  To mitigate the SARS-CoV-2 pandemic, officials have employed social
distancing and stay-at-home measures, with increased attention to room
ventilation emerging only more recently. Effective distancing practices for
open spaces can be ineffective for poorly ventilated spaces, both of which are
commonly filled with turbulent air. This is typical for indoor spaces that use
mixing ventilation. While turbulence initially reduces the risk of infection
near a virion-source, it eventually increases the exposure risk for all
occupants in a space without ventilation. To complement detailed models aimed
at precision, minimalist frameworks are useful to facilitate order of magnitude
estimates for how much ventilation provides safety, particularly when
circumstances require practical decisions with limited options. Applying basic
principles of transport and diffusion, we estimate the time-scale for virions
injected into a room of turbulent air to infect an occupant, distinguishing
cases of low vs. high initial virion mass loads and virion-destroying vs.
virion-reflecting walls. We consider the effect of an open window as a proxy
for ventilation. When the airflow is dominated by isotropic turbulence, the
minimum area needed to ensure safety depends only on the ratio of total viral
load to threshold load for infection. The minimalist estimates here convey
simply that the equivalent of ventilation by modest sized open window in
classrooms and workplaces significantly improves safety.
