in late winter/early spring. U.S. background (USB) includes all contributions from NB and NAB plus all human-caused emissions from Mexico and Canada. In the publication of the most recent NAAQS for ozone, EPA generically defined background ozone as USB: The term "background" O 3 is often used to refer to O 3 that originates from natural sources of O 3 ( e.g., wildfires and stratospheric O 3 intrusions) and O 3 precursors, as well as from man-made international emissions of O 3 precursors. Using the term generically, however, can lead to confusion as to what sources of O 3 are being considered. Relevant to the O 3 implementation provisions of the CAA, we define background O 3 the same way the EPA defines USB: O 3 that would exist in the absence of any man-made emissions inside the U.S. Mexican and Canadian emissions primarily impact locations on the borders of those two countries, with maximum contribution estimates from one study of about 30 ppb to border cities. Model estimates of the contribution of USB to total ozone range from 25 ppb to 50 ppb, with the highest values in the inter-mountain West, based on an EPA review of model data. The contribution of background ozone to total local ozone concentrations varies from location to location, day to day, and even hour to hour. These variations are driven both by changes in pollutant emissions from sources and by changing weather patterns that influence the chemistry and physical transport of the pollutants. There is also uncertainty associated with measuring or estimating these driving forces behind background ozone contributions. As mentioned, estimates of source contributions to background ozone rely on computer models that simulate atmospheric chemistry and transport and the resulting pollution. These models rely on large datasets of emissions inventories and meteorological data, both with detailed hourly and location-specific data. These input data are often not available at the temporal and spatial resolution needed, and so estimates and/or simplifications must be made, which increases uncertainty. Atmospheric measurements of ozone concentrations are then compared to model output to evaluate how well the model is performing. However, measurement data of the pollutants being modeled are also limited, which increases the challenge associated with evaluating the performance of the model in capturing ozone formation and movement. Modeling studies estimating background ozone and source attribution often present results as seasonal averages or represent time periods that may not be of specific use to regulators. See text box "Pollution Exposure Averaging Metrics" for additional information about averaging metrics. Retrospective studies face all the challenges mentioned. Forecasting studies face the additional challenge of attempting to model the future based on historical patterns and current conditions, adding another level of uncertainty. According to EPA, NO x emissions from electricity generating units have decreased 81% nationally compared to 1990 levels due in part to the acid rain program and ozone transport rules. While these reductions have resulted in total ozone decreases across most of the United States, models suggest that temperature increases in many areas of the United States during that same time frame have negated what would have been additional ozone benefits. Decreasing trends in total ozone concentrations measured at regulatory monitors between 1990 and 2010 generally occur in the eastern United States. Many western monitors do not show similar trends in total ozone over the same time period despite similar reductions in NO x from the power sector and individual personal vehicles. The lack of a decreasing ozone trend at many monitors in the West could be due to a number of causes: increasing seasonal average temperatures, an increase in incidents of fire since 1986, emissions from oil and gas development, increasing contributions from international transport of air pollution, and increasing global methane concentrations. According to the 2017 U.S. National Climate Assessment, on average since 1986 in the United States, the number of wildfires and the burn duration have both quadrupled. The number of acres burned has increased six-fold, compared to 1970 to 1986. The report also indicates that total NO x emissions from fire events are expected to increase with fire temperature, duration, and area burned. Figure 3 shows the annual count, and total area burned of all wildfires larger than 1,000 acres from 1984 through 2015. While emission inventories from Asia are highly uncertain—and outdated in some modeling cases—several sources of data suggest that Asian emissions, and potentially their impact on U.S. air quality, have peaked. Projections of fossil fuel combustion in Asian countries suggest uncertainty about peaking, although the Chinese government recently announced a target of a 15% reduction in NO x emissions by 2020 compared to 2015 emissions. A review of measurements of baseline ozone taken at monitors along the western coast of North America show that, after two decades of increasing trends, ozone flowing into the continental United States from the west stopped increasing in the mid-2000s and has begun to decrease. However, since about 2000, ozone levels measured at a rural site in Alaska have been increasing, with the source suspected in part to be transport of East Asian air. Demonstrations of high ozone directly related to wildfires and stratospheric intrusions may be eligible for exclusion from an ozone attainment calculation under the Exceptional Event Rule of the CAA. In order to facilitate successful demonstrations of exceptional events, EPA released a final guidance document in 2016 for preparations of exceptional events demonstrations for wildfires. A draft guidance document was released in August 2018 covering the preparation of exceptional events demonstrations for stratospheric ozone intrusions. These documents outline expectations, but they do not provide the specific modeling platforms or tools required to conduct the successful demonstrations. State, local, and tribal co-regulators recommended to EPA that the agency develop a similar guidance document for international emissions. Under Section 179B of the CAA, a demonstration of contribution by international sources may reduce attainment demonstration requirements in SIPs but does not provide regulatory relief from a potential nonattainment designation. Several issues may arise for congressional deliberations with regard to background ozone pollution. Stakeholders suggest challenges with meeting the NAAQS, in part due to the difficulty of reducing ozone in areas with potentially large contributions from background sources and in part due to lack of data availability to conduct demonstrations of those background contributions. Congress may seek to understand the progress of research on background ozone as EPA revisits the NAAQS for ozone. EPA released a state of the science background ozone white paper to stakeholders and requested feedback on major issues. The House Committee on Science, Space and Technology revisited the issue when its Subcommittee on Environment held a hearing on background ozone on June 21, 2018. The following four points summarize the opinions and policy and scientific challenges brought forth both by stakeholder responses to the EPA white paper and by the testimony from invited stakeholders at the recent background ozone hearing: 1. Some states, especially states in the western United States, have asserted that natural and non-U.S. sources of ozone and precursors have increased the ozone concentrations in their states. 