  The question of how SARS-CoV-2 is transmitted remains surprisingly
controversial today, especially with reference to airborne transmission. In
fact, despite a large body of scientific evidence, health and regulatory
authorities still require direct proof of this mode of transmission. To close
this gap, we measured the saliva viral load of SARS-CoV-2 of an infected
subject located in a hospital room, as well as the airborne SARS-CoV-2
concentration in the room resulting from the person breathing and speaking. As
the next step, we simulated the same scenarios to estimate the concentration of
RNA copies in the air through a novel predictive theoretical approach. Finally,
we conducted a comparative analysis (i.e. a metrological compatibility
analysis) of the differences between the experimental and theoretical results
by estimating the uncertainties of these two approaches. Our results showed
that for an infected subject's saliva load ranging between 2.4x106 and 5.5x106
RNA copies mL-1, the corresponding airborne SARS-CoV-2 concentration was not
detectable when the person was breathing, but was 16.1 (with an uncertainty of
32.8) RNA copies m-3 when speaking. The application of the novel predictive
estimation approach provided average concentrations of 3.2 (uncertainty range
of 0.2-8.3) and 18.5 (uncertainty range of 4.5-43.0) RNA copies m-3 for
breathing and speaking scenarios, respectively, thus confirming that for the
breathing scenario, the airborne RNA concentration would be undetectable, being
below the minimum detection threshold of the experimental apparatus (< 2 RNA
copies m-3).
