Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
PREPUBLICATION COPY â Uncorrected Proofs 1 Summary The U.S. general aviation (GA) sector, with a fleet that consists mostly of piston-engine aircraft, serves many important functions. Aerial observation, medical airlift, pilot training, and business transport are examples of important GA functions that are applied across the country, while some functions, such as crop dusting, aerial firefighting, search and rescue, and air taxi service, have particular significance to communities in rural and remote locations. Piston-engine aircraft are critical to performing all of these functions, and they are also the predominant aircraft used for personal and recreational flying, typically in the smallest, most basic airplanes. The vast majority of piston-engine airplanes and helicopters are powered by aviation gasoline (avgas). Nearly all of the countryâs approximately 170,000 active piston-engine aircraft burn a grade of avgas, designated â100LL,â that contains lead. The number â100â refers to 100LLs octane rating and âLLâ stands for âlow lead.â Lead is added in the form of tetraethyl lead to 100LL to achieve the octane rating needed for the safe operation of those high- performance aircraft with high-compression engines, which account for about one-third of the fleet and an even larger percentage of fleet fuel consumption. Because 100LL can be used by all kinds of piston-engine aircraft, this single grade is the only type of fuel consistently available to GA aircraft operators. Consequently, 100LL is also the only fuel that most existing piston-engine aircraft are certified to use by the Federal Aviation Administration (FAA). A highly toxic substance, CDC concluded that there is no known safe level of lead in blood. Because of the susceptibility of the developing nervous systems, exposure to low concentrations of lead, including prenatal exposure, has been linked to decreased cognitive performance in children. Since the use of lead additives in automotive gasoline was banned in 1996, avgas has become the countryâs primary source of lead emissions. For more than 25 years, FAA, the GA industry, and fuel developers have been searching unsuccessfully for an unleaded âdrop-inâ replacement fuel for 100LL that can satisfy the performance requirements of the entire piston- engine fleet, including those high-performance aircraft that require avgas with an octane rating of 100 or higher.1 During that time, the American Society for Testing and Materials (ASTM) adopted specifications for a second grade of leaded avgas, 100VLL (âvery low leadâ), which has the same octane rating as 100LL but nearly 20 percent less lead. In addition, ASTM issued specifications for unleaded (UL) avgas with lower octane ratings, which can be used by lower- performance aircraft. While only one fuel, UL94, that meets the ASTM specifications for unleaded avgas is currently produced, and is available at a select number of airports, 100VLL is not being produced. Section 177 of the FAA Reauthorization Act of 2018, called on FAA to commission this study by a National Academies committee. The study was to consider (a) ambient lead concentrations at and around airports where piston-engine aircraft are used, (b) existing non- leaded fuel alternatives to avgas used by piston-engine general aviation aircraft; and (c) mitigation measures to reduce ambient lead concentrations, including increasing the size of run- up areas, relocating run-up areas, imposing restrictions on aircraft using avgas, and increasing the use of motor gasoline. 1 A full replacement for leaded avgas is sometimes referred to as a âdrop-inâ fuel because it would not require any changes to the existing piston-engine fleet, any FAA certification approvals for use in existing aircraft and engines, any modifications to future engines and aircraft, or any new investments in fuel storage and dispensing capacity.
