Thread by @CorsIAQ

1/ I agree with @Don_Milton. It is important to distinguish between near- and far-field aerosol particles. In the near field (close contact) there is potential for much higher concentration of aerosol particles in the breathing zone of a receptor.

https://twitter.com/Don_Milton/status/1332370621028831239

2/ The actual concentration depends on extent of emissions from infector, body positioning of infector and receptor, mixing conditions between infector and receptor (which depends on a number of other factors), and DISTANCE between infector and receptor.

3/ Aerosol particles do not vanish beyond the near field. They exist in the far field and will accumulate until an approximate steady-state is achieved (more on this later) as long as the infector(s) stay in the space.

4/ The concentration in the far field will be lower than in the near field, but in a large indoor event there will be many more people exposed in the far field than in the near field, unless the infector(s) actively moves around and speaks to many people.

5/ A key factor is the ratio of near-field to far-field aerosol particle concentrations (expressed as volume for appropriate dose analyses), which depends on a large number of factors, largely building related (which is why building scientists should play a role in all of this).

6/ As but ONE OF THOUSANDS (caveat!) of different scenarios, I had a PhD student over a decade ago looking at breathing zone concentration vs. far-field (volume-averaged large chamber) concentration of cleaning chemicals during the act of cleaning w/ different body positions.

7/ The ratio of breathing-zone vs. far-field in those experiments ranged from 7 to 10 for 4 to 5 ft distancing in a 70 m3 chamber with ACH a little over 0.8/hr.

8/ For these SPECIFIC CONDITIONS (caveat) a 5 min near-field dose is equivalent to a 35 - 50 min far-field dose, certainly realistic exposure times in each. Seems entirely plausible that inhalation dose of virus-laden aerosol particles in near- and far-fields are BOTH important.

9/ Why pit the two against one another as if it has to be one or the other? Both are relevant and we should try to reduce both. Our respiratory system does not distinguish between or exclude aerosol particles based on where we are when we inhale them.

10/ More details on inhaled deposited dose at corsiaq.com. This is intended to be a roadmap in terms of factors that influence inhalation dose so that we can focus on approaches to reduce dose, whether near-field or far-field.