1/ On new CDC guidelines that move from single-event exposure time for close contact to 24-hour integrated exposure time over 24 hours. My take ....
2/ First, while exposure time is an important factor, it is incomplete. At corsiaq.com, I describe inhalation dose. Time is an important factor, but so is concentration of aerosol particles in the breathing zone & respiratory minute volume.
3/ Concentration (C) decreases with distance from the source, and so 10 ft is better than 6 ft, etc. Take advantage of this, but also know that some dose occurs in the far field as well. Further, C is taken in breathing zone, e.g., air inside mask. Thus masks reduce C!
4/ Respiratory minute volume (RMV) (volume or air breathed per minute) is also important. This can be 10 to 15 x greater for someone doing aerobic exercise relative to someone at rest. So, while not quite linear due to differences in particle deposition as RMV varies, ....
5/ Spending time in close contact with an infector while working out is effectively the same as taking a 15 minute exposure time for someone at rest and turning it into a 150 minute exposure time (roughly). Running to class & breathing heavily for first 10 min ...
6/ A guideline 15 min exposure time implies a threshold dose, which I do not believe has been shown. It seems more likely that there is a dose-response relationship w/ low (but not zero) risk at low dose and increasing risk with dose. See @ProfCharlesHaas tweets for good info.
7/ The 15 min exposure time, like 6 ft rule, is a non-exact guideline. The specific situation - nature of infector, where they are in infection, how many particles they generate, mixing conditions in space - which can affect close contact, mask usage, etc., are all important.
8/ The best way to reduce your risk in bldgs you do not control (e.g., not your home) is to always wear a good mask, avoid non-essential buildings or where others are not wearing masks, distance as much as possible from others, minimize your time in the space, & good hygiene.
9/ Be smart. Be patient. Don't let your guard down. Understand that we are going to a very bad place this late fall and winter. Be responsible to yourself AND to others, as you may be the unknowing infector. We all need to buckle up & do the right thing to crush this virus.

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More from @CorsIAQ

13 Oct
1/ Important tweet by @linseymarr.

Aerobic activities in gyms are a bad idea during this pandemic, and it is difficult to get risk down to what I believe are acceptable levels. Results of a gym simulation I did several months ago are shown here w/ specs next .... Image
2/ In this simulation I used Infector X in a restaurant in China with a significant outbreak of COVID-19. By analyzing metadata for the restaurant it is possible to ask the following question ... more ...
3/ If infector X is placed in a gym as a staff member, how much volume of virus laden aerosols would someone in the gym inhale relative to those who were infected in Restaurant X? The value "omega" on the vertical axis shows this ratio for different conditions. more ..
Read 6 tweets
12 Oct
1/ Starting to post hand-written notes and (soon) slide presentations for undergraduate and graduate indoor air quality and exposure science courses that I taught at UT Austin for decades. At corsiaq.com under teaching. Will take awhile to populate. More ....
2/ I almost always provided handwritten notes to students well in advance of class meetings. The "blank" sections were for problems or derivations that I did on the board so that students could write in the notes I provided during lecture. More ...
3/ Some notes are admittedly "cleaner" than others. You can tell by the handwriting when I was up until 3 a.m.!

These notes are primarily intended for fellow academics who might be developing such courses, but might also be of interest to others who wish to know more about ...
Read 5 tweets
12 Oct
1/ The rebreathed fraction of air is the fraction of air that one inhales that came out of the collective respiratory systems of others in an indoor space. If a rebreathed fraction is 0.05 (a high value), every breath you take contains 5% of the collective breath of others.
2/ An important parameter is the average rebreathed fraction while you are in a space with others. The average rebreathed fraction (f) can be determined using CO2 measurements as f = (Cavg - Cout)/Cbreath.
3/ Cbreath is the CO2 concentration exhaled and is approximately 38,000 ppm (some variation with diet, etc.). Cavg is the average indoor CO2 concentration during the time you are in an indoor space in ppm, and Cout is the CO2 concentration outdoors in ppm.
Read 12 tweets
9 Oct
1/ At very beginning of this pandemic (late February/early March) we placed CO2 monitors w/ data loggers in several locations of the library @Portland_State, counted occupants on several floors, and flux in/out of the library to get an average time of approximately 2 hrs/visit.
2/ We used the data to determine the average rebreathed fraction of air on floors & occupancy numbers & average occupancy time to estimate reproductive numbers if infectors were present (w/ a range of quanta generation rates) using the Rudnick-Milton model. Upshot ...
3/ BIG differences in rebreathed fraction between floors, but small in general. However, w/ large # of occupants, even small rebreathed fractions can lead to predicted site-specific reproductive # between 1 and 5 for a single infector in teh space depending on quanta gen rate.
Read 4 tweets
5 Oct
1/ Many years ago I was trying to get permission to do extensive IAQ sampling in a large school district in Texas. After weeks of trying I finally got to meet the then superintendent. I think I wore him down. In the end he looked at me and said ......
2/ "Alright. Do it. But if you find ANYTHING wrong I never heard about it." I was shocked. My team did find a lot that was wrong (w/ woefully poor ventilation being the root of many problems). We informed facilities staff and specific school officials.
3/ The want of anonymity was common between all five school districts that I worked w/ over 20 years. But some districts were more accommodating, were receptive to help, and in ONE case took our findings and acted positively to improve indoor air quality in their schools.
Read 4 tweets
4 Oct
1/ It is important to recognize that as long as an infected individual is in an indoor space, virus-laden aerosol particles will accumulate to higher concentrations in that space until (in a well-mixed space) an approximate steady-state concentration occurs. more ....
2/ The steady-state condition is reached when the rate of emissions from the source is equal to the rate of removal by ventilation, deposition onto surfaces, & filtration. At this point the rate of change of concentration with time is zero. more ...
3/ The time to achieve 95% of steady-state is 3/B (where B is the sum of removal mechanisms, each w/ units of inverse hours). I have observed K-12 classrooms with outdoor air change rates as low as 0.5/hr. w/o filtration and little deposition this = 6 hrs to steady-state. ....
Read 4 tweets

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