Single-pass wind tunnel testing for recirculating virus aerosol control technologies

A number of recirculating flow aerosol control technologies have been commercialized to mitigate aerosol-transmitted virus infections. Many of these technologies incorporate filters for particle collection and some may also incorporate technologies for virus inactivation. Given the wide variety of commercially available aerosol control technologies to consumers, it is extremely important to develop standardized methods to characterize their performance in bioaerosol removal and inactivation, such that technologies can be compared on an “equivalent-test” basis. However, no standard procedures have been established to evaluate the effectiveness of bioaerosol removal and inactivation in recirculating aerosol control technologies. We propose the use of a single-pass tunnel to assess the performance of bioaerosol control technologies, as single-pass wind tunnels can be sealed with well-controlled velocity and particle concentration profiles. Here, we specifically describe the construction of a single-pass wind tunnel and apply it to three recirculating aerosol control technologies, incorporating UV-C sources, filters, and electrostatic precipitators, respectively. We utilize a porcine respiratory coronavirus (PRCV) challenge aerosol, generated via pneumatic nebulization of a high titer (∼10⁷ TCID₅₀ mL⁻¹) viral suspension. Following guidelines similar to those used in the ANSI/ASHRAE Standard 52-2 test procedure for HVAC filters, in single-pass wind tunnel tests, velocity uniformity and particle uniformity are first monitored across the cross-section of the tunnel. The size distribution of viable particles is additionally determined in advance of tests by the collection of particles in the wind tunnel using a cascade impactor, with both RT-qPCR and titration used to quantify viruses collected on each impaction stage. We show that the viable particle size distribution follows the volumetric size distribution of the nebulized virus-laden suspension, and that this distribution can be tuned to be similar in shape to the observed distribution of aerosol from human respiratory activities. Following tunnel and virus aerosol characterization, for each tested technology, using triplicate tests, the single-pass log reduction based on RT-qPCR and viable virus titration is determined by simultaneously collecting virus aerosol particles upstream and downstream of the control technology. The tested technologies in this study have titration-based single-pass log reductions in the 1.5–4.0 range. Overall, design and testing suggest that the single-pass wind tunnel approach is a tractable method to examine the efficacy of aerosol control technologies in removing and inactivating viruses in aerosols, and suggest that such technologies should be described by their single-pass log reduction and operating flow rate, with the test virus size distribution reported alongside test results. In addition, we examine the limits of detection in single-pass wind tunnel tests in comparison to chamber tests, and in doing so find that for most control technologies, the wind tunnel test will yield higher concentrations downstream or during sampling, and hence clearer results for the log reduction.