1/ Increased ventilation is critical. Period. But remember, it is NOT everything. Inhalation dose = D = C x B x t x f. Ventilation affects C (concentration of virus-laden aerosol particles in air - in #/L). A tripling of ventilation will reduce C by a maximum factor of 3.
2/ If ventilation is increased and that causes a person to stay in a building for more time (t in minutes), that will obviate some of the benefit of increased ventilation.
3/ If a person goes into a building and does aerobic exercise and increases their respiratory minute volume (B in L/min) by a factor of 10 they will receive a greater inhalation dose than if they were in the building at rest at the lower ventilation rate.
4/ It's inhalation dose that matters (repeat). Increasing ventilation is important for lowering C and should be job #1. Improved central filtration and portable HEPA filtration can also lower C. Required masks can dramatically lower C (often more so than any of the above).
5/ It is important to continue to use these as layered inhalation dose (and therefore risk) reduction. But we should NOT forget about the other important factors - time (t) and respiratory minute volume (B).
6/ The latter (B) has been woefully overlooked in this pandemic. See my blog from last August for more detail --- corsiaq.com.
Should have added here that C is the time-integrated concentration over exposure period t. It should be taken as size-dependent, as the term "f" is faction deposited in different parts of respiratory system, which varies by particle size. Details at corsiaq.com.

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More from @CorsIAQ

13 May
1/ "There is nothing remotely similar to the CAA (Clean Air Act) for nonindustrial indoor environments. In the opinion of the author, the time is long overdue to explicitly address indoor air quality in future CAA amendments ..." (next)
2/ "and to formally address what may be the most important and relatively overlooked environmental issue of our time." Note that "our time" was over two decades ago.
3/ Almost 20 years ago I postulated that we could do more to reduce population exposure to harmful air pollution of outdoor origin than several decades of (highly beneficial) efforts to improve outdoor air quality, by designing, constructing, & operating buildings correctly.
Read 4 tweets
8 May
1/ Primer on Inhalation Dose

Remember that whether it is via close contact or far field (same indoor space but not close contact) inhalation dose associated with virus laden aerosol particles is defined by the same variables:
Dosei = Ci x B x t x fi

Let's take a closer look.
2/ For aerosol particles (as opposed to actual virions in those particles), Dosei is the number of particles in size range i that are inhaled and deposited in the respiratory system.
3/ These size-fractionated particle numbers can then be converted to size-fractionated particle volume or total volume in different parts of the respiratory system, as we have done with the safeairspaces.com model.
Read 25 tweets
8 May
1/ I have been asked extensively during interviews and by the public about my own personal decisions during this pandemic. I never answer questions like "which air cleaner should I buy?" but do give guidance on those that are proven and what to look for. Some examples.
2/ Do you use a mask outdoors?
From the start of the pandemic I have carried a mask with me when I go outdoors but only wear it if it appears I might come in close contact with someone. This is perhaps 5% of the time. I have avoided crowds for the past 15 months.
3/ Do you have a portable air cleaner in your home?
Yes, we have two very good portable HEPA air cleaners (each with CADR greater than 300 scfm). We purchased these long before the pandemic.
Read 18 tweets
8 May
1/ Excellent article by @zeynep. My take below ...

Why Did It Take So Long to Accept the Facts About Covid? nytimes.com/2021/05/07/opi…
2/ At the start of this pandemic transmission by direct contact, close contact via LARGE respiratory droplets, & fomites were emphasized. ...
3/ Airborne transmission by aerosol particles (in near or far fields) was ignored & even downplayed by @WHO and @CDC, despite the fact this pathway represented THE nightmare scenario. This effectively opened the gate for THE nightmare scenario to occur.
Read 12 tweets
1 May
1/ Simple answer = very little for aerosol particles but possibly (in some cases) dispersion that reduces close contact dose. But depending on flow conditions, there can be a low pressure zone on the downstream side that leads to some accumulation of particles.
2/ The risk of people letting their guards down thinking that barriers are somehow effective for aerosol particles is much greater than any benefit, IMHO. Technical answer next.
3/ Is there a benefit in terms of far field exposure? Very minimal. Here's why. Indoor aerosol particle decay rates (k) to (integrated) indoor surfaces range from approx 0.2 to 10/hr for 0.3 to 10 um particles, respectively (higher for larger diameter particles).
Read 9 tweets
24 Apr
1/ Inhalation dose occurs in both the near field (close contact) and far field in the same indoor space. It is reasonable to assume that near field concentrations in the breathing zone are < 2 to 8 x the far field based on measurements & modeling.
2/ The actual magnifier depends on distance, whether and type of masks worn, mixing conditions (TKE) between infector and receptor, mode of emissions (cough vs. speak vs. breath), body orientation of infector & receptor and controls in the far field.
3/ Assuming the magnifier is 4 x, then 15 minutes in close contact with an infector is the same as 60 minutes in the far field. In each case the dose is the same and the probability of infection from those doses should be the same.
Read 7 tweets

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