  In the past two decades, novel viruses capable of airborne transmission have
emerged with alarming frequency, including SARS-CoV-1 in 2003, H1N1 in 2009,
MERS in 2012, and SAR-CoV-2 in 2019. Yet, in many countries, controls on
airborne transmission were not widely adopted until the COVID-19 pandemic. To
optimize strategies for curbing the transmission of existing airborne viruses
and to prepare for outbreaks of novel viruses in the future, the efficacy of
three key controls -- face masks, ventilation, and physical distancing -- must
be well understood. In this study, we used the new Quadrature-based model of
Respiratory Aerosol and Droplets (QuaRAD) to quantify the efficacy of controls
across thousands of scenarios that represent the wide variability in factors
governing airborne transmission. We show that, while the efficacy of any
individual control was highly variable among scenarios, the combination of
universal mask-wearing and distancing of at least 1~m reduced the median risk
of initial infection in the susceptible person by 99\% relative to a close
(0.5~m), unmasked conversation. Increasing ventilation rates by 4~air changes
per hour led to further reductions in the median risk of infection by more than
70\% if the two people were distanced by 2~m or more. The combination of face
masks, distancing, and increased ventilation reduced the risk of infection by
more than 98\% in more than 95\% of scenarios. These findings suggest that
layering controls is highly effective for reducing transmission of airborne
pathogens and will be critical for curbing outbreaks of novel viruses in the
future.
