Options for Reducing Lead Emissions from Piston-Engine Aircraft (2021)

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1 Introduction In the 1970s, the aviation industry converged on a standard for the aviation gasoline (avgas) used in piston-engine aircraft, commonly called 100LL, which remains unchanged to this day. The “100” refers to the octane level of avgas, which is even higher than the octane level of high-tier automotive gasoline. The “LL” stands for “low lead,” reflecting the fact that avgas’ higher octane is created by the addition of tetraethyl lead (TEL).1 The addition of lead to boost octane enables the reliable operation of high- compression piston engines at the wide range of altitudes and climates in which small aircraft operate. An important function of the lead additive is to prevent early detonation of fuel in the cylinder. Detonation in a gasoline engine is often called “knock” because of its characteristic sound. Because knock can lead to the failure in flight of critical engine components, it must be avoided. Since 100LL became the universal grade of avgas, the harmful health consequences of lead pollution have become better understood. A highly toxic substance, lead is known to have profound adverse effects on the de- velopment of infants and children, and it can remain in the human body for decades, causing lasting harm. Furthermore, it is a persistent pollutant. As a mineral naturally found underground, once lead is extracted and released by human activity, it stays in the environment and its levels accumulate with additional emissions. 1  100LL is specified to have a maximum of 0.56 grams of lead (0.875 grams of TEL) per liter and a minimum of 0.28 grams (0.437 grams of TEL) per liter. 15 

16 OPTIONS FOR REDUCING LEAD EMISSIONS FROM PISTON-ENGINE AIRCRAFT Compared to other historic sources—heavy industry, early military aviation, and automobiles before their transition to unleaded gasoline 40 years ago—piston-engine aircraft have not been the largest contributor of the lead that has persisted in the environment. However, these other sources have been eliminated or greatly reduced, making avgas one of the few major sources of a pollutant whose environmental concentrations are not natu- rally dissipating over time. Thus, continued emissions from aircraft engines can add to lead concentrations that may already be presenting concerns in some locations, particularly at and near the roughly 13,100 airports where most piston-engine aircraft operate. The lead in avgas can also present an occupational health hazard to those who refuel and maintain piston-engine aircraft. Thus, “low-lead” is a misnomer in the sense that any amount of lead in fuel can be too high from a human health standpoint, prompting interest in reducing aviation’s reliance on leaded gasoline. The Clean Air Act (CAA) requires the U.S. Environmental Protection Agency (EPA) to set National Ambient Air Quality Standards (NAAQS) for principal air pollutants (known as criteria pollutants), which are wide- spread ambient air pollutants that are reasonably expected to present a danger to public health or welfare (see 42 U.S.C. 7408–7409).2 Lead is one of the criteria pollutants subject to regulation.3 In cases where a regulated pollutant may reasonably be anticipated to endanger public health or wel- fare, EPA can propose standards that apply to aircraft engine emissions; however, it must consult with and obtain approval from the Federal Avia- tion Administration (FAA) to issue any proposed standard that may affect aviation safety. Moreover, EPA does not have regulatory authority over which fuels may be used by aircraft. The fuels used in specific engine and aircraft types are defined by the engine and aircraft manufacturers and by ASTM International specifications controlling the composition and physi- cal properties of purchased fuel. FAA is responsible for certifying engine and aircraft types based on the manufacturer’s testing of the engine and aircraft when using a defined ASTM International fuel specification. Thus, even FAA does not directly approve the fuels used, but rather certifies that a given type of engine or aircraft is permitted to operate on a fuel defined by 2  Within the context of the CAA, welfare effects include effects on soils, water, agriculture, forests, wildlife, fabricated materials, atmospheric visibility, and climate. 3  In 1976, EPA listed lead under CAA section 108, making it what is called a “criteria pollutant.” As part of the listing decision, the agency determined that lead was an air pol- lutant, judged to have an adverse effect on public health or welfare. In 1978, EPA [under section 109(b)] issued primary and secondary lead NAAQS to protect public health and welfare. The lead NAAQS level is now 0.15 μg/m3, averaged over a 3-month averaging period. 

