Tuesday, August 12, 2014

Debunking Airborne Ebola: What You Need To Know About Aerosols, Droplets and Fomites


I briefly discussed aerosols and droplets in my first Ebola post, but because there is still so much online discussion about Ebola transmission and because it's relevant to all infectious diseases, I decided to move that section of the post here and expand upon it so that those interested specifically in this topic wouldn't have to wade through the rest of that riveting post  to find it (although I highly recommend it). There is still some minor redundancy between the posts, but redundancy when dealing with BSL4 pathogens is a very good thing. References are listed at the end of the post.


First, What Does "Airborne" Mean?
Let me clarify. Bodily secretions that make it into the air from various orifices (e.g., nose, mouth) are called droplets and are classified based on size and distance traveled. The smaller the droplet, the longer it stays suspended in the air, the farther it travels and the deeper into the respiratory tract it can go upon inhalation by the person sitting down the aisle from you on the airplane. Teeny-tiny droplets (less than 5 microns) are generally referred to as "aerosols" and can be generated by a cough, a sneeze, exhaling, talking, vomiting, diarrhea, passing gas etc. Aerosols can also be generated mechanically by things like flushing a toilet, mopping, or rinsing out a bloody wash cloth. When aerosols are infectious, they transmit disease when they are inhaled by an organism and its called "aerosol transmission". When droplets are larger than 10 microns they are called "large-droplets" and if infectious, they transmit disease by inhalation if the organism being infected is close enough to inhale the particles before they settle out of the air. They can also transmit virus if someone gets showered with droplets from, for example, a sneeze, or touching a droplet that is on the surface of an object(fomite) or someone's skin and it's called "large-droplet transmission".

When we say that a virus is airborne, we specifically mean it is capable of aerosol transmission via inhalation, even when not in close proximity to the source of the aerosol. For example, someone two aisles over at the market has the measles and coughs up a lung. When you get into that aisle you inhale the teeny aerosol droplets that are still hanging out in the air and they begin depositing virus particles in your respiratory tract. These particles then enter your cells and begin to replicate. You are infected. I hope you were vaccinated!

Large-droplet transmission is not considered airborne. An ebola patient sneezing directly on you will get virus particles on you, but those large droplets that landed on your lip weren't technically airborne, they settled out of the air and onto your lip fairly quickly and they weren't inhaled. If you're close enough for this, then I'm sure you are inhaling some aerosol sized droplets as well, and at this close range they are likely infectious. However, to be considered "airborne", those aerosol particles would have to remain infectious while they hang out or move through the air. Evidence indicates that while Ebola is found in the respiratory tract and theoretically could produce infectious aerosols and be airborne, for unknown reasons we don't see this happen between primates/humans in studies or in outbreaks (for detailed discussion of these studies read this post).

Not all viruses can form infectious aerosols. It depends on where the virus goes in your body and what happens when it gets there. Aerosol infectivity of a virus is determined by how long the virus remains infectious in the air, how deep into the lungs it can travel, and how many virus particles are actually in each droplet compared to how many are required to actually establish an infection. If a viral infection generates aerosols containing 10 virus particles per droplet, but it takes 1000 virus particles per human cell to establish an infection, then those aerosols are not infectious, even though they contain virus. In addition, while airborne, aerosols begin to lose water content by evaporation and virus particles, especially enveloped particles like Ebola, can be affected by other environmental conditions such as humidity, air currents, and sunlight. These particles are also subject to the laws of physics and mechanical forces. A good example of a virus for which these characteristics have been better defined is influenza and this is an excellent article that really explains the different kinds of aerosols and how they are transmitted.

One question we got repeatedly during the Twitter #Ebolachat session was, "If it's not airborne, then why are health care workers not only wearing head-to-toe protective gear, but dying in spite of it?"

