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PREPUBLICATION COPY â Uncorrected Proofs 33 3 General Aviation Lead Emissions and Their Potential Health Impacts This chapter addresses the portion of the committeeâs Statement of Task that calls for an assessment of ambient lead concentrations at and around airports where piston-engine general aviation (GA) aircraft are used. The chapter begins with a general overview of environmental lead dynamics, routes of human exposures, and human health risks attributable to lead exposures. The chapter then considers lead emissions resulting from the combustion of leaded avgas by GA aircraft and aircraft activities at airports that contribute to those emissions and nearby ambient lead concentrations. In addition, the chapter discusses various issues related to lead exposures to people in communities around airports as well as worker exposures at those airports. The chapter concludes with findings and recommendations. ENVIRONMENTAL DYNAMICS AND ROUTES OF LEAD EXPOSURE Although lead is present in the environment naturally, most elevated concentrations in air, water, and soil result from the past and current societal uses of lead (e.g., for transportation fuel additives, plumbing pipes, and paint). According to the World Health Organization, the widespread use of lead has resulted in extensive environmental contamination, human exposure and substantial public health problems in many parts of the world.1 Lead can be released to the environment at any point in the sequence from ore mining to the use of finished products containing lead, and recycling processes (e.g., recovery of lead from discarded lead-acid batteries). As discussed in EPA (2013), airborne lead is usually released to the environment in an inorganic form and as a component of particulate matter. Lead can deposit in soil, water, and other surfaces at various distances from an emission source. After deposition, lead particles can be resuspended and redeposited multiple times. The deposition patterns differ by size fractions. Combustion of leaded fuel by piston-engine GA aircraft is a major source of lead released into the environment. Weathered or chipped lead-based paint from buildings and other structures contribute lead to soils. Lead can be released from lead pipe or solder that comes into contact with acidic water. Lead compounds in the environment can be transformed biologically and chemically, and those changes affect transport in soil and uptake by vegetation. Lead may move from soil into surface water or groundwater, depending on the type of lead compound and the characteristics of the soil. In locations with large amounts of precipitation, lead tends to leach from soils to water. Organic matter in soils tends to retain lead by adsorption and hinder its release to water. After entering surface waters, lead can deposit to sediments and perhaps become resuspended into the water column. Amounts of lead that come into contact with humans are influenced by rates of transport within and between air, surface water, soil, and sediment. The possible routes of human exposure to lead are through inhalation, ingestion, and dermal absorption (see Figure 3-1). In addition to 1 See https://www.who.int/news-room/fact-sheets/detail/lead-poisoning-and-health.
PREPUBLICATION COPY â Uncorrected Proofs 34 exposure to airborne lead through inhalation, lead deposited from air on plant materials or in water becomes available for human consumption. Lead exposures that do not originate from atmospheric deposition include ingestion from lead-containing consumer goods, contact with dust or chips of lead-containing paint, and ingestion of drinking water contaminated by lead leaching from water pipes or lead-containing solder. In addition to exposure to inorganic forms of lead, workers can be exposed to lead in its organic form (e.g., tetraethyl lead [TEL] in aviation gasoline [avgas]). FIGURE 3-1 Conceptual model of multimedia lead exposure representing the movement of lead through various environmental pathways and routes of exposure. SOURCE: EPA, 2013, xxx. NATIONAL TRENDS IN AIRBORNE LEAD CONCENTRATIONS AND EMISSIONS For decades, lead in gasoline for on-road motor vehicles had been the primary source of environmental lead. In 1975, the U.S. Environmental Protection Agency (EPA) began to phase out the use of TEL as a gasoline additive. The phaseout culminated with a ban in 1996 on the sale of gasoline with added lead for on-road vehicles. EPA allowed the continued sale of leaded gasoline for piston-engine GA aircraft. Between 1970 and 2014, estimated nationwide lead emissions decreased by 99.7 percent (about 220,000 tons), mostly due to elimination of lead additives for gasoline.2 Sharp declines in annual nationwide lead emissions from 1990 to 2014 are illustrated in Figure 3-2. Between 1999 and 2014, lead emissions from metals industrial processing, fuel combustion, and other sources all decreased by approximately 90 percent. The largest remaining source category is nonroad vehicles and engines, which accounted for 63 percent of the anthropogenic (human-induced) lead emissions in 2014. Lead emissions from piston-engine GA aircraft is the primary source within the nonroad category.3 2 See https://www.epa.gov/air-emissions-inventories. 3 See https://www.epa.gov/air-emissions-inventories.
PREPUBLICATION COPY â Uncorrected Proofs 35 The percentage contribution of aviation-related lead emissions has increased because of dramatic reductions in other sectors, as illustrated in Figure 3-2. EPA reports that in 2017 piston- engine GA aircraft comprised the largest single source of lead air emissions in the United States (see Figure 3-3). Those aircraft accounted for 468 tons of emissions, which was roughly 70 percent of total lead emissions to air in the United States.4,5 The estimate includes lead emissions from aircraft on the ground and in-flight. Although requirements for unleaded gasoline do not allow for lead additives, federal regulations allow unleaded gasoline to contain up to 0.05 grams of lead per gallon (40 CFR 80.2). Because lead occurs naturally in crude oil, trace amounts might be present after the refining process. FIGURE 3-2 Anthropogenic lead emissions in the United States by source category, 1990-2014. NOTES: EPA notes that the trend shown in the figure from 1990 to 2014 reflects changes in the methods used to develop emission estimates as well as actual changes in the emissions over time. Therefore, real- world changes in emissions from year to year could have been larger or smaller than those illustrated in the figure. EPA obtained the data from the 2014 National Emissions Inventory, Version 2. Accessed 2018. SOURCE: https://www.epa.gov/report-environment/outdoor-air-quality. 4 See https://www.epa.gov/air-emissions-inventories/2017-national-emissions-inventory-nei-data. 5 The EPA emissions inventory uses 2.12 grams Pb per gallon which is 3.3 grams TEL per gallon.