PREPUBLICATION COPY â Uncorrected Proofs 2 Those mitigations could involve actions targeted at reducing lead emissions or reducing elevated concentrations of airborne lead in specific locations (hot spots). Findings with respect to each of these items in the study request are summarized next. The committee came to realize that currently there is no individual, certain solution to the aviation lead problem, and therefore a multi-pathway mitigation approach offers the greatest potential for tangible and sustained progress. The pathways that comprise this approach are described and recommendations are made for facilitating their pursuit. AMBIENT LEAD CONCENTRATIONS AT AND NEAR AIRPORTS Environmental health studies commonly rely on blood lead levels as a metric of exposure to the pollutant. Because lead does not appear to exhibit a minimum concentration in blood below which there are no health effects, there is a compelling reason to reduce or eliminate aviation lead emissions and sources of exposure. Airborne lead, which is usually in the form of particulate matter, can be inhaled by people in communities surrounding airports. In addition, particles containing lead can deposit onto soil and other surfaces and be ingested through activities, such as hand-to-mouth contact with surfaces where the particles have deposited. Deposited lead also can be resuspended into the air as dust and inhaled. Therefore, past emissions from piston-engine aircraft that deposited to soil and other surfaces can contribute to present-day lead exposures within and near airports. Lead exposures to workers at airports present another area of concern warranting mitigation. For example, the practices and protections of some airport personnel and aircraft technicians may need to be modified to reduce occupational and take-home exposures from lead deposits and residue in aircraft engines, oil, and spark plugs. Assessing the feasibility and effectiveness of the airport-specific application of potential mitigations would benefit from an improved understanding of individual airport characteristics. Airports differ in traffic activity, layouts, meteorological and topographical conditions, and proximity to the local population. They serve as bases for different types and numbers of aircraft that provide different functions within the community. Therefore, additional analyses are needed to take into account airport-specific conditions and attributes, including the geographic distribution of lead around the airport. Such analyses would inform the selection, design, and effectiveness assessment of lead mitigations at individual airports. EXISTING UNLEADED FUEL ALTERNATIVES The only specified and available unleaded avgas, UL94, has the potential to be used in about half to two-thirds of existing piston-engine aircraft, although these are generally the lower- performance aircraft that account for less fuel burn and flight hours. Aircraft would need to acquire FAA certification approvals to use UL94, which many newly produced aircraft do not have. However, many low-performance models are technically capable of using this fuel and continued innovations in engine design could soon enable many future high-performance aircraft to use it as well. The potentially significant obstacle to the greatly expanded use of UL94 (or other unleaded lower-octane fuels meeting ASTM standards) is that thousands of small airports would need to invest more than $100,000 in a second avgas storage and dispensing system to accompany existing systems for supplying leaded avgas to aircraft that require fuel with enhanced octane. Because the piston-engine fleet has very low annual turnover, the supply of an
PREPUBLICATION COPY â Uncorrected Proofs 3 avgas containing lead additives for higher performance aircraft will likely need to be supplied for many decades, unless a high-octane unleaded alternative can be developed and made widely available. OTHER POTENTIAL LEAD MITIGATION MEASURES Three specific potential lead mitigations are called out in the legislative request for this study: (a) imposing restrictions on aircraft using avgas, (b) increasing the use of motor gasoline in piston- engine aircraft, and (c) changing the locations within airports where pilots perform their engine run-ups during pre-takeoff checks. Imposing Restrictions on Aircraft Using Avgas The imposition of restrictions on aircraft using leaded avgas could take different forms, including limits on their use or on the fuels they burn, possibly by imposing costs (e.g., taxes or surcharges) on the use of leaded fuels, coupled with other measures, such as providing financial incentives for enabling private and public operators of aircraft and airports to change to lower- lead or unleaded fuels. Restricting the use of the high-performance piston-engine aircraft that require leaded avgas would have far-reaching ramifications for the many critical functions served by that portion of the fleet. Although it was not feasible to assess the ramifications on the industry, it is the judgment of the committee that restricting the use of this large and important component of the GA fleet would not be a viable mitigation. By comparison, restrictions on the fuel used by piston-engine aircraft, such as requirements for the use of UL94 by some or all low- performance aircraft that do not need to use high octane fuel, would require that substantial numbers of airports establish the requisite fuel storage and dispensing capacity to ensure the unleaded fuelâs widespread availability sufficient that aircraft operators could be confident that the fuel will be available along their routes of flight. For thousands of small airports, including many that are privately owned or operated by municipalities or other entities with limited revenues and financial capability, the cost of adding this capacity is likely to be prohibitive. Increasing the Use of Motor Gasoline The use of automotive, or motor, gasoline is not a viable unleaded alternative to avgas for appreciable lead reduction. Supplies dispensed at just about all automotive filling stations are blended with ethanol, which is corrosive to aircraft components. The use of automotive gasoline prior to adding ethanol also is not viable because the octane levels of pre-blended supplies are formulated with the expectation that octane-enhancing ethanol will be added before dispensing the fuel. Therefore, octane ratings of fuels exiting refineries that are intended to be used by automobiles are too low for most aircraft including many that do not have high-compression engines. Changing the Locations Within Airports Where Pilots Perform Engine Run-Ups It is standard safety practice for pilots to stop on a ramp or a taxiway just before takeoff to conduct an engine ârun-upâ (i.e., confirm that the engine can safely attain full power to produce takeoff thrust. They do this in part by briefly advancing the throttle to confirm the engine