INTRODUCTION 17 the manufacturer based on its testing. Aircraft owners are not permitted to use fuels that are not specified in an aircraft type certificate (TC) approved by FAA. More details on relevant FAA and EPA statutory and regulatory authorities and their interconnections are provided in Box 1-1. BOX 1-1 EPA and FAA Authorities Pertaining to Aircraft Emissions and Aviation Fuel Properties Endangerment Finding Section 231 (a)(2)(A) of the Clean Air Act (CAA) requires the U.S. Environmental Protection Agency (EPA) Administrator to “issue proposed emission standards applicable to the emission of any air pollutant from any class or classes of air- craft engines which, in [the Administrator’s] judgment, causes or contributes to air pollution which may reasonably be anticipated to endanger public health or welfare.” According to a 2010 advance notice of proposed rulemaking from EPA, the term endangerment finding is often used as a short-hand reference to such a judgment. It is notable that in instructing the Administrator to consider whether emissions of an air pollutant cause or contribute to air pollution, the law does not require the Administrator to find that emissions from any one sector or group of sources are the sole or even a major part of an air pollution problem. Moreover, the requirement does not contain a modifier such as “significant” or “major” to the term “contribute” and thus does not appear to set the magnitude of the contribu- tion as a criterion for an endangerment finding. Thus, EPA has broad authority in exercising its judgment regarding whether emissions from certain sources cause or contribute to air pollution, which may reasonably be anticipated to endanger public health or welfare.a Emission Standards Section 231(a)(2)(A) of the CAA grants EPA authority to propose standards ap- plicable to the emission of any air pollutant from any class or classes of aircraft engines judged to cause or contribute to air pollution, which may reasonably be anticipated to endanger public health or welfare. EPA is given discretion to issue the standard over a period of time that permits development and application of the requisite technology, considering the cost of compliance within that period. In doing so, however, the agency must consult with the Secretary of Transportation. In addition, Section 231(c) states that EPA’s regulations regarding aircraft “shall not apply if disapproved by the President, after notice and opportunity for public hearing, on the basis of a finding by the Secretary of Transportation that any such regulation would create a hazard to aircraft safety.” If a proposed emission standard is finalized by EPA, Section 232(a) of the CAA directs the Secretary of Transportation to issue and implement regulations to ensure compliance with the emissions standards, including aviation fuel standards. continued 

18 OPTIONS FOR REDUCING LEAD EMISSIONS FROM PISTON-ENGINE AIRCRAFT BOX 1-1 Continued Fuel Standards CAA section 216 defines “motor vehicle,” “nonroad engine,” and “nonroad ve- hicle.” Section 211(c)(1) allows EPA to regulate any fuel or fuel additive used in motor vehicles and nonroad vehicles or engines where emission products of the fuel of fuel additive either: (1) cause or contribute to air pollution or water pollution that reasonably may be anticipated to endanger public health or wel- fare, or (2) will impair to a significant degree the performance of any emission control device or system in general use, or that the Administrator finds has been developed to a point where in a reasonable time it will be in general use were such a regulation to be promulgated. This section of the CAA was used as basis for eliminating lead from fuel used in motor vehicles. However, in the CAA, aircraft are not defined as nonroad vehicles and aircraft engines are not defined as nonroad engines. Accordingly, EPA’s authority to regulate fuels under section 211 does not extend to fuels used exclusively in aircraft, such as leaded avgas (EPA, 2010). Fuels used in aircraft engines are regulated by the Federal Aviation Admin- istration (FAA) under section 232 of the CAA and 49 U.S.C. § 44714 (Aviation Fuel Standards). Under section 232, the Secretary of Transportation is to consult with the administrator of EPA regarding implementation of EPA standards and is to modify aircraft type certificates as appropriate and necessary. In linking back to the CAA provisions governing emissions standards, 49 U.S.C. § 44714 requires FAA to prescribe standards for the composition or chemical or physical properties of an aircraft fuel or fuel additive to control or eliminate aircraft emis- sions that EPA decides under Section 231 of the CAA endanger the public health and welfare and to issue regulations providing for carrying out and enforcing those standards. An addition to 49 U.S.C. § 44714 (from section 565 of the FAA Reau- thorization Act of 2018) gives FAA authority to allow the use of an unleaded aviation gasoline in aircraft as a replacement for leaded aviation gasoline if the agency: (1) qualifies the unleaded gasoline as a replacement for ap- proved leaded gasoline, (2) identifies the aircraft and aircraft engines eligible to use the unleaded gasoline, and (3) adopts a process, other than the tradi- tional means of certification, to allow eligible aircraft and aircraft engines to operate using the qualified replacement unleaded gasoline in a manner that ensures safety. (The law creating this addition states that existing regulatory mechanisms by which an unleaded aviation gasoline can be approved for use in an engine or aircraft will also remain in effect.) See Appendix C for additional details. 