PREPUBLICATION COPY â Uncorrected Proofs 36 FIGURE 3-3 2017 U.S. Emissions Inventory for Lead. NOTE: Other sources include chemical production and petroleum refining. SOURCE: https://www.epa.gov/air-emissions-inventories/2017-national-emissions-inventory-nei- data. Responding to the phaseout of lead in gasoline, average airborne lead concentrations decreased 99 percent from 1980 and 2016 (see Figure 3-4). EPA used monitoring data from seven sites in seven counties to illustrate the trend shown in the figure, as they were the only monitoring sites that provided sufficient data to assess trends from 1980. None of those sites reported annual maximum 3-month average lead concentrations greater than the 2008 National Ambient Air Quality Standard (NAAQS). 468 68 98 34 Lead Emissions to Air, tons Aircraft Metal Industrial Processing Boilers & Process Heaters Other
PREPUBLICATION COPY â Uncorrected Proofs 37 FIGURE 3-4 U.S. ambient lead concentrations; 3-month averages; 1980 to 2016. NOTES: The current lead NAAQS provides context for the magnitude of the concentrations. Measurements were collected from 7 monitoring sites in 7 counties. EPA obtained the data from EPAâs Air Quality System. Accessed 2017. SOURCE: https://www.epa.gov/report-environment/outdoor-air-quality. HEALTH EFFECTS FROM LEAD EXPOSURES Lead serves no biological function and has been understood to be a powerful toxicant since ancient times (Daley et al., 2018). Australian pediatricians Gibson and Turner described health effects resulting from childhood exposure to lead in paint at the end of the 19th century (Gibson et al., 1892). Despite the awareness of its toxicity, the physical and chemical qualities of lead made it attractive for a variety of applications. As an example, despite known toxicities documented well before, it was not until 1978 that lead-based paints were banned for residential use in the United States. While it is beyond the scope of this report to provide an exhaustive compilation of the health effects associated with lead exposure, this section provides a broad overview of leadâs profound and negative health impacts. Adverse health effects of lead have been observed in multiple organ systems because the mechanisms that induce lead toxicity are common to all cell types and because lead is widely distributed throughout the body through blood (ATSDR, 2020; EPA, 1986, 2013; NTP, 2012). Much of what is known about the health effects of lead exposure to humans comes from extensive work carried out since 1970, when studies of exposures to airborne lead from motor vehicle exhaust were being vigorously pursued through EPA, National Institute of Environmental Health Sciences (NIEHS), and other organizations. That effort was prompted by the realization that airborne lead concentrations, especially in large metropolitan cities, were especially problematic for young children. Atmospheric lead concentrations exceeded 8 Âµg/m3 in
PREPUBLICATION COPY â Uncorrected Proofs 38 the late 1980s (see Figure 3-4), while pre-industrial concentrations of airborne lead from natural origins are estimated at 0.0006 Âµg/m3 (Patterson, 1965). In cellular based studies, lead was found to disrupt mitochondrial function and cellular metabolism. The most extensively studied health outcomes are neurological, renal, hematological, immunological, reproductive, and developmental effects for children (ATSDR, 2020). In addition, lead and lead compounds have been listed as a potential carcinogen, although few if any direct links in humans have been reported (NTP, 2016). Research suggests that significant adverse health effects occur at blood lead levels (BLLs) below the current reference level (level of concern) set by the Centers for Disease Control and Prevention (CDC) (Gatsonis and Needleman, 1992; Lanphear et al., 2005; Schwartz, 1993, 1994). Typically expressed in units of Î¼g/deciliter (dL), BLLs reflect recent exposures to lead in the environment or workplace and releases from bone (which is a major reservoir for lead in the body). Learning and behavioral deficits may occur at BLLs lower than 5-10 Âµg/dL, including attention-related behavioral problems (Canfield et al., 2003; Froehlich, 2009; Nigg, 2008). Exposure to low concentrations of lead, including prenatal exposure, has been linked to decreased performance on standardized IQ tests for school-aged children (ATSDR, 2020). Bellinger (2012) estimates that lead exposure accounts for up to 23 million lost IQ points in a six-year birth cohort of U.S. children. Lead exposure has also been linked with higher levels of aggression, delinquent behavior, and criminal behavior (e.g., Beckley et al., 2018). Some investigators observed that criminal behavior in urban cities has dropped in parallel with the reduction of airborne lead following the elimination of leaded fuels for motor vehicles (Nriagu, 1990, Wakefield, 2002). In addition, in a New Zealand cohort, lead exposure during childhood was associated with lower socioeconomic status in adulthood (Rueben et al., 2017). Lead is taken up in developing children through metabolic pathways normally dedicated to calcium uptake. Because their blood-brain barrier is not fully developed, much of the lead is concentrated in the brain where it can interfere with nerve function and development of neuronal pathways. Lead exposure in children has been documented in brain scans (Rueben, 2020). In contrast, approximately 95 percent of the lead that enters the body tissues of adults is deposited in bone at equilibrium. Lead deposits in the bones of pregnant women can be transferred to the fetus during the normal process of supplying calcium from the motherâs bones for fetal development (ATSDR, 2020). Whether through ongoing exposure or through release of lead from bone, higher lead levels during pregnancy result in increases in preterm birth and lower birthweight (Taylor et al., 2014). There are various subgroups of adults that exhibit increased susceptibility to lead-related health effects. Lead exposure has been linked to pregnancy-induced hypertension and hypertension and cardiovascular disease among other adults, particularly among post- menopausal women, when the process of calcium loss from bone also causes the release of bone lead (ATSDR, 2020). Thus, increases in BLLs due to release from bone could exacerbate the normal loss of neurons during the aging process. In a meta-analysis, Chowdhury et al. (2018) found that exposure to lead was associated with increased risk for cardiovascular and coronary heart disease. In general, because health effects associated with lead exposure have been observed in every organ system, any pre-existing condition that compromises physiological functions could render a person more susceptible to leadâs effects. Although much progress has been made in reducing lead exposures to humans, childhood lead poisoning remains a critical environmental health concern. Epidemiological studies commonly rely on BLLs as a metric of exposure. Since the late 1970s, mounting evidence has