PREPUBLICATION COPY â Uncorrected Proofs 4 operates properly at high power. Studies indicate that the exhaust from engine run-ups can create geographic areas with higher lead concentrations, such as when situated to combine with exhaust from aircraft taking off at full power. However, the magnitude, frequency, and dispersion of these concentrations and their proximity to people are airport- and context-specific, depending on factors such as the level of traffic activity, meteorological and topographical conditions, and the location and orientation of runways and areas where pilots perform their pre-takeoff checks in relation to buildings and people. Hence, to assess whether changing the location of run-up areas will achieve appreciable benefits in mitigating hot spots for ambient lead concentrations requires detailed information on specific conditions at individual airports, and particularly those that have moderate to high traffic activity, which number in the hundreds or more. A MULTI-PATHWAY APPROACH FOR MITIGATING AVIATION LEAD Achieving continuing, and potentially full, reductions of lead from aviation is a challenge for which there is currently no single known technical solution that is certain to be available in the near term. The advent of an unleaded drop-in fuel could greatly reduce or even eliminate aviation lead. However, the formidable technical challenges and associated uncertainties about whether and when such a fuel could be developed and deployed suggest that it should not be relied upon as the sole mitigation measure. Thus, a multi-pathway approach that pursues lead emission and exposure reductions is needed in which the development of a drop-in fuel proceeds as a part of broader mitigation pathway focused on the development and deployment of lead-free fuels and new propulsion technologies, in combination with mitigation pathways focused on airport operations and practices and on existing fuels and aircraft. Implementation will require the participation of many across a diverse industry involving private, corporate and public entities, including pilots; airport managers and personnel; fuel suppliers; and aircraft engine, propulsion, and airframe manufacturers. As illustrated in Table S-1, the pathways are complementary to one another and would be pursued simultaneously. However, they differ in their potential to yield near-term reductions in lead emissions and exposures, implementation complexities and requirements, and certainty of being effective. For some pathways, candidate policy options are easy to identify while, for others, the most suitable policies are difficult to define because they could involve combinations of financial assistance and incentives, regulatory requirements, support for technology research and development, and other potential interventions to motivate and enable the desired response. Some mitigations would confer ancillary benefits that would justify their pursuit, even where the lead reductions could be relatively modest, such as increasing awareness in the aviation industry about the degree of lead exposure that aviation causes to encourage widespread change. Pursuing them together would account collectively for reduced lead in aviation, and would increase the probability of significant technical breakthroughs sufficient to achieve the ultimate goal of no leaded avgas.
PREPUBLICATION COPY â Uncorrected Proofs 5 TABLE S-1 Candidate Pathways for Aviation Lead Mitigation Measures Considerations Airport Operations and Practices Existing Specified Fuels and Fleet New Lead-Free Technologies (FuelsâPropulsion Systems) Aircraft Operations at Airports Pilot and Airport Personnel Practices 100VLL Used by All Aircraft or with Some Using UL94 UL94 Used by Low- Performance Aircraft UL94 in New Aircraft Including High- Performance Aircraft 100+UL in All Aircraft New Propulsion Systems (new aircraft and retrofit some legacy aircraft) Potential Reduction in Lead Exposuresa Small and variable, depends on individual airport conditions, activity, and hot spotsb Small and variable, but could be particularly important for aircraft technicians Up to 20% reduction (could exceed 40% if combined with UL94 use by low- performance aircraft) Up to 30% reduction (could exceed 40% if combined with 100VLL use by all other aircraft) ~0.5% reduction per year 100% reduction ~0.5% reduction per year Time Frame for Lead Reduction Benefits If Started Soon Near-term Near-term Near- to mid- term Mid-term Far-term for appreciable reductions and will require technical advances Unknown, may require technical breakthrough Far-term, pace of reduction depends on cost, rate of innovation, and extent of applicability to GA fleet Focus of Implementation Airport management FAA flight standards, pilot instruction and training Fuel supply chain, especially refiners Fuel supply chain and airports, especially fuel storage and Engine and aircraft makers Fuel supply chain, especially fuel developers; Technology developers, aircraft manufacturers, aircraft owners
PREPUBLICATION COPY â Uncorrected Proofs 6 programs, GA community dispensing capacity engine and aircraft makers Possible Policy Actions for Facilitating Implementation Provide data and tools for analysis and identifying operations changes Provide training and education materials, engage in awareness campaigns Directives and/or incentives, perhaps focused on refiners Incentives and financial assistance for airports to add fueling capacity, eased FAA certification Directives and/or incentives applicable to GA industry Publicâprivate collaborative (PAFI-like) for R&D, testing, and certification R&D support, FAA certification, incentives for aircraft owners to incur expense Main Sources of Uncertainty in Achieving Effective Implementation Variability in airport- specific factors Potential to affect practices Refiner capacity to meet tighter lead specifications Feasibility of adding fuel supply chain (refiners and airports), certification Ability to design suitable engines for all high- performance aircraft Potential to meet fuel performance requirements Rate of innovation, certification challenge, cost and owner interest Ancillary Benefits and Concerns Greater lead awareness and interest in lead-free fuels and propulsion Greater lead awareness and interest in lead-free fuels and propulsion Environmental and health impacts related to other fuel components Changes in pollutants, including greenhouse gases over life cycle a Where percentages are given, they refer to estimates of reductions in lead from the total fuel consumed by the piston-engine fleet. b Hot spots often refer to a spatial zone of emissions impact where the airborne lead concentration is significantly elevated above background.