INTRODUCTION 19 The U.S. active piston-engine fleet totals some 170,000 airplanes and helicopters. Unfortunately, no unleaded replacement fuel exists for aircraft that require high octane levels to operate safely, which comprise the roughly one-quarter of the fleet with the highest-performance engines that are used the most intensely and thus are estimated to consume more than half of all avgas. The remaining aircraft that are candidates for using lower octane grades of fuel are those with lower performance and that operate at lower altitudes, many of which were originally designed to allow for the use of avgas with lower octane before 100LL became the industry standard about 50 years ago. Thus, one possible approach for achieving early reductions in leaded avgas consumption is to transition the fleet to use two gasoline grades—a lower octane unleaded grade for those aircraft that can safely perform with it, and 100LL for those that require higher octane fuel to resist knock and ensure safe performance. This approach would lower overall lead emissions by the piston-engine fleet, depending on how many aircraft could operate with the lower octane fuel and how often those aircraft are used. However, the transition would potentially require the testing and recertification of a large number of aircraft and engines, some of which were designed and built decades ago, and whose manufacturers may no longer exist. Aircraft owners interested in switching to unleaded fuels may find this recertifica- tion option prohibitively expensive, except in cases where a supplemental type certificate (STC) is already available at moderate cost. Moreover, fuel consumption by aircraft in the piston-engine fleet varies widely because air- craft serve a range of general aviation (GA) purposes, such as pilot training, transport for small and remote communities, emergency medical transport, aerial surveying, and crop dusting. The higher-performance aircraft, which are used disproportionately for such non-recreational purposes, consume high quantities of fuel, but cannot be satisfied by existing lower octane unleaded avgas. The primary challenge associated with the two-fuel option is that it would require investments in a second supply chain for an unleaded fuel, including refinery and distribution capacity. Inasmuch as the supply of avgas is already a highly specialized element of the total gasoline market, accounting for roughly 0.1 percent of the total volume of gasoline refined each year in the United States, splitting its market in two could be prob- lematic from an economic standpoint. In addition, many airports across the country, including thousands of very small facilities that are privately owned or operated by municipalities, counties, and other entities, would need to establish a second fuel storage and dispensing system to accommo- date a second fuel at significant and potentially prohibitive expense. Because unleaded automotive gasoline is widely available and relatively inexpensive, it is sometimes proposed as an option for reducing lead use by 

20 OPTIONS FOR REDUCING LEAD EMISSIONS FROM PISTON-ENGINE AIRCRAFT the portion of the piston-engine fleet that can use lower octane fuel. Indeed, thousands of piston-engine aircraft were granted STCs during the 1980s that allowed them to use motor gasoline (MOGAS), which presumably referred to the unleaded automotive gasoline being produced and dispensed at automobile filling stations at that time. Many of these aircraft remain in the fleet; however, current formulations and properties of unleaded auto- motive gasoline do not resemble those earlier supplies. Gasoline containing ethanol cannot be used in aircraft because of its corrosive effects. Moreover, automotive gasoline delivered from refineries does not achieve designated octane levels until after ethanol, which boosts octane, is added at gasoline storage and distribution terminals prior to delivery to filling stations. Ab- sent the addition of ethanol, the octane levels of the automotive gasoline exiting refineries would be too low for use in most aircraft, including many lower-performance aircraft. In recognition of the many challenges associated with having multiple grades of avgas, FAA has been working with fuel suppliers and aircraft manufacturers and operators to develop a higher-octane unleaded drop-in fuel that can safely be used by all piston-engine aircraft currently using 100LL without requiring any modifications to engines or operations. Most recently, the Piston Aviation Fuels Initiative (PAFI), a collaborative formed by FAA and the GA industry in 2013, has established testing standards for new fuels, as well as a qualification test program to confirm that compliant fuels work in a broad range of existing aircraft gasoline engines. Further- more, PAFI has established mechanisms for public–private cooperation to help overcome the logistical, economic, and policy challenges to transition- ing to a drop-in fuel. The aim of the collaborative, which is ongoing, is to provide a solution that would allow the current piston-engine fleet and fuel supply chain to transition to unleaded fuels without prohibitive costs. PAFI’s efforts build on prior FAA and industry work to identify fuel ad- ditives to replace TEL (see, e.g., CRC, 2010). While this earlier work was unsuccessful in finding a replacement additive, it shed light on the many important factors that must be considered for a drop-in fuel. Low toxicity and prevention of knock and engine shutdowns are essential requirements, but so too are compatibility with a wide range of engine and fuel system materials and high performance with respect to many other capabilities such as freeze resistance, hot and cold starting, and transport and storage stability. Of course, a longer-term strategy to reduce aviation lead could include the development and introduction of small, GA-type aircraft that do not need gasoline. Diesel and turbine engines, whose use has traditionally been limited to larger, more complex aircraft, have been demonstrated in smaller 