PREPUBLICATION COPY â Uncorrected Proofs 39 demonstrated that lead causes irreversible, asymptomatic effects at BLLs far below those previously considered safe. Thus, CDC incrementally lowered its level of concern for BLLs from 60 to 5 Î¼g/dL over the last 40 years (CDC, 2020). Because lead does not appear to exhibit a threshold concentration for health effects, CDC concluded that there is no known safe level of lead in blood and refers to 5 Î¼g/dL as a reference level. That level was determined by examining the data on children, ages 1-5 years, in the National Health and Nutrition Examination Survey (NHANES) and selecting the BLL at the 97.5 percentile (CDC, 2020). In response to the committeeâs Statement of Task, the remainder of this chapter focuses on environmental and occupational aspects related to lead emissions from the use of avgas by piston-engine GA aircraft. LEAD EMISSIONS FROM GA AIRCRAFT Lead emissions from piston-engine GA aircraft at and near airports arise from numerous aircraft activities that can have different contributions to airborne particulate matter containing lead. Ground-level activities include idling at hangars, taxiing, run-up, takeoff roll, after-landing roll, and maintenance operations. Aloft activities include climb-out, local flying, and approach. These activities occur under different engine load and different times in mode, and differentially contribute to total lead emissions. A special type of landing and takeoff (LTO) cycle is a touch- and-go; it is a common flight training practice and involves a landing, ground roll, and takeoff without the other activity modes common to a conventional LTO cycle (e.g., no taxiing and run- up). Understanding the impacts of leaded fuel combustion requires estimating emissions and ideally also resulting airborne concentrations. EPA (2010, 2020a) describes the development of emission inventories for specific airports by using: â¢ Piston-engine aircraft activity data; â¢ Aircraft-specific fuel consumption rates during the various modes of a landing and LTO cycle; â¢ Time spent in each mode (run-up, taxi/idle-out, takeoff, climb-out, approach, and taxi/idle-in); and â¢ Assigned values for the lead content in the fuel, and the retention of lead in the engine and oil. In-flight lead emissions are estimated by taking the difference between the total nationwide emissions (based on avgas sales) and the sum of emissions estimated for each airport (EPA, 2010). National and airport-specific estimates are updated every 3 years as part of EPAâs National Emissions Inventory (NEI). The documentation of changes to the emissions estimation methodology informs examinations of trends over time. The development of specific emission inventories for each airport in the nation involves the use of assumptions and approximations that add uncertainties to the inventories. For some airports, LTO data are not available and they are estimated using equations that include the number of based aircraft at the airport and county population with adjustments for airports located in Alaska. Because many airports have both jet-engine and piston-engine aircraft operations and only the latter type emits lead, a fraction of LTOs attributable to piston-engine aircraft is assigned. That fraction is estimated using the numbers of aircraft based at an airport.
PREPUBLICATION COPY â Uncorrected Proofs 40 However, actual operations can vary dramatically, such that a small number of aircraft conduct a disproportionately large number of LTOs, as is often the case at airports with flight schools. Emission factors (grams of lead emitted per piston-engine LTO) are estimated from fuel burn rates which vary by aircraft with large differences for single-engine versus twin-engine aircraft. Again, these splits are estimated using the number of based aircraft and this may not reflect actual activity. Five percent of lead in the burned fuel is assumed to be retained in the engine and oil. Time-in-mode can differ from the assumed national defaults, depending on the airport configuration, how operations are managed, and pilot behavior. Finally, the lead content of fuel is estimated using national sales volume for each grade of aviation gasoline (with sales now dominated by 100LL) and assuming the lead content for each grade is at its maximum allowable concentration (e.g., 2.12 grams per gallon for 100LL) (EPA, 2010). The cumulative impact of these assumptions and approximations on airport-specific emission inventories is not clear. NASEM (2015a) provides a methodology and spreadsheet tool to prepare refined airport- specific lead emission inventories. Table 3-1 shows the activity-specific contributions to total lead-bearing particulate matter emissions for three airports using a refined emission inventory methodology with on-site data collection. These estimates exclude emissions during local flying, which is an important aspect because of the environmental persistence of lead. While the percentage contributions from some activities are relatively constant (e.g., taxiing and takeoff), for other activities there are large airport-to-airport differences (e.g., run-up and touch-and-go). TABLE 3-1 Estimated Contributions of Piston-Engine GA Aircraft Activities to Particulate Matter Lead Emissions at Three Airports Percentage of Total Emissions (%)a Source Group RVS APA SMO Run-up 22% 12% 13% Taxiways 12% 12% 15% Takeoff 5% 7% 6% Climb-out 26% 21% 29% Approach 17% 12% 27% Landing 1% 1% 2% Touch and Gob 11% 29% 1% Hangarsc 6% 6% 6% Helicoptersd 1% 0% 1%
PREPUBLICATION COPY â Uncorrected Proofs 41 a RVS is Richard Lloyd Jones Jr. Airport in Tulsa, Oklahoma; APA is Centennial Airport in Denver, Colorado; SMO is Santa Monica Municipal Airport in Santa Monica, California. b Touch-and-go operations for fixed-wing aircraft consist of an approach, brief ground roll (landing), an immediate takeoff, and a climb-outâall of which occur without exiting the runway. c Includes all emission activities within a hangar area, such as taxiing and idling. d Includes all phases of helicopter operation. NOTES: On-site data were collected for nominally 1 month at each airport in 2013. Estimates do not add up to 100 percent for each airport. SOURCE: NASEM, 2015b. The EPA 2017 NEI includes more than 19,000 airports that are estimated to have lead emissions.6 The total amount of lead emissions from those airports was estimated to be 224 tons. Approximately 25 percent of the airport lead emissions is attributed to about 178 airports (less than 1 percent). About 50 percent of the lead emissions is attributed to 587 airports (approximately 3 percent). Those airport emissions estimates are based on aircraft fleet and activity information, rather than the volume of avgas combusted, which is the basis used for the aircraft lead emission estimate in Figure 3-3. AIRBORNE LEAD CONCENTRATIONS FROM EMISSIONS AT AIRPORTS Estimates of airport-specific lead emissions provide general estimates of lead released to the nearby environment. Airport-specific estimates can also be used to identify lead hot spots, which are localized, relatively high concentrations of airborne lead relative to background concentrations. Mitigation efforts can seek to reduce hot spots, reduce total emissions, or both. Hot spots can occur distant from the public, such as in restricted access zones within the airport boundaries. The airborne lead concentration is dispersed as it travels downwind. Hot spots can also occur at locations where people are present on or near to the airport footprint. For example, emissions near an airport boundary can cause hot spots that extend beyond the footprint. For airports in densely populated areas, modeling results suggest these localized, relatively high concentrations can extend into residential neighborhoods (NASEM, 2016). Hot spots tend to arise at locations where multiple activities contribute lead emissions, such as downwind of run-up areas near the end of a runway with taxiing/idling and ground rolls before takeoff. Figure 3-5 shows contours of modeled airborne lead concentrations at Richard Lloyd Jones Jr. Airport in Tulsa, Oklahoma. The top-left panel presents total lead concentrations while the other three panels are the contributions from taxiways, run-up areas, and takeoffs. Hot spots are located at the ends of runways and an aircraft maintenance area (although the latter is likely ill-characterized and possibly biased high). Run-up areas tend to be important contributors to hot spots. Taxiing and idling while awaiting clearance for takeoff can also be significant. 6 See https://www.epa.gov/air-emissions-inventories/2017-national-emissions-inventory-nei-data.
PREPUBLICATION COPY â Uncorrected Proofs 42 FIGURE 3-5 Modeled airborne lead concentrations at Richard Lloyd Jones Jr. Airport in Tulsa, Oklahoma for a nominally 1-month period in 2013 using on-site activity data collection. NOTES: Airport property boundaries are designated by a thick black line; dark interior lines indicate runways. 1,000 ng/m3 equals 1 Âµg/m3. SOURCE: NASEM, 2015b. Building on the observation that hot spots tend to occur near run-up areas (not just because of run-up operations but because of the confluence of emissions from run-ups, queuing before takeoff, and takeoff operations), EPA modeled one airport and developed empirical relationships to estimate near-field lead concentrations using aircraft class (single versus multi- engine) and operations cycle activity (landing and takeoff versus touch-and-go) as the predictor variables (EPA, 2020a). Model-extrapolated estimates of lead concentrations were generated for hot spots at more than 13,000 airports nationwide. (EPA refers to hot spots as zones of maximum
PREPUBLICATION COPY â Uncorrected Proofs 43 impact.) Numerous assumptions were made to conduct the nationwide analysis because airport- specific activity data are limited. Therefore, for those airports with model-extrapolated lead concentrations greater than or within 10 percent of the lead NAAQS concentration (0.15 Âµg/m3), sensitivity analyses were conducted by varying key influential parameters to constrain the estimate concentrations. According to EPA (2020a), this screening analysis identified four airports having âmodel-extrapolated lead concentrations potentially greater than the lead NAAQS at the maximum impact area with unrestricted areas [to public access] within 50 meters.â It is important to note the exposure to airborne lead at concentrations less than the NAAQS can result in health effects depending on the susceptibility of the individual at any given time and the magnitude, duration, and frequency of the exposures (EPA, 2013). EPA (2020a) focused mainly on possible NAAQS exceedances. Nationwide results were presented as model-extrapolated lead concentrations stratified by LTO ranges across the 13,000 airports, without also identifying how many and which airports fall into each of the LTO ranges. AIRBORNE LEAD PARTICLE SIZES FROM PISTON-ENGINE GA AIRCRAFT EMISSIONS The exhaust of engines burning gasoline consists of gases including carbon monoxide, nitrogen oxide, and carbon dioxide, carbon in the form of soot, and carbonyl hydrocarbons (such as formaldehyde, a known carcinogen) (Rindlisbacher, 2007). In addition, if the fuel is leaded, the exhaust will contain lead dibromide particles. The addition of TEL to avgas would result in lead deposits that foul engines, unless the lead is scavenged following combustion. This is accomplished by adding organohalides (such as ethylene dibromide [EDB]) to gasoline, which react with the volatilized lead to form lead halide particles (EPA, 1986, 2013; Nriagu, 1990). When TEL and EDB are added to gasoline, lead dibromide particles are formed in the exhaust gas. In prior studies of motor vehicle exhaust, lead dibromide particles were shown to range in size from about 20 to 100 nanometers (nm) in diameter with a mean near 50 nm (for example, Little and Wiffen, 1977). Griffith (2020) confirmed this size distribution (mean size found to be 38 nm) in exhaust from a motor vehicle engine burning 100LL fuel and further demonstrated that these particles consist primarily of collections of 5 to 10 (or more) lead dibromide beads (4 nm diameter per bead) embedded in a hydrocarbon matrix (see Figures 3-6 and 3-7). Biological studies (Button et al., 2012; Kesimer et al., 2013) of the ability of nanoparticles of different size to penetrate the lung defenses have demonstrated that particles of greater than 40 nm are for the most part unable to pass the mucus barrier in the lung to gain access to the epithelial cells. Thus, it might be expected that most of the lead dibromide particles inhaled in the past from motor vehicle exhaust would have been flushed from the lungs by the mucosal system. However, if the 4 nm lead-dibromide beads were easily released from the larger assemblies, the smaller lead beads would rapidly transit the lung defenses and gain access to the epithelial cells. Griffith (2020) reports that exhaust particles collected in flight from a single piston-engine aircraft burning 100LL fuel were found also to consist of 4 nm lead-dibromide beads embedded in a hydrocarbon matrix. However, the particles in the aircraft exhaust were found to be much smaller (13 nm average diameter) and each particle contained only 1 or 2 lead dibromide beads. Such particles have the potential of rapidly penetrating the lung defenses either as the 13 nm particles or 4 nm beads. In addition, in the nasal passage, such small particles could gain direct