PREPUBLICATION COPY â Uncorrected Proofs 7 RECOMMENDATIONS Having evaluated these different, but complementary, pathways for reducing aviation lead, the committee concludes that concerted efforts are warranted to initiate and sustain progress across them all. The committeeâs recommendations are given next, grouped within each pathway with a summary of their desired outcomes. Airport Operations and Practices â¢ FAA should coordinate its efforts to reduce lead pollution and exposures at airports with those of other federal agencies that have key responsibilities for protecting public health, safety, and the environment at airports, including the Occupational Safety and Health Administration (OSHA) as well as EPA. FAA should collaborate with these agencies to explore the regulatory and programmatic means within their respective jurisdictions that can be brought to bear and combined in a complementary manner to reduce lead emissions and exposures at airports (Recommendation 4.1). â¢ FAA, in partnership with prominent organizations within the GA community, should initiate an ongoing campaign for education, training, and awareness of avgas lead exposure that is targeted to GA pilots, aircraft technicians, and others who work at airports. Informed by research on the most effective approaches for reaching these audiences, the campaign should be multi-pronged by ensuring that information on lead risks and mitigation practices is prominent in relevant manuals, guidelines, training materials, and handbooks for pilots, airport management, and aircraft technicians. Where appropriate, it should also be covered in relevant certification and licensure examinations. In addition, the information should be featured on FAA and GA organization websites and included in written materials distributed at GA industry conferences, tradeshows, and fly-ins (Recommendation 4.2). â¢ FAA should update its guidance on the location of run-up areas to reflect the results of research since the latest interim guidance was issued in 2013, including the need to account for both the emissions of engine run-ups and of takeoffs when analyzing the geographic distribution of lead emissions at the airport. This analysis should support decisions of whether to move run-up areas to reduce peopleâs exposure to lead emissions while also accounting for other concerns including safety and aircraft noise (Recommendation 4.3). The outcomes envisioned from these recommendations will result from increasing awareness by the many individuals needed to effect change in everyday operations. Pilots and aircraft owners, airport managers and personnel, and aircraft technicians would understand the hazards created by leaded avgas to themselves and the local community, and would follow best practices for containment during refueling, locating and timing engine run-ups, proper disposal of inspected fuel samples, and exposure protections. Airports would purposefully move pre- takeoff run-up areas to reduce the proximity of lead concentration hot spots to people where airport location, traffic activity levels and exhaust interactions warrant such a response.
