New militarized flu spreading like wildfire in Mexico City

tallaght01 wrote:

That's just wrong advice. The P2 (N5) masks are the recommended flu masks. You need to be careful about what you're advising people.

Review of Aerosol Transmission of Influenza A Virus

http://www.cdc.gov/ncidod/EID/vol12no11/06-0426.htm

Influenza Virus Aerosols

By definition, aerosols are suspensions in air (or in a gas) of solid or liquid particles, small enough that they remain airborne for prolonged periods because of their low settling velocity. For spherical particles of unit density, settling times (for a 3-m fall) for specific diameters are 10 s for 100 μm, 4 min for 20 μm, 17 min for 10 μm, and 62 min for 5 μm; particles with a diameter μm essentially do not settle. Settling times can be further affected by air turbulence (10,11).
The median diameters at which particles exhibit aerosol behavior also correspond to the sizes at which they are efficiently deposited in the lower respiratory tract when inhaled. Particles of >6-μm diameter are trapped increasingly in the upper respiratory tract (12); no substantial deposition in the lower respiratory tract occurs at >20 μm (11,12). Many authors adopt a size cutoff of <5 μm for aerosols. This convenient convention is, however, somewhat arbitrary, because the long settling time and the efficient deposition in the lower respiratory tract are properties that do not appear abruptly at a specific diameter value. Certainly, particles in the micron or submicron range will behave as aerosols, and particles >10–20 μm will settle rapidly, will not be deposited in the lower respiratory tract, and are referred to as large droplets (10–12).
Coughing or sneezing generates a substantial quantity of particles, a large number of which are <5–10 μm in diameter [reviewed in (10)]. In addition, particles expelled by coughing or sneezing rapidly shrink in size by evaporation, thereby increasing the number of particles that behave as aerosols. Particles shrunken by evaporation are referred to as droplet nuclei (10–12). This phenomenon affects particles with a diameter at emission of <20 μm, and complete desiccation would decrease the diameter to a little less than half the initial diameter (10). Droplet nuclei are hygroscopic. When exposed to humid air (as in the lungs), they will swell back. One would expect that inhaled hygroscopic particles would be retained in the lower respiratory tract with greater efficiency, and this hypothesis has been confirmed experimentally (11,12). Aerosols, though heavily diluted, remain airborne and thus can be carried over large distances, which may create a potential for long-range infections. The occurrence of long-range infections is affected by several other factors. These include the infectious dose, the amount of infectious particles produced, the duration of shedding of the infectious agent, and the persistence of the agent in the environment (11). Inferring an absence of aerosols because long-range infections are not frequently observed is incorrect.
Humans acutely infected with influenza A virus have a high virus titer in their respiratory secretions, which makes generation of virus aerosols possible. The viral titer measured in nasopharyngeal washes culminates on approximately day 2 or 3 after infection and can reach up to 107 50% tissue culture infective dose (TCID50)/mL (13,14). The persistence of the infectivity of influenza virus in aerosols has been studied in the laboratory. In experiments that used homogeneous aerosolized influenza virus suspensions (mean diameter 6 μm), virus infectivity (assessed by in vitro culture) at a fixed relative humidity undergoes an exponential decay; this decay is characterized by very low death rate constants, provided that the relative humidity was in the low range of 15%–40% (15,16). These results are consistent with those of an older study (admittedly performed in a more rudimentary manner) in which infectious influenza viruses in an aerosol could be demonstrated for up to 24 h by using infection in mice as a detection method, provided that the relative humidity was 17%-24% (17). In all these studies, the decay of virus infectivity increased rapidly at relative humidity >40%. The increased survival of influenza virus in aerosols at low relative humidity has been suggested as a factor that accounts for the seasonality of influenza (15,16). The sharply increased decay of infectivity at high humidity has also been observed for other enveloped viruses (e.g., measles virus); in contrast, exactly the opposite relationship has been shown for some nonenveloped viruses (e.g., poliovirus) (11,15,16).

Do Surgical Masks Stop Swine Flu? Probably not.

By Jon CohenPosted Monday, April 27, 2009, at 3:15 PM ETClick here to read more from Slate on the swine flu.

As the swine flu outbreak deepens, panicked citizens of North America are donning face masks. In Mexico City's downtown square, the Zócalo, for example, the army handed out face masks, and customs officials on U.S. borders are wearing protective personal gear, such as gloves and masks. (Click here for images.) But do surgical masks offer effective protection against viruses? In 2003, Jon Cohen wrote that the SARS virus, which is just 100 nanometers in size, can easily pass through such barriers. And there's every reason to believe that swine flu, at 80 to 120 nanometers, can, too. The original article is reprinted below.



The dramatic photos of surgical-masked people walking the streets of Asian cities hit by severe acute respiratory syndrome pose the question: Do the masks offer them any meaningful protection against the disease?Viruses, including the coronavirus that scientists believe may be the cause of SARS, are so tiny that they can easily pass through such barriers. Several studies even have shown that surgical masks fail to prevent transmission of the much larger mycobacterium tuberculosis, which causes TB. While the U.S. Centers for Disease Control and Prevention advises that people who have SARS wear these masks, they do not even recommend them for people in contact with those patients unless the infected person can't wear one. Wearing surgical masks outdoors, where virus-laden particles easily disperse, has even less value.

CDC does advise health-care workers working with SARS patients to wear a special mask called an N-95 respirator. But even these masks offer limited protection from coronaviruses. The name of the mask says it all. The "95" means the mask, if properly fitted—and that "fit factor" presents a big if—can filter out particles down to .3 microns 95 percent of the time. (A human hair is roughly 100 microns in diameter.) Human coronaviruses measure between .1 and .2 microns, which is one to two times below the cutoff.

The University of Cincinnati's Sergey Grinshpun has studied N-95 respirators and says it all comes down to "collection efficiency." N-95s made by different manufacturers have different collection efficiencies below the .3 cutoff. In other words, one company's mask, if properly fitted, might filter out 92 percent of coronaviruses, while another might catch only 50 percent.

"It seems to offer better protection than nothing," Grinshpun says. And he notes that viruses often travel on top of larger carrier molecules—like globs of mucus—making it easier to filter them. That's why CDC Director Julie Gerberding last week noted that covering your face with a T-shirt might help if you come in close contact with an infected person.

To efficiently protect yourself from coronaviruses, you would need to wear a full-faced mask with a high-efficiency particle air filter. But such HEPA filter masks cause what Grinshpun calls "quite a discomfort" in short order.
Any mask clearly wards off one bug: fear. Confoundingly, the sight of so many people wearing masks also spreads fear. And there's no measure of collection efficiency or fit factors that can help humans out of that pickle.

Explainer thanks Sergey Grinshpun, Julie Gerberding, and Web sites at the National Institute for Occupational Safety and Health and the International Society for Respiratory Projection (Americas section).

http://www.slate.com/id/2217045/

Blindpew wrote:

I shouldn't have brought Fianna Fail into this thread as they are an old an dying virus which has infected us all for years. Our immunity to them will be proved at the next election.
Thanks to nodolan for his mask advice. I'm going to go with the N99 mask as well as he seems to know more about this than I do. That said I'm not ruling out the wearing of a full head gas mask and NBC suit. Better safe and silly looking than coughing your guts up in an A&E ward.