Abstract

Size-fractionated aerosol particles were collected in a hospital emergency department to test for airborne influenza virus. Using real-time polymerase chain reaction, we confirmed the presence of airborne influenza virus and found that 53% of detectable influenza virus particles were within the respirable aerosol fraction. Our results provide evidence that influenza virus may spread through the airborne route.

Influenza is a highly contagious respiratory illness that results in >250,000 deaths annually worldwide [1]. Currently, influenza virus is known to be spread from person to person by at least 2 mechanisms: direct and indirect transfer of respiratory secretions and contact with large droplets that settle onto fomites [2]. In addition, influenza virus may also be transmitted by inhalation of small airborne particles [3], but this potential route is not well characterized and remains controversial.

Coughing, sneezing, talking, and breathing generate a cloud of airborne particles with diameters that can range from a few millimeters to <1 µm. Large droplets (diameter, >50 µm) settle on the ground almost immediately, and intermediate-sized droplets (diameter, 10–50 µm) settle within several minutes. Small particles (diameter, <10 µm), including droplet nuclei from evaporated larger particles, can remain airborne for hours and are easily inhaled deeply into the respiratory tract [4]. Alford et al. [5] revealed that humans could contract influenza by inhaling an experimental small-particle aerosol containing low levels of influenza virus. Studies involving mice, ferrets, and guinea pigs have demonstrated airborne animal-to-animal transmission [6, 7, 8]. Observational and epidemiological studies suggest that airborne influenza transmission occurs among people [9, 10], although these studies have been unable to clearly delineate a causal relationship.

The purpose of the present study was to measure the amount and size of airborne particles containing influenza virus in a health care facility. Size-fractionated aerosol samples were collected in a hospital emergency department during the February 2008 influenza season and were analyzed using real-time PCR. The results demonstrate that influenza virus was present in airborne particles in the respirable size range.

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