INTRODUCTION 21 airplanes. Furthermore, battery electric aircraft are now being developed, as are hybrid-electric aircraft in which a small onboard generator supplements the electricity stored in a battery to power an electric motor. However, these latter propulsion systems are, at this time, generally limited to single flight demonstrators operated as experimental rather than fully certified aircraft. While technology developments under way in electric ground vehicles may have application to small aircraft (e.g., the improved storage capacity and decreased cost of batteries), the aviation sector has unique demands for very low-weight technologies and very high reliability and safety assurance. Moreover, the very slow annual turnover of the piston-engine fleet means that it could take decades for the introduction of new, lead-free technologies to have an appreciable effect on aviation lead emissions. The challenge in reducing aviation lead emissions is therefore complex and multi-faceted. Meeting the challenge may require approaches that go beyond the development and introduction of new fuels and aviation tech- nologies to include a nearer-term focus on the way piston-engine aircraft are used and operated at airports where lead emissions can be more con- centrated and where pilots, aircraft technicians, and aircraft and airport maintenance personnel may have greater exposure to lead. Therefore, in addition to sponsoring research to evaluate and find possible drop-in fuels, FAA has sponsored a number of studies, including several by the Airport Cooperative Research Program (ACRP) (see, e.g., NASEM, 2015, 2016), to better understand how the lead emitted from the burning and vaporization of avgas disperses and concentrates at airports and contributes to human exposure. The studies have also examined potential mitigation measures at airports, including reconfiguring and relocating where pilots perform their engine run-ups during pre-takeoff checks, and changes in practices to ensure that avgas liquid and vapor are contained during refueling and after pilots inspect the quality of sampled fuel prior to flight (TRB, 2014). Although not sponsored by FAA, a small number of studies have also been undertaken to gain a better understanding of how aircraft technicians and airport maintenance personnel can be exposed to the lead from avgas, and how such exposures may be mitigated (see Chapter 4). EPA started studying lead emissions and concentrations at airports in 2010, when it began an assessment as part of a rulemaking activity to determine whether lead emissions endanger public health or welfare (EPA, 2010). The assessments have included air quality monitoring at and near airports and demographic analyses of the population residing near airports. In addition, because it would be impracticable in terms of time and resources to obtain monitored data for every airport, EPA used computational modeling to estimate airborne lead concentrations at other 

