Per- and polyfluoroalkyl substance (PFAS) aerosols inhibit lung surfactant function: estimation of aerosol deposition and effect of carbon chain length.
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Background and Purpose:
Lung surfactant (LS) critically impacts respiration by creating an air-liquid interface that reaches near-zero surface tension (ST) during compression. Inhibition of LS function is a molecular initiating event in an adverse outcome pathway resulting in decreased lung function via alveolar collapse. Previous studies used a constrained drop surfactometer (CDS) for screening chemicals and nanoparticle mixtures for in vivo toxicity by measuring LS inhibition in vitro. Past work, however, has not investigated aerosolized PFAS compounds which are ubiquitous in the atmosphere, and CDS systems may lack dosimetric characterizations to understand deposition of aqueous aerosols. The current study characterizes deposition in a newly developed CDS model using fluorescent tracers and assesses the inhibitory effects of PFAS compounds with varying carbon chain lengths on LS function.
Methods:
In a heated (37°C) 400 cc environmental chamber, a 10 mL droplet of Curosurf LS (2.5 mg/mL) on a knife-edge pedestal was oscillated at a rate of 20 cycles/minute. A tensiometer equipped with a high-resolution, high-speed camera monitored ST, surface area, and volume of the droplet. Following a 60s baseline period (minimum ST <5 mN/m), a test compound (fluorescent tracer or PFAS chemical) in 0.1% saline was aerosolized into the chamber. Deposition was complete by 60 seconds, and mass deposited on the droplet was assessed by measuring its fluorescence. In PFAS tests, the drop continued to oscillate, and the first 10 cycles following complete deposition were used to determine an average minimum ST. LS inhibition was defined as an average minimum ST > 10 mN/m. Tested PFAS compounds included perfluorooctane sulfonic acid (PFOS), N-ethylperfluorooctanesulfonamidoethanol (EtFOSE), perfluorohexane sulfonic acid (PFHxS), and trifluoroacetic acid (TFA), and each concentration of PFAS was tested in triplicate.
Results:
Tracer tests showed that nebulizing 30 µL and allowing a 60-second deposition period were ideal for consistent deposition between runs, with good inter-day reliability. On average, the deposition of negatively charged fluorescein on the 10 µL drop was 0.0604% of the input, while rhodamine, a positively charged tracer, demonstrated a slightly lower average deposition (0.0336%). Taken together, a deposition factor of 0.05% was used to estimate the final droplet PFAS concentrations. Using 0.2%, 0.63%, and 2% PFAS compound solutions, we estimated that 30, 94.5, and 300 nanograms, respectively, deposited on the droplet. PFOS, EtFOSE, and PFHxS inhibited LS at their highest test concentrations. While EtFOSE did not inhibit LS at lower concentrations, a 2% EtFOSE solution induced an average minimum ST of 17.89 mN/m that was significantly higher than the saline control. Solutions of 0.2%, 0.63%, and 2% PFOS elevated average minimum ST to 9.83, 10.31, and 10.96 mN/m, all of which differed significantly from saline controls while only the latter two inhibited LS. 2% PFHxS raised minimum ST to 10.59 mN/m, inhibiting LS function and increasing ST significantly from the saline control, while TFA did not significantly change minimum ST.
Conclusions:
PFAS compounds that inhibited LS were sulfonic acids or sulfonamides (PFOS, EtFOSE, PFHxS). These compounds had longer carbon chains than the ultrashort-chain TFA, indicating that chain length or presence of a sulfoxide group may influence a compound’s ability to inhibit LS. Additionally, final drop concentrations in runs that produced inhibition were much lower (8-14x) than the lowest adverse effect concentrations reported for PFOS and PFHxS in a study that directly mixed these compounds with LS. Our findings emphasize the importance of interactions at the air-liquid interface in inhibiting LS, which may have important health implications for airborne exposure to PFAS. (This abstract does not represent U.S. EPA policy)