Fair question and one I discuss in another post but I will reiterate here. Ebola patients exude many types of droplets, of all sizes from all orifices, and sometimes large volumes are lost violently through vomiting or diarrhea. These events generate a lot of droplets/aerosols/fomites with high viral loads, often in close proximity to HCW. Once deceased, victims continue to lose fluids for some time. It's a real problem. Protection is critical. Mucus membranes and broken/abraded skin are susceptible and the infectious dose of Ebola is low: 1-10 individual virus particles. Protecting every potentially susceptible inch is the goal. But protective gear, if available, is only as good as technique. Those suits are extraordinarily hot and exhaustion sets in quickly. Wipe sweat from your eye with a contaminated glove? You're infected. Accidents and mistakes happen easily in these conditions. Cleaning up messes also generates more droplets unless you're specifically trained to minimize that risk. And remember, many HCW are caring for people even without gear. They are not abandoning the sick. They are reaching out with ungloved hands to offer comfort and ease suffering. For these heroes, the only thing worse than reaching out without gloves...is not reaching out. For a truly remarkable account of what this is like in a field clinic, read emails from Dr. William Fischer II, a physician from UNC who has been on the front lines in Gueckedou, Guinea since May. [EDIT 8/15/14 A new story in the Wall Street Journal illustrates this point all too poignantly]

So, although Ebola laden fluids are infectious and can transmit virus, they are not considered airborne. Larger droplets splatter and splash, even in tiny amounts and contaminate surfaces creating fomites. Puddles, droplets and fomites, in a situation with limited personnel, training and resources, are the concern. Ebola is not airborne. If it were, the casualties in this epidemic would be far higher.


Please feel free to ask questions or make comments. Discussions are always welcome.

Cheers,

Heather


Update 8/20/14: Check out these links if you want to make a difference:
Africare
Global Giving
Direct Relief
Action Aid
Caitlin Rivers blog has a ton of links to help! Thanks Caitlin!!


References
Tellier R. Review of aerosol transmission of influenza A virus. Emerg Infect Dis. 2006 Nov [accessed 8/12/14]. 

Nicas M, Nazaroff WW, Hubbard A. Toward understanding the risk of secondary airborne infection: emission of respirable pathogens. J Occup Environ Hyg. 2005;2:143–54. 


Knight, V. Viruses as agents of airborne contagion. Annals of the New York Academy of Sciences. 1980; 353: 147–156.

Judie Alimonti, Anders Leung, Shane Jones, Jason Gren, Xiangguo Qiu, Lisa Fernando, Brittany Balcewich, Gary Wong, Ute Ströher, Allen Grolla, James Strong & Gary Kobinger. Evaluation of transmission risks associated with in vivo replication of several high containment pathogens in a biosafety level 4 laboratory. Sci Rep. 2014; Jul 25; 4:5824.










28 comments:

  1. So why the newest CDC guidelines say:

    Probable Case is a contact of an EVD case with either a high or low risk exposure [...]
    Casual contact is defined as a) being within approximately 3 feet (1 meter) or within the room or care area for a prolonged period of time (e.g., healthcare personnel, household members) while not wearing recommended personal protective equipment (i.e., droplet and contact precautions–see Infection Prevention and Control Recommendations);

    ? In plain language: you sit close for some time, and you become a "probable cause". Thus airborne, without getting into semantics.

    And why do they protect the patient with multiple layers when transporting, plus the negative pressure etc. and not only, say, two?
    See the recent article about preparations in Germany.

    Some quotes:
    I don’t believe the problem was a failure in the established protocol,” David Writebol [husband of the infected experienced Western nurse] said in press briefing from Liberia, where his wife got the disease. “… She had prepared herself.

    Ditto for the other local doctors, medical heroes, some of whom have been very careful.

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    1. Hi Zezen, Thanks for your question. Actually the "semantics of airborne" are quite important here, that is why the CDC guideline specifically states "droplet and contact precautions". In plain language, you must be near enough to the patient to be exposed to droplets that are not actually "airborne" (see airborne definition above), but rather move through the air by the force of the act that generates the droplet: cough, vomit etc. Unfortunately, people think they are being careful and yet it's not careful enough because you can't always see where the infectious droplets are. In these situations, infectious bodily fluids are often found where they are not expected, so people inadvertently touch something that was contaminated because they don't think it was contaminated.