PREPUBLICATION COPY â Uncorrected Proofs 44 access to the brain. Based on grams of lead emitted into the air for the particle size range considered by Griffith (2020), there may be 5 to 10 times more single lead-containing particles than from legacy motor-vehicle emissions. Whether this translates to a higher relative toxicity is unclear and further research would be valuable. Although larger particles may be unable to pass through the mucus barrier of the lung, lead absorption can occur through other mechanisms. Particles larger than 2.5 Âµm that are deposited into nasopharyngeal and tracheobronchial regions can be moved to the esophagus by mucociliary transport and swallowed. Particles smaller than 2.5 Âµm can deposit in the alveolar region and be absorbed following extracellular dissolution or ingestion by phagocytic cells (ATSDR, 2020). In addition to observing a high relative abundance of lead-bearing particles with a diameter less than 20 nm, Griffith (2020) also noted the presence of larger lead-bearing particles (35 nm or greater) at less frequency. Although those larger particles were fewer in number, they would likely dominate measurements of lead mass concentration, because mass scales with the cube of the diameter. However, smaller sized particles dominate particle number concentrations. Therefore, particle number concentrations may be a more meaningful metric than mass concentration for health studies of particle exposures less than 100 nm.
PREPUBLICATION COPY â Uncorrected Proofs 45 FIGURE 3-6 Images of exhaust particles from aircraft and automotive engines burning 100LL fuel. NOTES: In a recent study (Griffith, 2020), exhaust particles from automotive and aircraft engines burning 100LL fuel and operated at 2400-2500 rpm were captured directly on electron microscopic supports and imaged without further contrast enhancement. (Left). Field of exhaust particles from a 1957 V-8 automobile engine burning 100LL gasoline. Particles consist of large irregular shaped aggregates of burned hydrocarbon matrix (light halo) containing many 4 nm lead beads (small white particles). Imaging by transmission electron microscopy and shown in inverted contrast. (Right). Field of exhaust particles captured in flight from an aircraft engine (O-320) burning 100LL fuel. Particles also consist of a burned hydrocarbon matrix but contain only 1 or a few lead beads. The large white particle is a single large lead containing particle. Imaging by high angle annular dark field electron microscopy. Magnification for both panels is the same. SOURCE: Image courtesy of Jack Griffith, committee member.
PREPUBLICATION COPY â Uncorrected Proofs 46 FIGURE 3-7 Size distributions of particle sizes in aircraft and automobile exhaust (C) and lead beads (D). (C) Size distribution of automotive and aircraft particles showing the much smaller size of the aircraft exhaust particles. (D) Size distribution of the lead beads from automotive and aircraft emissions showing the same ~4 nm diameter. See Griffith (2020) for details. SOURCE: Griffith, 2020.
PREPUBLICATION COPY â Uncorrected Proofs 47 ENVIRONMENTAL DYNAMICS OF PARTICULATE MATTER LEAD EMISSIONS Both airborne concentrations and particle deposition locations depend on the emission height and particle size. The lead-bearing exhaust particles emitted close to the ground (such as, ground- based aircraft operations) have higher deposition rates and are transported over shorter distances in the atmosphere relative to particles emitted from aircraft aloft. Regarding particle size, particles with diameters less than about 100 nm have deposition velocities that increase as particle size decreases. In contrast, particles with diameters greater than about 500 nm have deposition velocities that increase with increasing size because of gravitational settling (Kruppa et al., 2019). As discussed in this chapter, lead-bearing particles in aircraft exhaust that are less than 20 nm in diameter have been observed to be in greater abundance than particles with larger diameters. Particles deposited onto surfaces, such as soil, can be resuspended by wind or mechanical action (e.g., traffic on paved and unpaved roads, agricultural tilling). In many cases, the resuspended particles will be agglomerates of the deposited particles and these larger particles have different transport properties and lung deposition patterns. Deposited particles can also undergo chemical or biological transformations. These processes do not affect the lead burden in the environment, but the chemical form can influence transport within soil matrices and uptake by vegetation. Lejano and Ericson (2005) found soil lead content to be markedly higher in areas close to major highways. Human exposure to lead particles from GA aircraft exhaust can occur via inhalation of airborne particles, inhalation of particles that deposit onto surfaces and are later resuspended, or ingestion through hand-to-mouth contact with surfaces where the particles have deposited (e.g., soil or locally grown fruits and vegetables). Further study of the complex processes involved in the environmental dynamics of lead will improve understanding of relationships between piston- engine aircraft emissions and human exposures. PAST STUDIES OF COMMUNITIES NEAR AIRPORTS Based on 2010 Census data, EPA (2020b) estimated that roughly 5.2 million people reside in a census block that intersects with a 500-meter buffer around an airport runway or a 50-meter buffer around a heliport.1 Of those people, 363,000 are children age 5 years and under. In addition, 573 public and private schools that enroll about 163,000 students (grades K-12) are located near an airport runway or heliport. The agency chose a distance of 500 meters because at that distance EPA estimated that airborne lead concentrations, averaged over 3 months (the averaging time used for the lead NAAQS) diminished to local background concentrations. EPA defined local background concentrations as the airborne lead concentrations that would be expected in the absence of a localized source, such as aircraft emissions (EPA, 2020c). However, because the estimated numbers of people are based on distance from airport runways, rather than distance from airport property boundaries (see Miranda et al., 2011), they likely underestimate the number of people living in residences or attending schools where relatively higher exposures are occurring. EPA 1 A census block is the smallest geographic area for which the U.S. Bureau of the Census collects and compiles census data, Populations were about 10 percent lower when restricting the analysis to end-of-runway buffers, instead of whole-perimeter runway buffers.
