Thread by @CorsIAQ

1. I learned today that a tweet that I posted nearly two months ago was used in an article in a well-known magazine in Canada. I am concerned that the tweet regarding elevators misrepresents my simple model results. The reporter never spoke with me. (more)

2. As such, I am hoping that this long thread of tweets provides correct context. Over 2 mos ago a friend who lives in a tall apt asked whether it is possible to breathe particles containing viruses in an elevator after someone infected with COVID-19 had used the elevator (more)

3. That night after all of my dean duties were completed I took 2-3 hours and did some engineering calculations based on specs found for residential elevators and aerosol emissions from coughing and speaking published in the peer-reviewed literature. (more)

4. The engineering calculations employed basic mass balances when an “infector” rides up 10 stories, the door opens, closes, and then returns to the 1st floor for the next rider. (more)

5. I used fundamental fluid mechanic calculations to estimate air flow in and out of the elevator when doors opened and closed. This is easy to check and I intend to do so in several elevators using a tracer gas in the future. (more)

6. My calculations showed that, indeed, a fair fraction of the aerosols emitted by an infector will still be in the air in the elevator when someone gets in it on the first floor. I stand by this rather simple and not surprising result. (more)

7. Importantly, I never said that the amount remaining would pose a high risk to someone who then got into the elevator and rode it up 10 floors over a 30 to 40 second time period. (more)

8. In fact, my guess is that while the risk may not be zero it is very low relative to other indoor environments. Significant assumptions and caveats were provided in the original tweet. (more)

9. Here is the important part of the story that I have discussed with journalists who have actually contacted and spoken with me about elevators. (more)

10. I explain the concept of deposited dose: Di = Ci*B*t*fi (where Di = dose for particles of size i, C = average concentration in breathing zone (amount of particles of size i and in future perhaps infectious viruses in integrated particles of size i per liter of air) (more)

11. B is breathing rate in L/min, t is time spent in the environment in min, and fi is the fraction of particles of size i that deposit in the respiratory system. I explain that the location of deposition may be very important. (more)

12. Risk (response) generally increases with dose (unless w/ a trigger threshold, e.g., for Sarin gas) but an actual dose-response curve for SARS-CoV-2 is not available, nor is the range or central tendency of (particle) size-dependent density of infectious viruses. (more)

13. An actual dose-response curve will differ between, say, an elderly individual and a healthy athlete. So, who might be at risk in the elevator is also important. (more)

14. The dose will also increase if someone has been out for a jog and returns to the elevator breathing at a rate that may be 10-20 x higher than someone who just woke up and got into the elevator. (more)

15. The highest risk in an elevator is when you are in it with someone who is infected and who speaks or (worse) coughs directly at you. (more)

16. While time of exposure is short, the avg concentration in the near field (close contact) may be very high in confined space of elevators. I believe we should be concerned about inhalation of very high concentrations of fine particles even over short times, (more)

17. and especially if someone falls into certain high risk categories. But we STILL DO NOT KNOW the extent of the risk.

So what can be (simply) done w/o much effort to be extra cautious? And if simple, why not just do it? (more)

18. Limit number who ride elevator. Require masks, as we should do in all public indoor spaces (including space where people wait for an elevator with proper social distancing). Have signs posted in the elevator not to speak (perhaps part of a new elevator etiquette). (more)

19. If possible, do not directly face others in the elevator (face away). To be extra cautious with fomites, use folded tissue paper if you push buttons and throw away after exiting. (more)

20. These rules should dramatically reduce whatever risks that there might be from riding an elevator for a short time with an infector. And please note that I am not defining the level of risk (see dose-response comments above).

21. Finally, I appreciate the many journalists who have contacted me, received the rest of the story, & did a good job of reporting and putting this issue in context.

That's my longest thread of tweets ever. Many thanks if you got this far! Now, back to late night dean work.