Coughs and sneezes staying airborne

Cold and flu pathogens may stay airborne for long
distances after a sneeze or a cough, researchers say.

The next time you feel a sneeze coming on, raise your elbow to cover up that multiphase turbulent buoyant cloud you're about to expel.

That's right: A novel study by MIT researchers shows that coughs and sneezes have associated gas clouds that keep their potentially infectious droplets aloft over much greater distances than previously realized.

"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."

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

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.

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."

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.

Source: EurekAlert! 

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NYPD to Conduct Anti-terrorism Airflow Study in New York City Streets and Subways

Photo:Damian Brandon/freedigitalphotos.net

The New York City Police Department and the U.S. Department of Energy's Brookhaven National Laboratory are scheduled to conduct this July the largest urban airflow study ever to better understand the risks posed by airborne contaminants, including chemical, biological and radiological (CBR) weapons as they are dispersed in the atmosphere and in the City's subway system.

The NYPD will use the data collected during the three days of research to optimize emergency response following an intentional or accidental release of hazardous materials.

"The NYPD works for the best but plans for the worst when it comes to potentially catastrophic attacks such as ones employing radiological contaminants or weaponized anthrax," said Police Commissioner Raymond W. Kelly, adding that, "This field study with Brookhaven's outstanding expertise will help prepare and safeguard the city's population in the event of an actual attack."

"Brookhaven Lab is a world leader in the use of tracer gases to study airflow, and we are excited about this opportunity to apply that expertise to enhancing the safety of New York City residents and emergency responders," said Brookhaven Lab Director Doon Gibbs.
Both agencies will be working with the Metropolitan Transportation Authority to test airflow through the subway system.

"The NYPD, in partnership with the MTA, is responsible for keeping more than 5 million daily subway customers safe and secure. This study will bolster the NYPD's understanding of contaminant dispersion within the subway system as well as between the subway system and the street, thereby improving its ability to better protect both our customers and the city population at large," said MTA Acting Chairman Fernando Ferrer. "We are glad they are joining Brookhaven National Laboratory for such an important effort, which will benefit the New York City subway without affecting its regular operations."

The Subway-Surface Air Flow Exchange (S-SAFE), as the project is formally known, was commissioned by the NYPD and funded through a $3.4 million Department of Homeland Security Transit Security Grant. It is the first of its scale to study airflow in a dense, complex urban environment both below and above-ground. Researchers from Argonne National Laboratory and Los Alamos National Laboratory, along with additional meteorologists and engineers, will support Brookhaven's scientists as they track the movement of harmless tracer gases detected by air sampling devices placed in select locations on the street and in the subway system.

There will be approximately 200 sampling devices deployed during this study. During the study, researchers will disperse low concentrations of harmless gases known as perfluorocarbons at select subway and street-level locations over three, non-consecutive days in July. Weather conditions will determine which days are selected for the tests, and will be announced to the public a day in advance. The research will be conducted during daylight hours in parts of the Bronx, Brooklyn, Queens, Staten Island and in Manhattan from 59th Street to the Battery.

Perfluorocarbon tracer gases (PFTs) present no health or environmental hazard. They are non-toxic, inert, odorless, and invisible, and have been used in airflow studies since the 1980s, including a 2005 Urban Dispersion Program (UDP) conducted in Manhattan. PFTs also are used in medical applications including eye surgeries and artificial breathing systems.

The NYPD and MTA worked closely together on planning and implementation of the study, which will include 21 subway lines and several dozen stations citywide, in addition to the street-level research. The field study is designed to have zero impact on commuting and other public activity. Members of the public may notice clearly marked boxes containing the air sampling equipment secured in subway stations, on street light poles, and hand-carried by researchers.

In addition to the study in Manhattan in 2005, previous airflow studies were conducted in subway systems in Boston, and Washington, D.C. - but none as extensive as the one planned for New York City in July.

While the study is focused on the airflow and dispersion of airborne contaminants resulting from the release of a CBR agent, the findings will also enable City agencies to better understand dispersion characteristics of other potential inhalational hazards, such as smoke or fumes from chemical spills.

The study is also expected to help police and other agencies decide where to best locate CBR detection equipment. Results from the study will help authorities refine evacuation or other responses in the event of an emergency. One of ten national laboratories overseen and primarily funded by the Office of Science of the U.S. Department of Energy (DOE), Brookhaven National Laboratory conducts research in the physical, biomedical, and environmental sciences, as well as in energy technologies and national security. Brookhaven Lab also builds and operates major scientific facilities available to university, industry and government researchers.