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TWiV 56

Joe writes:


Guys, what a great podcast. I am a chemical engineer with a MS in Environmental Management. I have been doing EH&S work in industrial settings for about 20 years after some years in R&D and manufacturing positions. I have always had a strong curiosity for all branches of science (physics, astronomy, electronics and biochemistry), as well as math and history. Emergency response and risk communication functions have always been part of my job over the years. Virology was relatively new to me when I found your podcast. I had spent quite a bit of time learning the state of the science related to avian flu and SARS so that I could gauge the level of preparedness my company should have for responding to a pandemic. I eventually concluded that we needed a plan and should be ready but that the H5N1 virus had some ways to go before it became a general threat to humans. That research had left me with an interest to learn more of the broader field of virology. When H1N1 hit this year in Mexico, I found your topical podcasts very helpful.

I have been rapidly catching up on your episodes as I drive (about 1 episode per weekday) and will be sad when I am forced to slow down to real time speeds. I don’t suppose you all have considered doing “Today in Virology” so that I will have a new episode each day!!!

To the point:

I strongly agree with your assessments of the limited value of face masks with respect to providing protection from airborne flu infections. Actual user practices leave many infection pathways open. Beyond just not getting a good seal, how do you train users to remove the mask without contaminating their hands and face. If you reuse the mask which would be necessary in an emergency, how do you keep the inside clean. When do you wear it and when do you take it off? I train people to wear respiratory protection in accordance with the OSHA requirements for fit testing, medical surveillance, industrial hygiene testing, etc. It will not be easy to translate those practices to an untrained inexperienced population.

I agree with Dick that the whole “hot and humid stops the spread of flu” dogma seems inconsistent with the facts. How do we know that flu does not show up in human stool? Do we have actual data? In Asia, could the transmission mechanism be airborne particulate avian feces? Could the reason the pandemic strains violate the seasonal rules be a function of increased intestinal transmission or conversely that their envelopes are more robust in withstanding environmental stresses? The great thing about science is that the more we learn the more questions we generate.

With respect to the study on flu transmission and absolute humidity, If I heard you correctly when you quoted the summary hypothesis of the paper, the authors proposed that the aerosol droplets float longer when dry. This set off my engineering “I doubt it” alarm! So I spent some time reviewing the basics of psychrometry and humidification. I have a new theory on why low absolute humidity correlates with transmission. I think the critical parameter is actually droplet temperature. Droplet temperature will be controlled by the Wet Bulb Temperature for any set of conditions. Wet bulb temp is the temperature of a wetted surface that is experiencing steady state evaporation into the local air conditions. The lower the Relative humidity, the faster the evaporation and more heat lost resulting in a cooler surface. Think swamp cooler in Phoenix vs. New Orleans. Let me give some data to elucidate:

(All data interpolated from Figure 12-2 of Perry’s Chemical Engineering Handbook 5th edition)

Take a volume of air with an absolute humidity of 30 grains of H2O per pound of dry air. (7000 grains = 1 pound) Sorry for the units, but I think in Fahrenheit!

@ 35 F = 100% Relative Humidity (RH) and the Wet Bulb Temp. (WBT) = 35 F (i.e., No cooling by evaporation)

@50 F = 55% RH WBT = 43 F

@80 F = 20% RH WBT = 55 F

As long as the absolute Humidity is low, the particle temperature will stay low until it dries out. In higher humidity, there will be little evaporative cooling and so a higher droplet temperature will be reached more quickly. My amateur virologist guess would be that the lipid coating has an upper temperature limit beyond which it loses strength and/or integrity. At higher temperatures and higher humidity the particle temperature gets too high for the virus to survive shear stresses related to contact with surfaces. In engineering terms it doesn’t bounce well if it is too hot! This would also be consistent with avian transmission routes via cold water bodies. I would expect drying rates at higher temperatures with low RH to be much faster and once the particles dry, their temperature would equilibrate to the bulk air temperature. This would be consistent with the data showing reduced transmission at higher temperatures. Do we have any data that would indicate if dried viruses are still infectious? The physics of these small particles is not straightforward. Also, none of this addresses how the properties of spit differ from pure water. If there is someone out there with the experience and the computer power, this could be an interesting research topic. Please include me in the Et. Al. part when you publish.

A couple of recommendations:

A great website for non biochemists wishing to understand the basics of DNA and RNA is DNAi.org. Great history, clear explanations and the coolest videos of translation and replication I have ever seen! When you watch an animated ribosome at work it makes you proud to be a DNA based life form!

A book for Dick: “The Great Mortality” by John Kelly; “An intimate history of the Black Death, the most devastating plague of all time” I found this when researching H5N1 and was struck by how it captures the social and environmental influences on the development of the plague and then the impacts of the plague on societies and their reactions. It puts some of the current responses to swine flu in perspective.

Future Topics:

Why all the differences in nucleic acid structures within viruses? What are the pros and cons of ssDNA vs. dsDNA vs. ssRNA+ vs. ssRNA- vs. dsRNA?

How does an encapsulated virus survive in the digestive system? My vote is for Hydrogen Bonding (the universal answer for all chemistry exams!)

Dick’s list of top ten vectors throughout history.

How and why retro viruses change our DNA. What’s in it for them? (yes I know I think they are alive!)

How about a session on rhino viruses since we can all relate to their impacts. Why so many, are the different strains new each year or just new to me, etc.?

PS. Doesn’t the Zoster vaccine qualify as one you take after infection like rabies?

Love the show and the latent humor



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