22 OPTIONS FOR REDUCING LEAD EMISSIONS FROM PISTON-ENGINE AIRCRAFT unmonitored airports. Following the release of its latest modeling and monitoring data in February 2020, EPA concluded that the results indicate that lead concentrations at and near airports are typically well below the lead NAAQS (EPA, 2020). Nevertheless, the monitoring did find some air- ports where lead concentrations exceeded the NAAQS in locations in close proximity to where pre-takeoff engine checks take place. Moreover, the agency has continued to express concern about aggregate exposures from all sources of lead, including low concentrations in air from piston-engine aircraft operations, and has therefore pointed to the importance of working to reduce lead emissions from aviation.4 At the time this committee’s report was authored, EPA had not pro- posed a formal determination, positive or negative, of whether lead emis- sions from the use of leaded avgas cause or contribute to air pollution, which may reasonably be anticipated to endanger public health or welfare. Further updates on the status of EPA’s deliberations could provide data and analyses that inform mitigation strategies and point to where more research and assessments are needed. While a formal EPA determination is not a prerequisite for introducing measures to mitigate aviation lead, it would add more clarity about the array of regulatory and non-regulatory means available for this purpose. STUDY REQUEST AND CHARGE Section 177 of the FAA Reauthorization Act of 2018 (P.L. 115-254) calls on the Secretary of Transportation, acting through FAA, to make appropriate arrangements with the National Academies of Sciences, Engineering, and Medicine (the National Academies) to convene an expert study committee to examine: (a) existing non-leaded fuel alternatives to the aviation gasoline used by piston-powered general aviation aircraft; (b) ambient lead concentrations at and around airports where piston-powered general aviation aircraft are used; and (c) mitigation measures to reduce ambient lead concentrations, including increasing the size of run-up areas, relocating run-up areas, im- posing restrictions on aircraft using aviation gasoline, and increasing the use of motor gasoline in piston-powered general aviation aircraft. The study committee’s Statement of Task (see Box 1-2) reflects the leg- islative request and emphasizes the importance of being as quantitative as possible, particularly when considering how candidate mitigation measures 4  See https://nepis.epa.gov/Exe/ZyPDF.cgi?Dockey=P100YG46.pdf. 

INTRODUCTION 23 could potentially improve air quality near airports in relation to EPA’s lead NAAQS. Those mitigations could involve actions targeted at reducing lead emissions or reducing elevated concentrations of airborne lead in specific locations (hot spots). While not obligated to make recommendations on the adoption of one or more of the mitigations identified in the legislative request, the study committee is nevertheless given the latitude to recom- mend near- and longer-term lead reduction mitigations that warrant further consideration, including recommendations on priority research needs for reducing future piston-engine aircraft lead emissions. STUDY APPROACH To fulfill its charge, the study committee reviewed the literature on the health impacts of lead in the environment and the many research reports on the contribution of piston-engine aircraft to lead concentrations, includ- ing the ACRP and EPA reports noted above. Federal ambient air and water quality standards, as well as standards pertaining to workplace health and BOX 1-2 Statement of Task The study of lead emissions from the consumption of aviation gasoline by piston- powered general aviation aircraft shall include an assessment of: • Existing non-leaded fuel alternatives to the aviation gasoline used by piston-powered general aviation aircraft; • Ambient lead concentrations at and around airports where piston-pow- ered general aviation aircraft are used; and • Mitigation measures to reduce ambient lead concentrations, including increasing the size of run-up areas, relocating run-up areas, imposing restrictions on aircraft using aviation gasoline, and increasing the use of motor gasoline in piston-powered general aviation aircraft. As part of assessing mitigation measures, the committee will consider potential improvements in air quality near specific airports in relation to the maximum allowable lead concentration established for the National Ambient Air Quality Standards. The evaluation methods should be quantitative to the extent possible. The committee is not asked to recommend the adoption of one or more mitigation measures. As appropriate and based on available scientific and technical information, the committee will recommend near- and longer-term mitigation measures that warrant further consideration by federal agencies. In addition, it will identify priority research needs for reducing future lead emissions from piston-engine aircraft. 

