The research, published in the journal Air Quality, Atmosphere & Health by a team led by Wei-Chung Su of Baylor University with colleagues at Boston University, the University of Texas Health Science Center at Houston, the University of North Texas Health Science Center, and Temple University, stands apart from most prior PAH risk assessments in a crucial way. The majority of published studies worldwide, from Xi’an and Tehran to Athens, Algiers, Hanoi, and Tula, have evaluated PAHs bound to larger particles such as PM2.5 or PM10, and nearly all of them implicitly assumed that every inhaled particle deposits entirely in the respiratory tract. That assumption overstates the dose actually delivered to lung tissue. The Houston team instead measured, in the field, the fraction of inhaled ultrafine particles that genuinely lodges in each region of the respiratory system, then combined those measurements with size-resolved chemical composition data to calculate the true deposited mass of each PAH.

The key instrument behind this advance is the Mobile Aerosol Lung Deposition Apparatus, or MALDA, a portable system containing a set of 3D-printed replicas of the human airways. MALDA reproduces the three major regions of the respiratory tract: the head airways, including the nasal and oral cavities; the tracheobronchial tree, extending down to the eleventh airway bifurcation; and the alveolar region, constructed from conductive porous foam. During the field campaigns, the apparatus was operated at an inspiratory flow rate of 30 liters per minute through the nasal airway, corresponding to a minute ventilation of 15 liters per minute, the breathing pattern of an adult engaged in light outdoor activity such as walking. Nanoparticle sizers connected to the MALDA inlet measured the size distribution of particles entering the system and the depleted distribution downstream, allowing the researchers to derive size-dependent deposition fractions for each airway region directly from real urban aerosol.

Sampling took place at three sites chosen to represent distinct exposure environments across the metropolitan area. Park Place Boulevard captured highway traffic emissions; Clinton Drive reflected a mix of refinery industries, truck exhaust, and the Port of Houston ship channel; and the School of Public Health site in the Texas Medical Center hospital district represented local vehicle exhaust. Measurements rotated among the sites from 2021 to 2022, spanning hot and cool seasons and yielding 39, 63, and 62 MALDA datasets at the three locations respectively. Across all sites and seasons, the peak in the ultrafine particle size distribution sat at roughly 40 nanometers. The industrial Clinton Drive site showed the highest average particle concentrations, while the hospital district showed the lowest, a contrast that would later prove central to interpreting the risk results.

To determine what chemicals the particles carried, the team deployed a rotating aerosol cascade impactor, the MOUDI, which sorts particles by aerodynamic diameter across eleven stages ranging from 56 nanometers to 18 micrometers. The three smallest stages, at 56, 100, and 180 nanometers, served as the ultrafine fraction. Because each size fraction collects only a tiny mass of material, accumulating enough particulate matter for reliable chemical analysis required extraordinary patience: roughly 300 to 400 hours of cumulative sampling, ten hours per day over 30 to 40 sampling days per period. The collected PAHs were extracted following the U.S. EPA TO-13A method, using a dichloromethane-methanol mixture and sonication, and then quantified by gas chromatography-mass spectrometry. The analysis produced size-dependent PAH-to-particle mass ratios, generally below the order of 10 to the minus 4 and mostly in the 10 to the minus 6 range.

An intriguing pattern emerged from those ratios. Several PAHs showed increasing mass fractions as particle size decreased, a signature of volatile components gradually evaporating from the particles and leaving the less volatile, often more hazardous PAHs enriched on the smallest ones. This matters because deposition in the lung is strongly size-dependent. The MALDA measurements showed that the alveolar region, the deepest gas-exchange zone of the lungs, captured a higher fraction of inhaled ultrafine particles than the head or tracheobronchial airways, with the highest deposition fraction observed at 15 nanometers. Since deposition patterns were similar across all three sites, the ambient particle concentration itself became the dominant factor determining how much PAH actually reached residents’ lungs.

Combining the deposition fractions, particle size distributions, and PAH mass ratios, the researchers calculated the hourly deposited mass of each PAH in the airways, values spanning roughly 10 to the minus 10 to 10 to the minus 8 milligrams per hour across sites. These deposited masses fed into standard U.S. EPA exposure equations, assuming three hours of daily outdoor exposure, 350 exposure days per year, a 30-year exposure duration, and a 70-kilogram body weight. For non-cancer effects, the team computed hazard quotients using reference doses, summing them into a hazard index because the PAHs examined produce similar respiratory effects. For cancer risk, they multiplied lifetime average daily doses by cancer slope factors, with one in one million as the conventional threshold of acceptability.

The results were striking in their margin of safety. Every hazard quotient fell below 1.0, and the summed hazard indices ranged from about 10 to the minus 5 to 10 to the minus 4, orders of magnitude below the threshold for concern. Estimated excess lifetime cancer risks ranged from 10 to the minus 13 to 10 to the minus 11, far beneath the 10 to the minus 6 acceptability line. A probabilistic Monte Carlo analysis with 5,000 iterations, propagating uncertainty in deposited mass and exposure hours, confirmed the picture: the highest 95th-percentile non-cancer risk was 2.3 times 10 to the minus 3 for benzo[a]pyrene at Clinton Drive, and the highest 95th-percentile cancer risk was 9.0 times 10 to the minus 9 for dibenz[a,h]anthracene at Park Place Boulevard. Even so, the authors caution that sensitive groups could face higher doses; children’s lower body weight may yield body-weight-normalized doses two to three times the adult estimate, and outdoor workers breathing 50 percent faster over eight-hour shifts could see doses up to four times higher, though the enormous safety margins suggest conclusions would hold even for these populations.

Compared with international studies, the Houston cancer risk estimates are several orders of magnitude lower than most reported values, which typically range from 10 to the minus 7 in Athens and Cyprus up to an unacceptable 10 to the minus 4 in Hanoi and Tula. The discrepancy largely reflects the different particle fractions studied, since particle mass scales with the cube of diameter and larger PM2.5 particles can simply carry far more PAH. The one available comparison for the ultrafine fraction, a study in southern Thailand, reported PM0.1 cancer risks of 10 to the minus 9 to 10 to the minus 8, matching the Houston findings. Importantly, when the team combined the present PAH results with their earlier companion study of toxic metals in the same ultrafine particles, both risk categories remained within acceptable limits, with metals consistently posing higher risks than PAHs. The framework does have limits: oxygenated, nitrated, and alkylated PAH derivatives, black carbon, and other hazardous organics were beyond its scope, and no regulatory reference doses exist for black carbon at all. Still, the deposited-mass approach offers a transferable methodology for environmental and occupational settings alike, and the findings suggest that, at least for the parent PAHs measured, everyday urban exposure in Houston is unlikely to drive PAH-related respiratory harm, a result that could inform emission standards and air quality advisories in cities worldwide.

Subject of Research: Health risk assessment of PAHs deposited from inhaled urban ultrafine particles in the Greater Houston Area

Article Title: PAH-associated health risk from ultrafine particles: an urban field study

Article References: PAH-associated health risk from ultrafine particles: an urban field study. (n.d.). https://doi.org/10.1007/s11869-026-02118-x

Image Credits: AI Generated

DOI: 10.1007/s11869-026-02118-x

Keywords: ultrafine particles, PAHs, respiratory deposition, health risk assessment, air pollution, Houston, MALDA, MOUDI, cancer risk, urban air quality, toxicology, environmental health