Aerosol transport measurements and assessment of risk from infectious aerosols: a case study of two German cash-and-carry hardware/DIY stores

Abstract

We report experimental results on aerosol dispersion in two large German cash-and-carry hardware/DIY stores to better understand the factors contributing to disease transmission by infectious human aerosols in large indoor environments. We examined the transport of aerosols similar in size to human respiratory aerosols (0.3μm-10μm) in representative locations, such as high-traffic areas and restrooms. In restrooms, the observed decay of aerosol concentrations was consistent with well-mixed air exchange. In all other locations, fast decay times were measured, which were found to be independent of aerosol size (typically a few minutes). From this, we conclude that in the main retail areas, including at checkouts, rapid turbulent mixing and advection is the dominant feature in aerosol dynamics. With this, the upper bound of risk for airborne disease transmission to a susceptible is determined by direct exposure to the exhalation cloud of an infectious. For the example of the SARS-CoV-2 virus, we find when speaking without a face mask and aerosol sizes up to an exhalation (wet) diameter of 50μm, a distance of 1.5me to be unsafe. However, at the smallest distance between an infectious and a susceptible, while wearing typical surgical masks and for all sizes of exhaled aerosol, the upper bound of infection risk is only ∼ 5% and decreases further by a factor of 100 (∼ 0.05%) for typical FFP2 masks for a duration of 20 min. This upper bound is very conservative and we expect the actual risk for typical encounters to be much lower. The risks found here are comparable to what might be expected in calm outdoor weather.

Face-masks save us from SARS-CoV-2 transmission

“…al. [8] of aerosol transport in large hardware stores. The decay rates measured in the large indoor space of hardware stores are size independent and τ is approximately constant for all measured particle sizes (0.3-10 µm) [8].…”

“…[8] of aerosol transport in large hardware stores. The decay rates measured in the large indoor space of hardware stores are size independent and τ is approximately constant for all measured particle sizes (0.3-10 µm) [8]. The small measured decay rates in large indoor spaces suggest that existing well-mixed contaminants in the environment do not play a critical role in infection risk calculations and that the strongest probability of infection is created by direct contact with an infected individual and their exhale particle emission.…”

“…al. [8] where τ is dependent on particle size. In such small spaces, fine particles linger in the environment for longer times and even large particles (>5 µm) have considerably long residence times as compared to well ventilated large spaces.…”

“…The advection and turbulent diffusion of the exhale cloud is strongly influenced by the ambient thermodynamical conditions and the airflow, e.g. the type and strength of the ventilation in indoor environments, or outdoors, as well as in large indoor spaces the air currents that carry away the exhale [8]. While being advected, particles may lose their volatile content due to evaporation and shrinkage, which is determined by their chemical composition, ambient thermodynamical conditions, exhale flow velocity, mixing with the ambient air, and the time they need to reach an equilibrium size [9,4].…”

“…Here, we report an analysis of airborne disease transmission, specifically COVID-19, based on new data and models combined with improvements to established models and include 1. the human exhalation particle database published in [24], 2. the total inward mask penetration measured on human subjects in this study, 3. the localized aerosol dispersion in large indoor spaces from recent investigations of [8], 4. the mono-/poly-pathogen infection risk model [3], 5. the established respiratory tract deposition model [37] refined to include time-dependent hygroscopic growth for particles entering the respiratory tract, 6. the particle shrinkage due to evaporation.…”