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 http://www.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.