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  2. 4 teachers came home to Beaufort SC and no precautions are being taken parents were asking teachers to self quarantine and stay home the 21 day period B.O.E says no. I think better safe than sorry I don't think this should be taken lightly I am deeply concerned for my grandsons , daughter and her family

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    1. While I understand your concern, you can take at least some comfort in several things regarding this situation. First of these is that Ebola patients cannot spread the disease unless they are having symptoms and I imagine none of those teachers would go near anybody, much less a child, if they even suspected the beginning of an illness. Also, Ebola is not spread as easily as is say, the measles, and in a country like the U.S. with very different standards of sanitation, hygiene, healthcare and resources, it is much easier to prevent transmission. In addition, the actual risk to those teachers may have been very low to none depending on their experiences while in Africa. I don't think anyone is taking this lightly. Chances are, they simply have information that you don't have, that helps them more accurately determine the risk. My best to you and your family.

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  3. thank you so much for the information just worried but your right thanks

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    1. You're very welcome. Please don't hesitate to ask if you have any other questions.

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  4. The last line of the 5th paragraph reads:

    "and this is an excellent article that really explains the different kinds of aerosols and how they are transmitted."

    The word 'article' is supposed to be a link, but it is malformed and doesn't work. Would you please correct that? I would like to read the referenced article.

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    1. Hi! Thank you so much for stopping by and I am so sorry about the link! I fixed it and it worked from here so it should be OK, but let me know if you still have problems with it.

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    2. Thanks. Link now works.

      Thank you for your efforts. There is a lot of scaremongering going on. I have been in contact with my elected officials, local, state and federal. This is a site I refer them to for sober expertise.

      A major hospital in my area has been reassuring the public they are "fully prepared" for Ebola. Looking at your posting on the equipment and training that's required, then talking to nurses who work at this hospital, there is no way that's true.

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    3. Hi! I'm glad the link works now, thanks! I understand your concern. It looks like the plan now is to transfer any suspected Ebola patients to hospitals that are known to be prepared to handle them (not just those that think they can), so hopefully this won't be an issue again.

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  5. Hi Heather - I planned a trip from PHX to DAL for my daughter's sweet 16 bday, which is this Sunday. The plan was to spend 2 days (18th and 19th) at Dallas ComicCon in Irving. With the recent developments in DAL (2nd person diagnosed), in addition to the inaccurate info from the CDC and Hospital involved in Mr. Duncan's care, my husband and I are in agreement that there are too many unknowns and we shouldn't risk exposure in case this "mutates" and becomes airborne. My daughter of course is heartbroken at the prospect of us cancelling her trip. With so much "group" activity (airports, airplanes, convention center) I feel it's best to not take any chances, esp considering my daughter has Early Onset Adult Polycystic Kidney Disease. Doesn't Ebola attack the kidneys? I've read your site regarding the Ebola facts / risks and I guess I'm still confused at to how likely it is to catch this virus vs. being, say, struck by lightning (!) Your thoughts?

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    1. Hi, and thank you so much for reading and for asking this question. I completely understand your fear and hesitation, especially with an already susceptible daughter. First let me say that there have been no symptomatic Ebola patients on flights coming to the US and Ebola patients really are not infectious - cannot spread the disease - unless they are showing symptoms, and this is why there are screens at airports (to keep people with symptoms off of planes - preventing their migration to other countries during incubation is a bonus but not guaranteed). I don't know what the stats are on lightning strikes but the only way you could catch Ebola on a plane from Phoenix to Dallas is if there was an Ebola patient with detectable symptoms on the flight AND you got some of his or her bodily fluids in a mucus membrane or open wound. Mr. Duncan had no symptoms on the plane, or even the next few days, so there is no way the plane was contaminated in any way from Mr. Duncan. It's very hard to catch Ebola. And as of yet, none of his close contacts under surveillance have shown any symptoms and the 21 incubation period will end on Monday. Health care workers are at high risk because as the patient gets sicker, they need more care and have a lot of terrible symptoms that spread the virus. As for your trip, you have to do what you're comfortable with and there's no shame in erring on the side of caution if that's what you feel you need to do. I recognize that those without my training and understanding of the virus have a different perspective and that's OK. If it were me, I would go ahead with my plans. I hope this helps.