24 OPTIONS FOR REDUCING LEAD EMISSIONS FROM PISTON-ENGINE AIRCRAFT safety, were reviewed to obtain a better understanding of how lead emis- sions and exposures are regulated. In considering the history of the use of leaded fuels for piston-engine aircraft, the committee examined reports on past technical research to find replacements for leaded avgas, some dating back decades, along with documents and articles on websites from the avia- tion industry, fuel suppliers, and the GA community. To find any references to best practices for controlling aviation lead emissions and exposures, the committee reviewed the many manuals, handbooks, and other procedural and instructional documents that are commonly used by GA pilots, airport operators, and aircraft technicians, including relevant FAA publications, circulars, and bulletins. During two meetings open to the public, the committee invited brief- ings from officials and representatives from FAA and EPA. They discussed the obligations and regulatory authorities of their agencies related to lead emissions from aircraft in the context of the CAA, including subjects such as EPA and FAA cooperation. EPA reported on the agency’s evaluation of the air quality impact of lead emissions from aircraft using leaded avgas and the status of rulemaking and endangerment assessments under the CAA. These briefings, and follow-on correspondence, provided the committee with both background and highly detailed information on the challenges associated with reducing and potentially eliminating lead from avgas and with controlling lead concentrations and exposures resulting from aircraft and airport operations. The committee also invited briefings from representatives of aircraft and engine manufacturers, airports, fixed base operators who dispense avia- tion fuel, small airplane operators, and suppliers and developers of aviation fuel. Along with FAA officials, they explained the technical demands of aviation fuel in providing sufficient octane and other properties essential for ensuring the safe operation of piston-engine aircraft. They provided infor- mation on the role of piston-engine aircraft in the national transportation infrastructure, the means by which engines and aircraft are certified and their fuels defined, and the operations and varied activities that take place at the thousands of small airports that serve most of the aircraft in the piston- engine fleet including their refueling. They also discussed the progress being made in the development of unleaded fuels and in aircraft gasoline engines and alternative propulsions systems. Numerous committee member ques- tions were fielded during these briefings, and they were often followed by more specific information requests handled through email correspondence. PAFI was the subject of several briefings by FAA and the program’s GA industry collaborators. Briefers’ explanations of the purpose, structure, accomplishments, and status of the collaborative were valuable to the com- mittee. Not only did they provide a fuller picture of the many technical 

INTRODUCTION 25 hurdles that must be overcome to develop a safe and effective drop-in fuel, but also insights into practical issues will need to be addressed if such a fuel is developed and promoted as a general replacement for leaded avgas. One can expect, for instance, that in addition to a candidate fuel’s techni- cal properties, questions about its eventual price, availability, proprietary control, and impact on fueling infrastructure would be concerns in a GA industry experiencing declining demand and activity levels. Indeed, because of PAFI’s emphasis on spurring private-sector fuel development, the propri- etary formulations of the fuels being evaluated under the program and their specific behaviors and performance when tested have remained confidential and are thus unknown to the study committee. The information on PAFI that is provided in this report, therefore, is essentially the same information contained on FAA’s public website.5 So informed, the study committee addressed specific aspects of the Statement of Task. It examined the lead emission rates from piston-engine aircraft, the chemical and physical states of the lead emitted by the aircraft, lead environmental transport and deposition, routes of lead exposure, and potential environmental and human health impacts related to lead emis- sions. These reviews included the consideration of completed studies on environmental lead concentrations from emissions at and around airports. The committee then identified a number of gaps in understanding of envi- ronmental dynamics, exposures, and potential health effects, and consid- ered how they might be filled by research, monitoring, education, and other means. The committee also considered how airport-related activities and operations, including refueling, pre-flight checks, and aircraft maintenance, could contribute to aviation lead emissions and exposures. Based on these reviews, it identified some potential ways to reduce their contributions. In addition, the committee considered the unleaded and lower-lead fuels approved for use by all or portions of the existing piston-engine fleet. The committee estimated the potential to reduce lead emissions by replacing 100LL with one or more of these fuels. It also examined the potential for MOGAS to play a meaningful role in reducing aviation lead. Finally, the committee focused on the promise of an unleaded drop-in fuel and lead-free propulsion technologies for application to aircraft in the existing and future fleets. As part of this focus, the committee reviewed the history, structure, and accomplishments of PAFI and considered the status—as much possible given information restrictions—of the fuels being tested under the collab- orative as well as outside of it by fuel suppliers. Having considered the challenges and opportunities for reducing avia- tion lead from multiple pathways (airport operations and practices, fuel availability and development, the characteristics and use of the existing 5  See https://www.faa.gov/about/initiatives/avgas. 

