Since writing the previous blog entry (“What is my risk?”) I have encountered additional information to refine the risk calculation I outlined.  I found a reference that provides a better value for the relative intensity of aerosol generation between the activities of talking and passive breathing: approximately 10X (compared to my placeholder value of 2).  When this weighting is applied to the social interaction duration histogram, the critical exposure is reduced from four person-intensity-hours to three.  

This does not seem like a large impact on critical exposure but the intensity level associated with talking now requires that all of those short interactions become shorter.  If the critical exposure is 3 hours at level 1 (silent breathing in the same room), and talking is 10 times more intense, then an exposure of 0.3 hours (18 minutes) in conversation with an infectious person will deliver the critical dose of virus-laden aerosols.  This suggests a limit of 15 minutes in any interaction with a stranger: the 15-minute rule.