PREPUBLICATION COPY â Uncorrected Proofs 48 (2020b) acknowledged that on individual days, the impact of aircraft lead emissions can extend to almost 1,000 meters downwind from the runway of a highly active airport. Miranda et al. (2011) compared BLL surveillance data for children (ages 9 months to 7 years) to place of residence in six counties in North Carolina to examine associations between BLLs and proximity of the residence to an airport. Controlling for potential confounding by exposure to deteriorating lead-based paint, the authors found that children living within 1,000 meters of the property boundary of airports at which planes use leaded avgas have statistically higher BLLs than other children do. The estimated effect on BLLs exhibited a monotonically decreasing doseâresponse pattern, with the largest change in BLL (4.4 percent higher) on children living within 500 meters of an airport boundary. Two different studies based on BLLs lower than 10 Âµg/dl have estimated economic impacts associated with children exposed to lead from aviation gasoline emissions. Wolfe et al. (2016) estimated the nationwide annual costs of IQ losses from aircraft lead emissions. The authors developed a general aviation emissions inventory, including emissions from aircraft aloft, for the continental United States and modeled changes in atmospheric concentrations of lead. They used those concentrations to quantify the impacts of annual aviation lead emissions on the U.S. population. The authors found that aircraft lead emissions contribute to $1.06 billion 2006 USD ($0.01â$11.6) in annual damages from lifetime earnings reductions. Zahran et al. (2017) studied time and space relationships between BLL data from more than one million children and their proximity to 448 airports in Michigan. The authors found consistent evidence that avgas use is significantly linked to elevated BLLs in children residing near airports. They estimate the social damages (IQ point loss and IQ point loss to future earnings) attributable to leaded avgas consumption to be at least $10 per gallon. WORKER EXPOSURES AT AIRPORTS In occupational settings at airports, workers can be exposed to inorganic lead through inhalation and ingestion of dibromide particles emitted from the combustion of leaded avgas. Workers can also be exposed to organic lead in the form of TEL in evaporative and refueling emissions from uncombusted avgas. TEL can be absorbed through the skin, eyes, and mucous membrane. Uncombusted avgas has additional toxic components of concern, including volatile organic compounds (VOCs) and the fuel additive EDB. Calculated estimates of evaporative emissions of those compounds at airports are presented below. Later, this section considers industrial hygiene and occupational health requirements that cover worker exposures to inorganic lead, TEL, and other hazardous components. Evaporative and Refueling Emissions Avgas is a volatile liquid fuel under EPA definitions for motor vehicle fuel.2 Under ASTM D910, its vapor pressure range is 38-49 kPa at 38Â°C; this is normally referred to as Reid Vapor Pressure (RVP). In many publications, this is presented in pounds per square inch (psi), with a range of 5.5-7.1 psi. Except at the high end, RVPs in this range are below that normally seen in automotive gasoline. The only exception would be gasolines used in the summer ozone months in California, areas where federal regulations require the use of reformulated gasoline, and a few 2 See U.S. EPA definition at 40 CFR Â§ 86.1803-01.
PREPUBLICATION COPY â Uncorrected Proofs 49 counties where the RVP is 7.0 psi.3,4 As a volatile liquid fuel stored in a rigid or even semi-rigid container such as an aircraft fuel tank, it is the basic nature of the fuel to evaporate into the air in any ullage volume (unoccupied usable volume in the tank). These volumes taken together are commonly referred to as the tank headspace and will vary by tank design and the tank fill amount at any given time. The various compounds in the avgas will evaporate into the headspace until it is saturated. This evaporative process depends on temperature, the vapor pressure, the mass fraction of the compounds in the fuel, and to a lesser degree elevation (i.e., atmospheric pressure). The evaporation continues until a liquid-vapor equilibrium occurs between the compounds in the liquid fuel and corresponding compounds in the headspace. These evaporative processes are relatively rapid, so most of the time the headspace will be saturated (Gauss, 1973). Table 3-2 below shows calculated headspace percent concentrations for VOCs at 25Â°C for sea level for 5.5 and 7.1 psi RVP aviation gasoline. Grade 100LL avgas contains two other additive compounds (TEL and EDB) with relatively lower vapor pressures, which evaporate during handling and storage. Using the ideal gas law and a molecular weight of 68 g/mole for the fuel vapor at 5.5-7 psi, the hydrocarbon concentration in the headspace ranges from 2.6-3.35 grams VOC per gallon headspace. The vapor pressure of TEL at 25Â°C is 0.00387 psi and for EDB it is 0.2322 psi (NIOSH, 2007).5 As mentioned above, the concentration in the headspace depends on the mass fraction in the liquid. According to Table 1 of ASTM D910, for Grade 100LL, the allowable range for the TEL dose in the liquid is 0.27-0.53 ml TEL/liter avgas (0.28-0.56 g Pb/liter avgas). Using the high end of this D910 range, this calculates to a dosing rate of 3.31 g TEL per gallon of Grade 100LL avgas. Taking the TEL vapor pressure at 25Â°C and the high end of this dosing rate, the headspace concentration is 0.036 mg/gallon for Grade 100LL avgas.6 Using a 1:1 molar ratio dosing rate for TEL and EDB, the ppm concentration of EDB in avgas would be the same as for TEL (3.31 g/gal), but due to the higher vapor pressure, the headspace concentration is 2.05 mg/gal. TABLE 3-2 Calculated Headspace Concentrations RVP (psi) VOC Percent Concentration in Headspace at Saturation VOC in Headspace (g/gallon) at Saturation TEL in Headspace (mg/gallon) at Saturation EDB in Headspace (mg/gallon) at Saturation 5.5 24.7% 2.60 0.036 2.05 7.1 31.9% 3.35 0.036 2.05 3 Reid Vapor Pressure (RVP) Control Periods for California Air Basins and Counties are provided at: https://ww3.arb.ca.gov/fuels/gasoline/rvp/rvp_controlperiod.pdf. 4 EPA provides information on gasoline Reid Vapor Pressure at https://www.epa.gov/gasoline-standards/gasoline- reid-vapor-pressure. 5 See pp. 136 and 302 of NIOSH (2007). 6 While aviation gasoline related hydrocarbon emptying and breathing loss emissions from storage tanks should already be incorporated into local emission inventories, the 0.0365 mg/gallon concentration value for TEL and the 2.05 mg/gallon value for EDB may be useful in calculating the toxic emission inventories for these storage tanks.