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    2. It sounds like aggressive droplet precautions with the wearing of gowns, gloves, and masks would have and could make the difference in the early management of contacts with early appearance of symptoms. Fancy negative pressure or "hazmat" like suits are not really needed and probably do not add to the safety for caregivers.

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    3. Hi Ronald! Thanks for reading and for the comment! I agree completely. If health care workers wore that protection with suspected cases, rather than waiting until they were confirmed, it would make a huge difference.

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    4. To compare catching a disease to being struck by lightning or attacked by a shark is problematic at best. For example: if you are sitting at your computer surfing the net, then you are far less likely to be attacked by a shark than someone who is surfing in New South Wales. Unfortunately, statistics will mostly measure the probability of ANYONE being attacked by a shark. Thus if there are six billion people in the world and only 1000 (just guessing) shark attacks, then the probability of an attack is really low. If only the number of those surfing is measured, then the probability of an attack is pretty high. There would also be the confounding variable of those surfers attacked by sharks whose remains were never found and the attack was not witnessed.
      Thus the analogy: Swim with sharks and you get bit. Walk amongst those infected with Ebola (any of the five sub-types, and do not believe the inaccurate statements concerning Ebola-Reston) and you get sick.

      R.L. Wolfe

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    5. Hi and thanks for that comment! Your insight into statistics is a great reminder of a really important principle.

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  6. Heather, the previous post mentioned the "fancy negative pressure or 'hazmat' like suits" as unlikely to add safety for the caregivers. The filovirus are usually 80 nanometers in diameter, or 0.08 microns, right? Wouldn't the N95 mask with a protection level of 0.3 micron be too permeable provide protection? Thank you!

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    1. Hi and thanks for reading! While you're right about the size of the virus, the masks don't need to protect against individual virus particles carried on a breeze through the air, they are protecting against tiny liquid droplets containing many virus particles, when they are forcefully expelled by a patient.

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  7. Hi Heather. Thank you so much for being here to answer questions. Are you saying that there is no chance of contracting this virus through inhalation? Basically saying it is ok to be with 3 ft of an ebola victim, breathing the same air? Appropriate attire would only be mask gloves and a gown?

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    1. Hi and thanks for stopping by! I'm saying there is zero evidence that a surgical mask is not enough respiratory protection. The epidemiological data we have tells us that surgical masks are enough and when are worn, along with gloves, skin and eye protection, transmission is prevented. If we get data that says otherwise, I'll be the first to acknowledge it.

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    2. Thank you! Will the virus go through intact skin?

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    3. Also, I don't understand why the CDC is tracking down the whole the the nurse traveled on, and not the people who were in her immediate area if the virus is not in the air. Thank you so much for your time!

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    4. the whole plane the nurse traveled on. sorry

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    5. Hi! I'm happy to answer your questions! :-) No the virus will not go through intact skin. As for the plane, I think that, because of public perception after the Dallas fiasco, they have to cover their butts, but they also need to investigate this situation thoroughly and find out what happened. We've heard conflicting stories of whether she had symptoms or not, and I think tracking down each person on board and interviewing them about the experience will go a long way to help determine the actual risk associated with that flight.

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  8. How long does the Ebola live on surfaces? I know that bleach kills it but is there anything we can use to kill it on our hands. Hand sanitizer?

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    1. Hi and thank you for reading! Ebola can live on surfaces for a few days if it's in a wet drop of fluid, but it's viability decreases as the drop dries out. All common disinfectants kill Ebola including hand-sanitizer because it's an enveloped virus. The envelope is susceptible to disinfectants and alcohol. Non-enveloped viruses are not susceptible to alcohol. For more on this you can read my post on it: Ebola, Karma and the Cell Membrane:

      http://www.pathogenperspectives.com/2014/08/ebola-karma-and-cell-membrane.html

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  9. Heather - I just want to thank you for the informative and timely replies to my questions. Your site is very interesting and I appreciate someone with your knowledge taking the time to explain things in greater detail and in language that I understand! :-)

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    1. Hi! And thank you so much for the very kind comment! I understand how difficult it can be when so much conflicting information is being thrown at you from every direction. I'm just really happy that this blog is helping with that, even a little. So thank you so much! :-)

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