PREPUBLICATION COPY â Uncorrected Proofs 8 Existing Fuels and Fleet â¢ FAA should research public policy options, which could be implemented as quickly as possible at the federal and state levels as well as by Congress, for motivating refiners to produce and airports to supply 100VLL. The objective would be to reduce lead emissions from the entire piston-engine fleet while unleaded alternatives are being pursued for fleetwide use (Recommendation 5.1). â¢ FAA should research public policy options that will enable and encourage greater use of available unleaded avgas by the portion of the piston-engine fleet that can safely use it. Possible options include (a) issuing a Special Airworthiness Information Bulletin that will permit such use and (b) providing airports with incentives and means to supply unleaded fuel, particularly airports that are eligible for FAA- administered federal aid as part of the National Plan for Integrated Airport Systems (Recommendation 5.2). â¢ A mechanism should be established for facilitating the increased availability of existing grades of unleaded avgas across the fleet of piston-engine aircraft. Fulfilling that need would likely require congressional involvement, such as by providing incentives for pilots to use existing unleaded avgas and for more small airports to add requisite fuel storage and dispensing capacity (Recommendation 5.3). The outcomes envisioned from these recommendations would motivate changes within the fuel supply chain to reduce lead in aviation fuels quickly. 100VLL would be made available for purchase and used by the entire piston-engine fleet, potentially leading to a 20 percent reduction in lead emissions after fully replacing 100LL without necessitating any changes in airport fueling capacity, aircraft or engines, and aircraft operations. Even as 100VLL would be made widely available, increasingly larger portions of the low-performance fleet would be certified to use a lead-free lower octane avgas, such as UL94, and many pilots of these aircraft motivated to use it. The fuel would be made available at many high- and medium-volume airports where supplemental fuel storage and dispensing capacity exists or can be economically added. This development has the potential to result in lead reductions that could exceed 40 percent when combined with 100VLL being used by legacy high-performance aircraft, although this full reduction may not be achieved due to expected limits on the availability of an unleaded grade at many lightly used airports where the cost of adding more fueling capacity may be prohibitive. Future Lead-Free Fuels and Propulsion Systems â¢ FAA should continue to collaborate with the GA industry, aircraft users, airports, and fuel suppliers in the search for and deployment of an acceptable and universally usable unleaded replacement fuel. The collaboration should be carried out through the Piston Aviation Fuels Initiative (PAFI) or an alternate holistic process for evaluating all the properties and conditions necessary for production, distribution, and safe use of the fuel, including the use of common test protocols and procedures and by making available the needed testing facilities for the development of the data required to support FAA approvals for the fuel to be used by existing piston-engine aircraft (Recommendation 6.1).
PREPUBLICATION COPY â Uncorrected Proofs 9 â¢ A clear goal should be established that all newly certified gasoline-powered aircraft after a certain point in time (e.g., within 10 years) are approved to operate with at least one ASTM-specified unleaded fuel. Also, an additional amount of time should be identified by which all newly produced gasoline-powered aircraft, including those currently produced with older type certificates, would attain that same goal. Congressional action to establish the goal and timeframes would ensure achievement of those important results. For example, that congressional action would promote the development of new engine variants compatible with existing unleaded fuels which could possibly yield prescriptions to support the eventual retrofit of some legacy aircraft and engines as they reach required overhaul milestones (Recommendation 6.2). â¢ FAA initiativesâincluding collaborations with industry and other government agencies such as the National Aeronautics and Space Administrationâshould be used to promote the development, testing, and certification of safe and environmentally desirable lead-free emerging propulsion systems (such as diesel, electric, and jet fuel turbine engine) for use in GA aircraft, including the requisite airport refueling and recharging infrastructure. Congressional encouragement and the provision of resources as required would ensure the success of those initiatives (Recommendation 6.3). The outcomes of these recommendations have the longest time frame and some are uncertain as to when they can be realized and at what cost â but they lead to a full transition of the aviation industry from the use of leaded fuels. If the technical challenges in creating a safe, unleaded aviation fuel can be overcome, it would become widely available and the entire gasoline-powered GA fleet would be able to use unleaded fuels, either through the development and supply of a single high-octane unleaded grade (e.g., 100+UL) that can serve the entire fleet or a mix of higher- and lower-octane (e.g., UL94) unleaded grades that can serve different portions of the fleet. While the former outcomeâa drop-in fuelâ would enable economies of scale in fuel production and reduce the need for multiple fuel distribution, storage, and dispensing systems, either outcome would greatly reduce or even eliminate lead from the GA sector. In parallel, within a decade or so, all newly produced gasoline- powered aircraft, including high-performance aircraft, would be certified to use an unleaded avgas grade, which may need to be a lower octane grade if a safe unleaded drop-in