PREPUBLICATION COPY â Uncorrected Proofs 50 While there are traditionally five sources of evaporative emissions for gasoline-powered motor vehicles, only two (diurnal and refueling) are important for GA aircraft. In 2017, there were approximately 172,000 active piston-engine GA aircraft based in the United States, which consumed about 192.43 million gallons of Grade 100LL and 3.87 million gallons of MOGAS (GAMA, 2018).7 Using an estimated weighted aircraft tank fuel volume of 87 gallons (70 percent at 60 gallons and 30 percent at 150 gallons)8 and a 10 percent ullage, gives a headspace of 52 gallons if tanks are half full. However, it is recommended practice that piston-engine aircraft refill after flight to reduce water vapor condensation into the stored fuel. If this full refill occurs one-half of the time, the average headspace would be 30 gallons. Using traditional EPA modeling techniques for diurnal and refueling emissions (EPA, 2014; Reddy, 1989), VOC, TEL, and EDB emissions in megagrams (Mg) can be estimated as shown in Table 3-3. TABLE 3-3 Estimated 2017 Nationwide Evaporative Emissions from Piston-Engine General Aviation Aircraft (25Â°C) Mg RVP (psi) Diurnal VOC Refueling VOC Diurnal TEL Refueling TEL Diurnal EDB Refueling EDB 5.5 2100 500 0.0681 0.00685 3.88 0.39 7.1 2485 645 0.0681 0.00685 3.88 0.39 The entries in the columns in Table 3-3 are not additive. For a best estimate for an annual inventory, the arithmetic average of the 5.5 and 7.1 psi RVP cases would seem representative. Taking one-half of the sum the diurnal and refueling VOC inventory values yield an annual value of 2,865 Mg. This is about 0.25 percent of the evaporative and refueling inventory for gasoline-powered highway motor vehicles.9 There is no corresponding TEL or EDB inventory for gasoline-powered highway motor vehicles, because TEL is not used in unleaded gasoline and EDB is not necessary. The combined evaporative and refueling inventory for TEL is estimated to be 0.075 Mg; this is only about 0.012 percent of the estimated amount of TEL added to Grade 100LL. The combined evaporative and refueling inventory for EDB is 4.27 Mg, about 0.67 percent of the estimated amount of EDB added to Grade 100LL in 2017. More information about EDB is provided in Appendix D. Industrial Hygiene and Occupational Health Requirements Health effects studies of exposure to elemental lead and lead-bearing compounds provide a basis for the current occupational exposure standards and guidelines (ACGIH, 2001a-c, 2017; ATSDR, 2020). Furthermore, NIOSH has published some excellent general reference materials that provide information on occupational lead exposures).10,11 The OSHA occupational exposure standards and related requirements (29 CFR Â§ 1910) apply to employees of fixed base operators (FBOs), repair and overhaul shops, and airports 7 See Tables 2.7 and 2.8 of GAMA (2018). 8 Aircraft Bluebook, Spring 2020, Vol. 20-01. Available at: https://aircraftbluebook.com/Tools/ABB/ShowSpecifications.do. 9 See pp. III-1 to III-19 of EPA (1999). 10 See NIOSH workplace lead publications at: https://www.cdc.gov/niosh/topics/lead/publications.html. 11 See the OSHA substance data sheet for occupational exposure to lead at https://www.osha.gov/laws- regs/regulations/standardnumber/1910/1910.1025AppA.
