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"When you cough or sneeze, you see the droplets, or feel them if someone sneezes on you," says John Bush, a professor of applied mathematics at MIT, and co-author of a new paper on the subject. "But you don't see the cloud, the invisible gas phase. The influence of this gas cloud is to extend the range of the individual droplets, particularly the small ones."
Coughs
Researchers have illuminated the flows of coughs with powerful lasers and fancy photo techniques through the use of powerful computers to model this flow of thousands of tiny particles. They've used heated manikins and cough machines in rooms filled with tiny droplets of olive oil or theatrical smoke to track how air moves, where breath goes, and how exposed we are to someone else's cough.
A typical cough starts with a deep breath, followed by a compression of air in the lungs and then a crackling burst as that air is forced out in a fraction of a second.
The average human cough would fill about three-quarters of a two-liter soda bottle with air -- air that shoots out of the lungs in a jet several feet long. Coughs also force out thousands of tiny droplets of saliva. About 3,000 droplets are expelled in a single cough, and some of them fly out of the mouth at speeds of up to 50 miles per hour.
Sneezing is even worse.
It starts at the back of the throat and produces even more droplets -- as many as 40,000 -- some of which rocket out at speeds greater than 200 miles per hour. The vast majority of the droplets are less than 100 microns across -- the width of a human hair. Many of them are so tiny that they cannot be seen with the naked eye.
"What happens to these droplets depends on their size," said fluid dynamicist Bakhtier Farouk of Drexel University in Philadelphia. He is working on software that models how microscopic droplets move around a room.
Most of the larger, heavier drops fall quickly to the floor under the influence of gravity. The smaller and lighter particles (those that are five microns or less across) are less affected by gravity and can stay airborne almost indefinitely as they are caught up in and dispersed by the room's airflow.
Cold temperatures allow most viruses to linger for extended periods and various mechanisms allow cold viruses to infiltrate our immune systems especially in the winter.
Spreading Up To 200 Times Further Than Previously Assumed
Indeed, the study finds, the smaller droplets that emerge in a cough or sneeze may travel five to 200 times further than they would if those droplets simply moved as groups of unconnected particles -- which is what previous estimates had assumed. The tendency of these droplets to stay airborne, resuspended by gas clouds, means that ventilation systems may be more prone to transmitting potentially infectious particles than had been suspected.
With this in mind, architects and engineers may want to re-examine the design of workplaces and hospitals, or air circulation on airplanes, to reduce the chances of airborne pathogens being transmitted among people.
"You can have ventilation contamination in a much more direct way than we would have expected originally," says Lydia Bourouiba, an assistant professor in MIT's Department of Civil and Environmental Engineering, and another co-author of the study.
The paper, "Violent expiratory events: on coughing and sneezing," was published in the Journal of Fluid Mechanics. It is co-written by Bourouiba, Bush, and Eline Dehandschoewercker, a graduate student at ESPCI ParisTech, a French technical university, who previously was a visiting summer student at MIT, supported by the MIT-France program.
Smaller drops, longer distances. Know how to cover up
The researchers used high-speed imaging of coughs and sneezes, as well as laboratory simulations and mathematical modeling, to produce a new analysis of coughs and sneezes from a fluid-mechanics perspective. Their conclusions upend some prior thinking on the subject. For instance: Researchers had previously assumed that larger mucus droplets fly farther than smaller ones, because they have more momentum, classically defined as mass times velocity.
That would be true if the trajectory of each droplet were unconnected to those around it. But close observations show this is not the case; the interactions of the droplets with the gas cloud make all the difference in their trajectories. Indeed, the cough or sneeze resembles, say, a puff emerging from a smokestack.
"If you ignored the presence of the gas cloud, your first guess would be that larger drops go farther than the smaller ones, and travel at most a couple of meters," Bush says. "But by elucidating the dynamics of the gas cloud, we have shown that there's a circulation within the cloud -- the smaller drops can be swept around and resuspended by the eddies within a cloud, and so settle more slowly. Basically, small drops can be carried a great distance by this gas cloud while the larger drops fall out. So you have a reversal in the dependence of range on size."
Specifically, the study finds that droplets 100 micrometers -- or millionths of a meter -- in diameter travel five times farther than previously estimated, while droplets 10 micrometers in diameter travel 200 times farther. Droplets less than 50 micrometers in size can frequently remain airborne long enough to reach ceiling ventilation units.
A cough or sneeze is a "multiphase turbulent buoyant cloud," as the researchers term it in the paper, because the cloud mixes with surrounding air before its payload of liquid droplets falls out, evaporates into solid residues, or both.
"The cloud entrains ambient air into it and continues to grow and mix," Bourouiba says. "But as the cloud grows, it slows down, and so is less able to suspend the droplets within it. You thus cannot model this as isolated droplets moving ballistically."
