Thread by @CorsIAQ

1/ Rebreathed Fraction (RF) and Schools.
The rebreathed fraction of air is the fraction of air that a person inhales that came out of the collective respiratoiry systems of others in the indoor space they are in. We'd like RF to be as small as possible!

2/ RF can be calculated as
RF = (CO2in - CO2out)/CO2breath
Here CO2in is CO2 concentration in the indoor space, CO2out is the outdoor CO2 concentration, and CO2breath is the typical CO2 concentration on human breath (around 36,000 to 38,000 ppm) w/ some variation by diet, etc.

3/ Earlier in the pandemic I estimated quanta generation rates based on outbreaks & used these w/ the Rudnick-Milton model to approximate an upper-bound acceptable RF(avg) of 0.008 in a model classroom. This leads to a CO2in = 698 ppm (say 700 ppm). Well before delta variant.

4/ That analysis assumed no masks or filters. Universal mask wearing and good filtration allows some relaxation of the RF threshold that I calculated. But again, this was before the delta variant.

5/ I have studied RF in hundreds of classrooms in Texas, including quite recently, and found median and mean RF of around 0.025 (4 x above what we'd like to see for orignal SARS-CoV-2 virus and much higher x than what we'd like to see for delta variant).

6/ I've seen rebreathed fraction of air in some Texas classrooms as high as 0.13 (13%) at peak & 0.075 (7.5%)% averaged over day. For the former, this equates to roughly 1 in every 8 inhaled breaths being a collection of the exhaled breath of every student & teacher in the space.

7/ In that horrific case, if there are 25 students & 1 teacher in the classroom with just ONE infector, 1 in every 200 inhaled breaths is equivalent to locking lips with the infector as they are exhaling and you are inhaling.

8/ The normal respiratory rate for 6 to 12 yo is about 18 to 30 breaths per min. Assume lower end. That means locking lips with the infector about every 11 minutes during the school day. If more than one infector in this worst-case classroom ... more frequent!

9/ For 12 to 18 yo it is reasonable to assume 12 to 16 breaths per minute. Assuming 12, that means locking lip with a single infector and inhaling every 17 minutes during the day.

10/ Example above is an extreme case. However, it is a real case. The majority of classrooms that I have studied in Texas are under-ventilated relative to ASHRAE 62.1-2019, which is for "normal" times (not a pandemic). Some classrooms WOEFULLY under-ventilated pre-pandemic.

11/ When I have asked school districts why they operate at such low ventilation rates I am usually told it is for reduced energy use and entirely for saving money. Second place is "we did not know."

12/ Even the median or best 25th percentile in our past studies is highly insufficient for a pandemic. In fact, looking back at past data indicates that only 1 of 46 classrooms in our recent study of seven high schools in TX comes close to a RF = 0.008, at 0.01.

13/ Sending children w/o masks back to school in TX, where population ctrs are on fire w/ delta variant, & without SUBSTANTIAL increases in ventilation & advanced filtration (no time to describe findings wrt filtration!) is IMO highly irresponsible at best & likely much worse.

Note - I am sure that some school districts in Texas have improved ventilation and filtration. Kudos to those that have. However, I am also fairly confident that many have not for a range of reasons, some beyond district control.