PREPUBLICATION COPY â Uncorrected Proofs 51 where exposures to lead bromide, TEL, or EDB may occur. The most common airport work areas with a potential for lead exposures are on and around the flight line and in repair and overhaul shops where GA aircraft that use avgas are maintained.12 The OSHA requirements also extend to employees who may be incidentally exposed (i.e., employees not involved in work which would routinely involve lead exposure but may work nearby or have very short-term or transient exposure periods). Potential exposures to flight crews are covered by FAA requirements instead of OSHA requirements.13 Responsible authorities in some states operate their own occupational safety and health programs under OSHA auspices (see Appendix E). Lead Dibromide Because lead dibromide is a combustion product found in engine exhaust, it could originate anywhere the aircraft engine is operating, including within maintenance and repair facilities and in operational areas on the airport grounds. With the large number of airports, aircraft operations, and the widespread use of leaded avgas, lead dibromide exposures are expected to occur commonly in workplaces involving ground support and maintenance operations or through incidental contact to those working nearby. As is discussed in detail in Appendix E, the expanded OSHA health standard for lead, 29 CFR Â§ 1910.1025, contains a permissible exposure limit (PEL) for inorganic lead (e.g., lead dibromide) and has very detailed and specific requirements. At a minimum, an employer is required to carry out an initial exposure determination for each employee or group of similarly exposed employees in the workplace to assess whether any employee may be exposed to airborne lead concentrations at or above an action level. The OSHA regulations cover personal exposure monitoring, education/training for any employee exposed to inorganic lead in the workplace and additional requirements for workplace controls, medical surveillance, and biological monitoring for exposures greater than the action level for employees within an exposure group. The regulations are also very prescriptive regarding communication with the exposed employees and recordkeeping. It is important to note that BLLs can remain elevated long after lead exposures have been reduced or eliminated, due to release of lead from adult bone into the blood. TEL Although the expanded OSHA standard for inorganic lead specifically excludes organic lead (e.g., TEL), it is covered by OSHAâs air contaminant standards (29 CFR Â§ 1910.1000 Table Z-1 and 29 CFR Â§ 1926.55). Inhalation, ingestion, and dermal exposures to TEL can occur as a result of activities, such as handling engine parts that are wetted with leaded avgas by mechanics, the dispensing and inadvertent spillage of avgas that is being dispensed by aircraft ground service operators, or the improper use of avgas as a shop solvent for parts cleaning or perhaps other purposes. 12 For an example of a comprehensive airport lead workplace exposure and program assessment, see Chen and Eisenberg (2013). 13 Personal communication, David Valiante, OSHA, June 17, 2020.
PREPUBLICATION COPY â Uncorrected Proofs 52 EDB Inhalation, ingestion, and dermal exposures to EDB may occur as a result of activities, such as those identified above for TEL. Exposure assessments and related requirements for EDB are covered under OSHAâs air contaminants standard (29 CFR Â§ 1910.1000 Table Z-2 and 29 CFR Â§ 1926.55). There are separate PELs and action levels for TEL and EDB and an assessment is required for TEL and EDB exposures, as applicable. However, the OSHA standards are less prescriptive for these contaminants relative to inorganic lead and may allow for a negative initial determination to be made through either personal exposure monitoring, application of data from similar workplaces conducting similar tasks, engineering evaluation, or worst case exposure calculations. In this case, the exposure determination may be either qualitative or quantitative. If the initial assessment indicates that the exposure is above the action level, this must also be documented and either more personal exposure monitoring or workplace controls, or both may be required. Either way, the basis for this determination must be documented and the records retained. The assessment and response to workplace lead is complicated because inorganic lead and TEL exposures have an additive health impact. This requires that in addition to the assessments and responses for exposures for each contaminant (inorganic lead and TEL) for those employees exposed to both, the assessment must also include evaluation of the exposure as a mixture (see Appendix E). FINDINGS AND RECOMMENDATIONS There are no known safe levels of human lead exposure, as measured by blood lead levels. Lead exposure can result in significant negative health effects, particularly among children, and unlike some other metals there is no known biological function of even trace amounts of lead in the human body. The importance of reducing lead pollution motivates the development and implementation of mitigation measures to reduce or eliminate lead emissions from GA aircraft (Finding 3.1). 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, 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 that 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 mitigation efforts at individual airports (Finding 3.2). EPA should conduct more targeted monitoring and enhanced computational modeling of airborne lead concentrations at airports of potential concern, as indicated by its recent screening study, to evaluate aircraft operations that are main contributors to lead hot spots and design airport-specific mitigation measures. (Hot spots often refer to a spatial zone of emissions impact where the airborne lead concentration is significantly elevated above background.) In addition to airports found to have airborne lead concentrations exceeding the concentration of the lead National Ambient Air Quality Standards (NAAQS),
PREPUBLICATION COPY â Uncorrected Proofs 53 additional monitoring and modeling should include airports found to have lead concentrations that are lower, but approaching, the NAAQS concentration (Recommendation 3.1). Past emissions from piston-engine aircraft that deposited to soil and other surfaces can contribute to present-day lead exposures at locations within and near airports (Finding 3.3). EPA and NIEHS 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) Lead in piston-engine aircraft exhaust has been observed to occur in the form of beads about 4 nanometers (nm) in diameter embedded in particles with diameters less than 20 nm. Those particles are smaller than the lead particles observed in automobile exhaust. Smaller particles may deposit and distribute in the body differently than larger-sized particles that have been the subject of more research in past. Thus, it is important to understand the particle size properties of lead emitted from aircraft and how those properties affect atmospheric transport and deposition as well as human exposure-response relationships (Finding 3.4). EPA and NIEHS should sponsor research to improve the understanding of the physical state of the lead-containing particulate matter emitted from various types of GA-aircraft piston engines, including turbocharged engines, using fuel formulations of different lead content, including an existing grade of avgas with a lower lead content (100VLL), to inform future studies of atmospheric transport and deposition, human exposure, and health risks of lead emissions form GA aircraft (Recommendation 3.3) Based on the nature of the workplace activities with GA aircraft, lead exposures are expected to occur for flight line and maintenance shop workers, including those employed by the airport itself, FBOs, and repair/overhaul facilities. Workplace lead exposures include not only inhalation of airborne emissions, but also inhalation, ingestion, and dermal absorption of the fuels additives TEL and EDB as a result of aircraft refueling and maintenance activities. OSHA regulations, including permissible exposure limits and related requirements, apply for each of these contaminants (Finding 3.5). REFERENCES ACGIH (American Conference of Governmental Industrial Hygienists). 2001a. Lead, Elemental and Inorganic. Documentation of the Biological Exposure Indices-BEIÂ®, 7th ed. Cincinnati, OH: ACGIH. ACGIH. 2001b. Lead and Inorganic Compounds. Documentation of TLVÂ® Chemical Substances, 7th ed. Cincinnati, OH: ACGIH. ACGIH. 2001c. Tetraethyl Lead. Documentation of TLVÂ® Chemical Substances, 7th ed. Cincinnati, OH: ACGIH.
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