Once airborne, viruses in these tiny droplets can survive for hours. Even if the droplets hit a surface, the viruses can survive and still spread disease if the droplets become airborne later. When a droplet lands on paper, its virus particles can survive for hours. On steel or plastic they can survive for days.
Once they are breathed in, the droplets settle onto cells at the back of the throat, where the virus attempts to enter these cells and begin replicating. This may or may not cause an infection. The body's natural defenses are designed to eliminate infections, and whether someone will fall ill depends on how much virus is breathed in and whether the person's immune system has encountered that virus previously, said Julian Tang, a clinical virologist in Singapore.
When people do get sick, the body tries to deal with the infection by bringing up mucus to help clear it. Some of this mucus is swallowed, carrying the virus down to be destroyed by stomach acid. Some viruses in the throat, though, will be expelled when we cough, and this coughing expels the mucus (and new virus) out of the body, thus beginning the whole process anew.
Most people know to cover their nose and mouth when they cough or sneeze, but it's inevitable that viruses will spread no matter how much you cover up, so quarantining yourself while sick is the best solution to prevent viruses from spreading exponentially. One should not cough or sneeze into one's hand. "The current thinking is one should only cough into the crook of the arm." Covering our nose and mouth can somewhat limit the dispersal of contaminated respiratory droplets, but when we cough into our hand, it becomes coated with virus that can then be transferred to everything from elevator buttons and light switches to gas pump and toilet handles.
Ready for a close-up
Other scholars say the findings are promising. Lidia Morawska, a professor at Queensland University of Technology in Brisbane, Australia, who has read the study, calls it "potentially a very important paper" that suggests people "might have to rethink how we define the airborne respiratory aerosol size range." However, Morawska also notes that she would still like to see follow-up studies on the topic.
The MIT researchers are now developing additional tools and studies to extend our knowledge of the subject. For instance, given air conditions in any setting, researchers can better estimate the reach of a given expelled pathogen.
"An important feature to characterize is the pathogen footprint," Bush says. "Where does the pathogen actually go? The answer has changed dramatically as a result of our revised physical picture."
Bourouiba's continuing research focuses on the fluid dynamics of fragmentation, or fluid breakup, which governs the formation of the pathogen-bearing droplets responsible for indoor transmission of respiratory and other infectious diseases. He aim is to better understand the mechanisms underlying the epidemic patterns that occur in populations. "We're trying to rationalize the droplet size distribution resulting from the fluid breakup in the respiratory tract and exit of the mouth," she says. "That requires zooming in close to see precisely how these droplets are formed and ejected." Funding for the study was provided by the National Science Foundation.
6 Ways To Keep Your Immune System Strong Against The Cold Virus
#1) Live in a warmer climate. Unfortunately, this is the number one and best preventive defense against the cold virus, but not much consolation to those living far from equator. Sorry Minnesota.
#2) Taking the sunshine vitamin is shown to reduce the risk of flu to a third of what it would otherwise be. The correct daily dose of vitamin D3 for adults is approximately 5,000 IU/day, not the 200 to 600 IU recommended by the Institute of Medicine, the National Institutes of Medicine and the FDA. You may even be shocked to know that there are many physicians in both Canada and the United States who prescribe as much as 50,000 IU of vitamin D daily as a treatment for a long list of chronic diseases.
#3) Stay away from sucrose. Its ability to impair and depress the immune system is unparalleled.
#4) Stay away from all vaccines, especially the flu shot. Flu vaccines still contain mercury and only work to depress the immune system. Regardless of what statistics your government has released, the actual chances of a flu vaccine preventing the flu are less than 4 percent.
#5) Use Virus-Fighting Herbs. Some of the best immune stimulants are anti-viral herbs. Virus-fighting herbs include purple coneflower, pot marigold and black elder. Other important antiviral herbs include yarrow herb (Achillea millefolium), hyssop herb (Hyssopus officinalis), lemon balm herb (Melissa officinalis), St. Johnswort (Hypericum perforatum) marjoram herb (Origanum majorana), oregano herb (Origanum vulgare), heal-all herb (Prunella vulgaris), rosemary herb (Rosmarinus officinalis) and blue vervain herb (Verbena hastata). It is important to begin taking the herbs as soon as you think you are getting sick. Take your formulation four to six times per day until you are better.
#6) Use Deep Acting Immune Tonics. Another group of herbs that help to improve and optimize immune function are the immune tonics. These herbs are deeper acting than immune stimulants, but take longer to work. They include North American ginseng root (Panax quinquefolius), lacquered polypore or reishi mushroom (Ganoderma lucidum), artist's conk (Ganoderma applanatum), Chinese milkvetch root (Astragalus membranaceus) and Siberian ginseng root (Eleutherococcus senticosus). Combine two or three immune tonics and take them three to four times per day for two to three months. Immune tonics are not suitable for treating infections in progress. They are used for preventive purposes or to optimize immune function and work best after first doing several cycles of immune stimulants.
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