Risk assessment for airborne disease transmission by poly-pathogen aerosols

Abstract

In the case of airborne diseases, pathogen copies are transmitted by droplets of respiratory tract fluid that are exhaled by the infectious and, after partial or full drying, inhaled as aerosols by the susceptible. The risk of infection in indoor environments is typically modelled using the Wells-Riley model or a Wells-Riley-like formulation, usually assuming the pathogen dose follows a Poisson distribution (mono-pathogen assumption). Aerosols that hold more than one pathogen copy, i.e. poly-pathogen aerosols, break this assumption even if the aerosol dose itself follows a Poisson distribution. For the largest aerosols where the number of pathogen in each aerosol can sometimes be several hundred or several thousand, the effect is non-negligible, especially in diseases where the risk of infection per pathogen is high. Here we report on a generalization of the Wells-Riley model and dose-response models for poly-pathogen aerosols by separately modeling each number of pathogen copies per aerosol, while the aerosol dose itself follows a Poisson distribution. This results in a model for computational risk assessment suitable for mono-/poly-pathogen aerosols. We show that the mono-pathogen assumption significantly overestimates the risk of infection for high pathogen concentrations in the respiratory tract fluid. The model also includes the aerosol removal due to filtering by the individuals which becomes significant for poorly ventilated environments with a high density of individuals, and systematically includes the effects of facemasks in the infectious aerosol source and sink terms and dose calculations.

Exhaled particles from nanometre to millimetre and their origin in the human respiratory tract

“…In particular, small particles, e.g. <10 µm, shrink by evaporation in a fraction of a second under typical ambient conditions and remain in the air for a long time [e.g., see [12][13][14]29]. In addition, the situation-specific particle size and concentration determine the protective properties of filtering face masks [see e.g.…”

“…, 14]. The shrinkage of these particles, defined as the particle diameter in the respiratory tract compared to the equilibrium size they reach at ambient relative humidity (RH), determines their deposition rates on surfaces and residence times in the air [13,14,[22][23][24][25][26][27][28]. In particular, small particles, e.g.…”

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

“…Ventilation rates and deposition in the respiratory tract of the susceptible individual are calculated based on the ICRP lung deposition model [52], where the inhaled aerosols undergo time-dependent hygroscopic growth and under best estimates are only composed of water and NaCl. The risk of infection is calculated under the assumption of an exponential model and monopathogenic aerosols [5]. Note that for the parameters studied here, the polypathogen model yields a lower infection risk [5] and therefore the monopathogen model is best for estimating the upper bound.…”

“…The risk of infection is calculated under the assumption of an exponential model and monopathogenic aerosols [5]. Note that for the parameters studied here, the polypathogen model yields a lower infection risk [5] and therefore the monopathogen model is best for estimating the upper bound. We also assumed that the initial absorbed dose to the susceptible individual was zero.…”

“…While moist exhaled aerosols larger than 50 µm settle by gravitational deposition on room surfaces before drying, smaller droplets dry rapidly, thereby shrink, and due to their small mass remain airborne for extended periods of time [3]. Such aerosols may contain single or multiple copies of pathogens when exhaled by an infectious, and when inhaled by a susceptible, there is a risk of infection given by the absorbed dose of pathogens [5]. In order to control and minimize airborne transmission risk by infectious aerosols, it is essential to further our understanding of aerosol dynamics and dispersion across their large spectrum of size distributions.…”