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Study points to flu inhalation risk
Virus particles remain airborne
Influenza virus particles remain suspended in the air, which suggests that the virus may be transmitted by an airborne route.1 That finding, from a study by the National Institute for Occupational Safety and Health (NIOSH) and West Virginia University, bolsters the recommendation to use N95 respirators rather than face masks during an influenza pandemic.
The transmission of influenza has been hotly debated. The Centers for Disease Control and Prevention considers influenza to be spread primarily through "large virus-laden droplets" that can enter the respiratory tract of people in close contact (within 6 feet) of an infected person or that can be transmitted by contact with infectious respiratory secretions.
"Airborne transmission (via small-particle residue of 5 microns or less of evaporated droplets that might remain suspended in the air for long periods of time) also is thought to be possible, although data supporting airborne transmission are limited," the CDC stated.2
Airborne spread ups the ante
If influenza is airborne, then preventing the spread of pandemic influenza would require N95s or even greater protection, according to respiratory protection experts and health care worker advocates.
"Now we know that every time someone coughs or sneezes, there is a significant fraction [of virus] that is airborne," says Bill Borwegen, MPH, occupational safety and health director of the Service Employees International Union. "It can stay airborne for some time and potentially cause a risk to people who are further than 3 feet or 6 feet away [the commonly recommended distance to reduce exposure risk]."
In the study, researchers collected air samples in the emergency department of West Virginia University Hospital in Morgantown in February 2008. More than half the viral particles detected by PCR testing were in the inhalable range of less than 4 microns, the authors said.
"A number of factors, including temperature, humidity, and severity of the influenza season, could influence the concentration of viral particles in an aerosol sample. Future studies will be needed to address the viability and infectivity of these viral aerosols and, ultimately, will shed light on the relative importance of airborne transmission of influenza," the authors concluded.
Small particle inhalation
The study also built upon recent findings that influenza patients exhale tiny viral particles. In three clinics in Hong Kong, researchers tested the exhaled breath of 12 patients and found influenza RNA in one-third of them. More than 87% of the exhaled particles were smaller than 1 micron.3
"These findings regarding influenza virus RNA suggest that influenza virus may be contained in fine particles generated during tidal breathing, and add to the body of literature suggesting that fine particle aerosols may play a role in influenza transmission," the authors concluded.
Those findings are not surprising to Mark Nicas, PhD, MPH, CIH, MS, adjunct professor of environmental health sciences at the University of California - Berkeley School of Public Health, who has studied the mathematical pattern of particle dispersion.
"I don't doubt that influenza can be transmitted by inhalation," he says. "I would call it an expected finding that you would find influenza virus in air [near infected persons]."
Nicas notes that doesn't reveal how infectious those particles are. Because you don't know what percentage of this virus is active, you can't make calculations of infection risk.
However, faced with a pandemic strain of influenza that has a high case fatality rate, hospitals would need to consider the risk of inhalation and use a respirator with greater protection than an N95, he says.
In its pandemic guidance, the U.S. Occupational Safety and Health Administration acknowledges the possibility that influenza could be spread through the airborne route: "No study has definitively established airborne transmission as a major route of influenza transmission, but multiple studies suggest that some airborne influenza transmission may occur."4
OSHA's guidance: "Given the potential severity of health consequences (illness and death) associated with pandemic influenza, a comprehensive pandemic influenza preparedness plan should also address airborne transmission to ensure that health care workers are protected against all potential routes of exposure.
1. Blachere FM, Lindsley WG, Pearce TA, et al. Measurement of airborne influenza virus in a hospital emergency department. Clin Infect Dis 2009. Available at: www.journals.uchicago.edu/doi/abs/10.1086/596478. Accessed Feb. 17, 2009.
2. Centers for Disease Control and Prevention. Prevention and control of influenza: Recommendations of the Advisory Committee on Immunization Practices (ACIP), 2008. MMWR 2008; 57:1-60.
3. Fabian P, McDevitt JJ, DeHaan WH, et al. Influenza Virus in Human Exhaled Breath: An Observational Study. PLoS ONE 2008; 3: e2691. doi:10.1371/journal.pone.0002691. Available at www.plosone.org/article/info:doi%2F10.1371%2Fjournal.pone.0002691. Accessed Feb. 17, 2009.
4. U.S. Occupational Safety and Health Administration. Pandemic influenza preparedness and response guidance for healthcare workers and healthcare employers. 2007. Available at www.osha.gov/Publications/OSHA_pandemic_health.pdf.