2. Current statutory and regulatory options to address natural and non-U.S. sources often require technical data, modeling, and analyses that may be cost prohibitive. 3. Modeling results that attempt to quantify levels of contribution from sources are uncertain, and they represent historical, or average, impacts. 4. In most locations, especially urban locations, many studies (including the whitepaper published by EPA ) have shown that local sources contribute a large part to total local ozone. Most recently, the U.S. Court of Appeals for the D.C. Circuit considered whether EPA should take background ozone into account when setting NAAQS. The case has not been decided at the time this report was published. A potential avenue for Congress to address gaps in the scientific understanding of background ozone is through research and development. Therefore, Congress may consider funding implications of the following recommendations, made by stakeholders in the scientific and regulatory communities, that are intended to improve the understanding of contributions from background ozone: International engagement and/or cooperation at the federal level and through research collaborations that may improve understanding of non-U.S. contributions to U.S. air quality and may increase cooperation for pollution reduction goals. Increased monitoring at ground level and at higher levels in the atmosphere, through state or federal regulatory air quality monitoring projects or research campaigns, to aid in analysis of background ozone trends and improve confidence in the performance of the models used to estimate background contributions. Additional research and development into model estimates of background ozone, representing additional weather patterns (i.e., El Nino/La Nina patterns can influence background ozone), and model simplification schemes to provide more information about the variability associated with background ozone contributions. Finally, Congress may consider the role that methane plays in air quality. Methane is a precursor to ozone, and so methane emission reductions have been suggested as one option to reduce global background concentrations of ozone. Additionally, ozone itself is a strong greenhouse gas. EPA's review of the ozone NAAQS is underway and set to be completed in 2020, with background ozone contributions suggested as a topic to be addressed. Congress may conduct oversight as EPA carries out this effort. 

Summary: Exposure to ozone, a common air pollutant, has been linked to early death, plant and crop damage, and damage to property. The U.S. Environmental Protection Agency (EPA) sets National Ambient Air Quality Standards (NAAQS) for ground-level ozone to protect human health and welfare with, by law, a "margin of safety." States that contain areas with ozone concentrations above these standards must develop plans to reduce emissions and improve air quality. However, states have direct control only over emission sources located within their borders. The Clean Air Act (CAA) requires EPA to re-evaluate the NAAQS every five years to include the latest science and technological advancements. Studies reporting the human health impacts of ozone increasingly suggest that ozone exposure may not be completely safe at any level. With the potential for a NAAQS re-evaluation leading to science-based recommendations for a tighter standard, some stakeholders have expressed increasing concern that future—and even current—ozone standards could be difficult to meet due to the contribution of "background ozone," which arises from a variety of sources described in this report. In some areas of the United States, background ozone may be approaching 70 parts per billion (ppb) on some days, the current level of the NAAQS. Some Members of Congress have expressed interest in adverse health effects that occur at or below the current standard, challenges some nonattainment areas may have in meeting current standards, and particularly the responsibilities for meeting the health standard, given interstate and international transport. EPA's review of the ozone NAAQS is underway and set to be completed in 2020, with background ozone contributions suggested as a topic to be addressed. Congress may have an interest in better understanding scientific capabilities, needs, and efforts to improve understanding of contributions from background sources, as well as options for regulatory responses. Defining Background Ozone Three terms are used for different types of background ozone, and distinguishing among them can be important for regulatory purposes. 1. Natural background. Ozone concentrations that would be present without any human influence or contribution from anywhere on the globe. Natural background includes contributions from wildfires, vegetation, lightning, ozone in the stratosphere, and global methane concentrations. Contributions to background ozone from wildfires and methane have been increasing over the past several decades. 2. North American background. Ozone concentrations absent human-caused emissions from North America. North American background includes all sources in natural background plus ozone from international sources outside North America. Studies suggest that Asian emissions may be contributing to ozone in the United States, especially in Western states, but that those contributions may be beginning to decrease. 3. United States background. Ozone concentrations absent human-caused emissions from the United States. U.S. background includes all sources in North American background plus ozone formed from emission sources in Mexico and Canada. Challenges in Estimating Background Ozone The CAA provides alternative regulatory options for areas that successfully demonstrate significant influence from some specific sources of natural background ozone on ozone exceedences. However, such demonstrations may be difficult to conduct and reliably assess, given data and analytical challenges: Emissions inventories. Current understanding of the amount, location, and type of pollutant emissions from many types of sources is insufficient. Therefore inventories typically provide estimations, which may not be precise enough for apportioning contributions. Weather data. Meteorological data (i.e., wind speed, wind direction, temperature, cloud cover, humidity, etc.) are not currently measured at a fine enough spatial scale to adequately represent relevant weather processes. Ambient air quality measurements. Data on pollutant concentrations are limited, which increases the challenge of understanding ozone formation and movement. Fine spatial and temporal measurements are needed both horizontally across the surface and vertically to higher levels of the atmosphere. Source contribution variability. Background ozone source contributions change by year, season, day, and hour and from location to location. This makes it difficult to project future contributions, including when contributions will be relevant to attainment status. This report provides information on sources of background ozone, presents key challenges in addressing these sources, and discusses potential options to overcome these challenges.