An impactor-based aerosol platform for probing indoor, short-range transmission dynamics: a Phi6 bacteriophage study

1. ABSTRACT Short-range transmission is a driver of airborne disease spread. However, limited knowledge exists on the immediate impact of host, environmental, and seasonal factors on viable pathogen-laden droplets (VPLD) shortly after release. This work modelled the effects of respiratory droplet size, airway mucus composition, viral loads, and seasonal variations in indoor relative humidity (RH) on the number of VPLD collected shortly after equilibration. Clinically relevant concentrations of the SARS-CoV-2 surrogate bacteriophage, Phi6, were prepared in solutions reflecting the solute content of the airway mucus in healthy and disease states. Low solute levels and viral loads, characteristic of the presymptomatic phase, increased VPLD counts (initial diameter range, 5.59 to 20.28 µm) under low indoor RH in temperate winter. Elevated solute levels and decreased viral loads, characteristic of the late symptomatic phase, reduced VPLD counts (initial diameter range, 1.73 to 20.28 µm) under intermediate RH in temperate summers. High viral loads, characteristic of the early symptomatic phase, resulted in comparable VPLD counts (initial diameter range, 1.73 to 20.28 µm) across solute levels and RH, indicating a buffering effect of high viral loads against inactivation. These findings, integrated with complementary lines of evidence, were used to describe potential mechanisms of short-range transmission dynamics. 3. SYNOPSIS This study investigated the effects of four factors including respiratory droplet size, mucus composition, viral load, and seasonal variations in indoor relative humidity, on the recovery of viable pathogen-laden droplets to understand short-range transmission dynamics.