replacement for current 100LL is not established in time. Aircraft operators could choose to transition their aircraft systems to use this fuel once it is sufficiently available along their routes of flight and at their home base airports. The capability to use a lower octane grade, which currently exists for at least a few high-performance engines in production, would gradually increase the demand for unleaded fuel, and motivation for airports to install appropriate fueling systems to provide it, if coupled with many legacy, low-performance aircraft using the fuel. High-performance engines capable of using unleaded avgas might also transition into the legacy fleet as older aircraft undergo major engine retrofits. Furthermore, the share of new aircraft using lead-free fuels, such as diesel and jet fuel, or non-petroleum propulsion systems, such as electric power, would be increasing steadily driven by innovation and consumer interest and uptake (to possibly include retrofits in some legacy
PREPUBLICATION COPY â Uncorrected Proofs 10 aircraft) to yield appreciable lead reductions and also could confer other environmental benefits such as reduced greenhouse gas (GHG) emissions. Over the coming decades, efforts to reduce GHG emissions from the production and use of transportation fuels may influence the availability and composition of petroleum-based aviation fuels and hasten the introduction of aviation propulsion systems that do not require petroleum. It will be important that the transition to using avgas with lower or no lead content also coordinate with efforts seeking to reduce or eliminate GHG emissions, given the shared concerns with developing and certifying new aircraft technology, the supply and distribution systems for GA aircraft fuels, and broad awareness within the GA community. ONGOING NEED FOR RESEARCH, DATA, AND ANALYSIS While ample evidence and knowledge exist about the harm caused by lead pollution to highlight the need to initiate a comprehensive set of aviation lead mitigations now, there also remains a compelling need for more research and data to inform the design and assessment of mitigations and to target them in the most effective manner. Recommendations for research to meet such needs are provided in Box S-1; for instance, mitigations can be better applied with better understanding of how environmental lead concentrations at and near airports vary according to differences in airport configuration, activity levels, and other characteristics. Additional recommendations and discussions are provided in Chapter 3 of this report. EPA has been at the forefront of efforts to further the needed research, data, and analysis largely within the context of its obligation to implement the CAA. Success in designing and implementing a lead mitigation strategy will require such an ongoing commitment to research, data collection, and analysis. BOX S-1 Recommended Research on Aviation Lead Pollution and Its Effects EPA should conduct more targeted monitoring and enhanced computational modeling of airborne lead concentrations at airports of potential concern, as indicated by its most recent screening study, to evaluate aircraft operations that are main contributors to lead hot spots and designing airport-specific mitigation measures. In addition to airports found to have airborne lead concentrations exceeding the concentration of the lead NAAQS, the additional monitoring and modeling should include airports found to have lead concentrations that are lower but approaching the NAAQS concentration (Recommendation 3.1). EPA and the National Institute of Environmental Health Sciences should sponsor research to enhance the understanding of lead exposure routes and their relative importance for people living near airports and working at them. The research should include studies, such as observations of blood lead levels among children, in communities representing a variety of geographic settings and socioeconomic conditions that are designed to examine the effectiveness of the lead mitigation strategies over time (Recommendation 3.2). CONCLUDING COMMENT There are no known safe lead exposures as measured by blood lead levels; leadâs adverse effects on human health, and particularly on the development of children, are well established. While
PREPUBLICATION COPY â Uncorrected Proofs 11 the elimination of lead pollution has been a U.S. public policy goal for decades, the GA sector continues to be a major source of lead emissions, largely because of the complex challenge of eliminating those emissions that is documented in this report. However, the evidence of lead pollutionâs hazard demands that those challenges not become an excuse for inaction, but instead become the subject of concerted, sustained, and multipronged efforts to find and implement mitigations. It is important to note that EPA, which sets National Ambient Air Quality Standards under authority of the Clean Air Act (CAA), has been studying airborne lead concentrations at airports for the past decade to determine whether lead emissions endanger public health or welfare. However, the agency has not yet proposed such a formal determination, positive or negative. Given the uncertainty of this development, CAA-specific regulatory tools were not considered in this study, but if they were to become available, they would almost certainly have a prominent role in a lead mitigation strategy. A key message of this report is that a lead mitigation strategy focused almost entirely on developing an unleaded drop-in fuel that would eliminate aviation lead emissions has a high degree of uncertainty of success given the formidable technical challenges. Additional mitigation measures are available that could be applied in the near and mid-terms to make progress in reducing lead emissions and exposures while other approaches having the potential for much larger impacts are being pursued.
PREPUBLICATION COPY â Uncorrected Proofs 12