I was recently asked the following by a colleague at MSF France:
- how long time can a droplet nuclei remained suspended in "regular/lab conditions" and what's the level of impact of certain measures on it (like humidity) ?
- I've found that the survival of mtb outside the host is: Sputum (cool and dark location) : 6 to 8 months, clothing : 45 days, paper - book : 105 days (this is the reference: http://www.biosafety.be/CU/PDF/Mtub_Final_DL.pdf) and m bovis its viability (half life) is around 1.5 hours after airborne http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WWR-4M3BCC4-1&_use...
but no info on airborne mtb. Any info on this?
There are only two papers on the viability of mycobacteria in air, both done in rotating drums with controlled humidity, etc., and they had very different results. One is old, done in the 60s by Loudon (Loudon RG, et al, Am Rev Resp Dis 1969; 100:165-171) and showed a half life for M. tuberculosis of about 6 hours. More recently, workers in the UK repeated the study with aerosolized organisms from actively replicating organisms from continuously fed cultures. They found at T 1/2 of about 20 mins. (Leve, et al, Letters in Appl Microbiol 2000; 31:238-241). We will try to post these references on GHDonline.
I tended to believe the 6 hr figure as the upper limit. Humidity was 50% I believe in the Loudon study, but 75% in the Lever study. Our own UV studies suggest that high humidity was protective for the organism against UV damage - possibly other insults as well. For respiratory viruses, however, high humidity is deleterious, so you cannot be sure. However, these studies are all artificially generated aerosols and human generated aerosols could be quite different.
Recent studies have shown that many organisms in the airways retreat to a spore-like inactive state that would likely favor survival. Most importantly, ventilation determines the presence of viable tubercle bacilli in air much more than viability. In a room with just 1 air change per hour, 63% of organisms are gone in an hour, 84% in two hours, and over 90% in 3 hrs. With 6 ACH, of course, organisms are gone in minutes. As you know, viability of organisms on surfaces is inconsequential since they must be inhaled to infect humans.
I hope this helps.
Edward A. Nardell, MD