Real-Time and Integrated Measurement of Potential Human Exposure to Particle-Bound Polycyclic Aromatic Hydrocarbons from Aircraft Exhaust - PAHs

Environmental Health Perspectives, Sept, 2000 by Jeffrey W. Childers, Carlton L. Witherspoon, Leslie B. Smith, Joachim D. Pleil

We used real-time monitors and low-volume air samplers to measure the potential human exposure to airborne polycyclic aromatic hydrocarbon (PAH) concentrations during various flight-related and ground-support activities of C-130H aircraft at an Air National Guard base. We used three types of photoelectric aerosol sensors (PASs) to measure real-time concentrations of particle-bound PAHs in a break room, downwind from a C-130H aircraft during a four-engine run-up test, in a maintenance hangar, in a C-130H aircraft cargo bay during cargo-drop training, downwind from aerospace ground equipment (AGE), and in a C-130H aircraft cargo bay during engine running on/off (ERO) loading and backup exercises. Two low-volume air samplers were collocated with the real-time monitors for all monitoring events except those in the break room and during in-flight activities. Total PAH concentrations in the integrated-air samples followed a general trend: downwind from two AGE units [is greater than] ERO-loading exercise [is greater than] four-engine run-up test > maintenance hangar during taxi and takeoff [is greater than] background measurements in maintenance hangar. Each PAH profile was dominated by naphthalene, the alkyl-substituted naphthalenes, and other PAHs expected to be in the vapor phase. We also found particle-bound PAHs, such as fluoranthene, pyrene, and benzo[a]pyrene in some of the sample extracts. During flight-related exercises, total PAH concentrations in the integrated-air samples were 10-25 times higher than those commonly found in ambient air. Real-time monitor mean responses generally followed the integrated-air sample trends. These monitors provided a semiquantitative temporal profile of ambient PAH concentrations and showed that PAH concentrations can fluctuate rapidly from a baseline level [is less than] 20 to [is greater than] 4,000 ng/[m.sup.3] during flight-related activities. Small handheld models of the PAS monitors exhibited potential for assessing incidental personal exposure to particle-bound PAHs in engine exhaust and for serving as a real-time dosimeter to indicate when respiratory protection is advisable. Key word: engine exhaust, human exposure, integrated-air samplers, JP-8 fuel, PAH, polycyclic aromatic hydrocarbons, real-time PAH monitors. Environ Health Perspect 108:853-862 (2000). [Online 31 July 2000]

http://ehpnet1.niehs.nih.gov/docs/2000/108p853-862childers /abstract.html

The potential exposure of maintenance personnel, flight crews, and passengers to aircraft fuels and exhaust is of concern to the military and the commercial airline industry. To address these concerns, the National Exposure Research Laboratory of the U.S. Environmental Protection Agency and the U.S. Air Force (USAF) Surgeon General's Office initiated a collaborative methods development program to characterize the exposure of military and civilian personnel to aircraft fuels and exhaust at or near airports, in maintenance hangars, and during flight-related activities.

To date, significant effort has been directed toward developing methods and making exploratory measurements to assess human exposure to volatile organic compounds (VOCs) in JP-8 aircraft fuel (1-3). Parallel efforts are currently under way to characterize exposure to VOCs, semivolatile organic compounds (SVOCs), and nonvolatile organic compounds (NVOCs) in aircraft exhaust. One class of SVOCs and NVOCs associated with aircraft exhaust that is of particular concern is polycyclic aromatic hydrocarbons (PAHs). These compounds are formed by the incomplete combustion of fossil fuels and other organic matter (4) and are distributed into the air in the vapor phase or the particulate phase through adsorption or condensation on the surface of respirable particles (5,6). Several PAHs are listed by the National Toxicology Program as "reasonably anticipated to be a human carcinogen" (7). PAHs ranked as probable human carcinogens are primarily associated with the particulate phase. Therefore, the characterization of incidental or chronic inhalation exposure to particle-bound PAHs in aircraft exhaust is critical to assessing the health risks related to aircraft support, maintenance, and usage.

PAH concentrations in ambient air or indoor microenvironments are typically determined by using integrated-air samplers to collect vapor-phase and particle-bound PAHs on a combination filter/sorbent cartridge (8). The collected PAHs are extracted from the cartridge with a suitable solvent and then quantified using an appropriate analytical technique such as gas chromatography/mass spectrometry (GC/MS) or high-performance liquid chromatography. This multistep sampling and analysis method provides chemical speciation of the PAHs in the air sample and can be used to determine the average exposure to specific PAHs during a given monitoring period. This method, however, does not produce a direct report of real-time PAH concentrations or a temporal record of acute exposures during episodes of high PAH emissions. In addition, integrated-air sampling laboratory analysis procedures are time-consuming, labor intensive, and expensive. These drawbacks led to the development (9-13) and evaluation of real-time PAH monitors (14-23).

One real-time monitor for measuring airborne particle-bound PAHs is based on a photoelectric aerosol sensor (PAS) (9-13). When particles coated with a submonolayer of PAH are irradiated with ultraviolet (UV) light that has an energy above the photoelectric threshold of the surface-bound PAH, the particle will emit a photoelectron and become positively charged. In a PAS system these positively charged particles are collected on a filter electrometer. The current measured across the electrometer is proportional to the number of charged particles created by the photoemission process. Strong correlations between the PAS response and total PAH (13-16,22,23) or individual PAHs, such as benzo[a]pyrene (17), have been documented in laboratory and field experiments. The PAS response is related to particle size, surface coverage, photoionization potential, molecular structure, and geometry of specific PAHs (12,16). The photoelectric threshold is lower for PAHs with a large [Pi]-electron system (12). Therefore, the photoionization process is more efficient for larger PAHs, i.e., those containing four or more fused aromatic rings, which are typically associated with the particulate phase (5, 6). Vapor-phase PAHs are not photoionized by UV light (22), and large particles have a high probability of recapturing the emitted photoelectron. The PAS system therefore is presumed to respond to surface-bound PAHs on ultrafine particles only.