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?"
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.
Update 8/20/14: Check out these links if you want to make a difference:
Caitlin Rivers blog has a ton of links to help! Thanks Caitlin!!
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.