Thread by @CorsIAQ

1/ Not surprising & consistent w/ comments I made in a recent article by @RobertRoyBritt. Exposure time is NOT the only factor that determines risk, whether in close contact or indoor far field. We should focus on inhaled deposited dose (IDD) & not just on distance or time.

https://twitter.com/DrEricDing/status/1319026171674243072

2/ IDD = Concentration (C) (#/L) x Respiratory minute volume (B) (L/min) x Time of exposure (t) (min) x Fraction deposited in respiratory system (f) (-). C & f are taken for specific particle sizes and the resultant size-specific IDD values summed (integrated) for total IDD.

3/ Concentration (C) depends on a number of factors, including distance, emission mode and strength, air flow conditions (e.g., cross-flow, mixing/TKE) & whether the receptor is wearing a mask (as concentration is in breathing zone inside mask), quality of mask, etc.

4/ C can vary widely just by the nature of emissions from the infector (up to two orders of magnitude difference in particles emitted per min), even larger differences in total viral load (#/mL), and order of magnitude differences in fraction of viruses that are infectious.

5/ Just the nature of emitter leads to many orders of magnitude differences in C (if C is based on virions), & when superimposed on other factors noted above even GR8er variation. It is entirely logical that at high C exposure times << 15 min lead to elevated infection risk.

6/ Respiratory minute volume (B) depends on the individual and their level of activity. Note that this can vary over an order of magnitude for someone at rest versus someone doing strenuous work or aerobic activity. I address here for gyms: radio.com/kcbsradio/podc…

7/ Time (t) is time of exposure (min). In many ways I would rather see B and t be combined into inhalation volume per exposure event so that others would acknowledge the need to account for B and not stick to simple and overly simplified individual factors like "time".

8/ The fraction of particles that deposit in the respiratory system (f) (and where particles deposit in system) depends on particle size, mode of breathing (nose vs. mouth), and (to some extent) B (see above). I have tweeted about this previously.

9/ I fully understand the desire to provide the public with simple guidelines as anything more can be confusing to many & thus counterproductive. But we need to UNDERSCORE that 6 ft and 15 minutes are GROSS generalizations and do NOT eliminate risk.

10/ Greater distance lowers risk. Less time lowers risk. Wearing masks lowers risk. Being outdoors lowers risk. Not breathing heavily near others lowers risk, & all other strategies that many of us have been stating ad nauseam for months on end lower risk. None eliminates risk.

11/ I've added more on inhaled deposited dose and the factors described above on my blog at corsiaq.com. IMHO, we need to become one with this concept. The greater the dose the greater the risk. Let's focus on lowering dose (& many steps for doing so) to lower risk.