26 OPTIONS FOR REDUCING LEAD EMISSIONS FROM PISTON-ENGINE AIRCRAFT piston-engine fleet, and aircraft engines and lead-free propulsion systems), the committee came to view the goal of fully eliminating aviation lead as being complex, multi-dimensional, and having uncertain potential to be at- tained soon, or at all, by focusing on a single approach such as the advent of an unleaded drop-in fuel. As a result of this conclusion, the committee considered combinations of pathways that can be taken to curb lead emis- sions and exposures along with initiatives aimed at developing lead-free fuels and technologies. While in many cases the relative advantages and disadvantages of choosing specific policy measures (such as, relative benefits and costs of regulations, taxes, or subsidies) to facilitate progress along each pathway could not be fully assessed in this study, such assessments would be needed to decide on the most appropriate mitigations to pursue. Additionally, with respect to the Statement of Task’s expectation that possible mitigations would be assessed with regard to their impact on meet- ing the lead NAAQS, EPA’s finding that lead concentrations are typically well below the lead NAAQS at airports suggested that such mitigation- specific quantification would not be fruitful, and probably not possible. In requesting this study, Congress did not ask for lead mitigation options to be considered in relation to the NAAQS or with the CAA’s jurisdiction and mitigation tools directly in mind. The study committee nevertheless notes EPA’s continuing concern about aggregate exposures from all sources of lead, and recognizes that key agency decisions, such as a formal endanger- ment determination, positive or negative, could have an important bearing on the prioritization and implementation of public policies that align with the mitigation pathways considered in this study. REPORT ORGANIZATION The remainder of the report is organized into six chapters. The next chapter (Chapter 2) provides background on the U.S. piston-engine aircraft fleet, its use characteristics, and the airports where the aircraft operate from and are based. Chapter 3 addresses the Statement of Task’s call for an assessment of ambient lead concentrations at airports where piston-engine aircraft are used. The chapter also includes an examination of the potential health effects of lead exposure and various aspects of aviation lead emissions. Chapter 4 considers how airport-related activities and operations may be contributing to lead emissions and exposures. It also discusses mitigation measures that may apply to those activities and operations, such as changes to engine run-up areas. The Statement of Task’s request for an assessment of existing lead-free fuel alternatives, including MOGAS, is addressed in Chapter 5 as part of a review of existing unleaded and lower-lead fuels to replace 100LL fully or partially. Chapter 6 reviews the potential for an unleaded drop-in fuel, and 

INTRODUCTION 27 considers PAFI and its progress. The chapter also considers the prospects for converting some of the existing fleet to lead-free technologies and of future lead-free propulsion systems making in-roads into the GA sector. In Chapter 7, the report concludes with a summary assessment of the findings and recommendations from the previous chapters. REFERENCES CRC (Coordinating Research Council). 2010. Research Results Unleaded High-Octane Avia- tion Gasoline. Final Report CRC Project No. AV-7-07. http://crcsite.wpengine.com/wp- content/uploads/2019/05/AV-7-07-Final-Report-6-18-10.pdf. EPA (U.S. Environmental Protection Agency). 2010. 40 CFR Part 87. Advance notice of pro- posed rulemaking on lead emissions from piston-engine aircraft using leaded aviation gasoline: Proposed rule. Federal Register 75(81):22440–22468. https://www.govinfo.gov/ content/pkg/FR-2010-04-28/pdf/2010-9603.pdf. EPA. 2020. Technical Update: Reports on the Impact of Lead Emissions from Piston-Engine Aircraft on Air Quality Near U.S. Airports. EPA-420-F-20-008. https://nepis.epa.gov/ Exe/ZyPDF.cgi?Dockey=P100YG46.pdf. FAA (Federal Aviation Administration). 2020. Aviation Gasoline: About Aviation Gasoline. https://www.faa.gov/about/initiatives/avgas. NASEM (National Academies of Sciences, Engineering, and Medicine). 2014. Best Practices for General Aviation Aircraft Fuel-Tank Sampling. Washington, DC: The National Acad- emies Press. https://doi.org/10.17226/22343. NASEM. 2015. Best Practices Guidebook for Preparing Lead Emission Inventories from Piston-Powered Aircraft with the Emission Inventory Analysis Tool. Washington, DC: The National Academies Press. https://doi.org/10.17226/22143. NASEM. 2016. Guidebook for Assessing Airport Lead Impacts. Washington, DC: The Na- tional Academies Press. https://doi.org/10.17226/23625.