The air we breathe contains microorganisms that can cause infectious respiratory diseases. After two occupants of an apartment were diagnosed with influenza in February of 2013, efforts were made to detect and isolate airborne influenza virus using two different types of active air samplers: a Sioutas Personal Cascade Impactor Sampler (PCIS) and an SKC BioSampler. The PCIS collects size-fractionated particles by impaction on polytetrafluoroethylene filters, whereas the SKC BioSampler collects airborne particles in liquid media. Influenza H3N2 virus was collected by both types of air samplers. The PCIS collected a range of particle sizes containing influenza virus near one of the sick individuals but only ultrafine particles when the samplers were positioned farther away. Viable virus was present in the liquid collection media of the SKC BioSampler and some PCIS filters. These findings suggest that influenza patients produce ultrafine aerosol particles that contain viable virus. 1. Introduction Airborne microorganisms are typically present in the air we breathe and are potential causes of infectious diseases. Influenza viruses are among the respiratory pathogens that can be transmitted by airborne routes [1–4]. Breathing, coughing, sneezing, and talking during the course of influenza generate a cloud of airborne particles containing influenza virus. The airborne particles have diameters that range from a few millimeters (large droplets formed during coughing and sneezing) to submicron (<1?μm) (formed during breathing). Small particles (≤5?μm) including droplet nuclei from evaporated larger particles can remain airborne for hours (these concepts and their relevance to influenza viruses are discussed in [1, 3–14]). Presently, the US Centers for Disease Control and Prevention (CDC) and the World Health Organization (WHO) state that influenza virus transmission occurs mostly by large-particle respiratory droplets that travel only a short distance from the source (1.829?m (6 feet) according to the CDC and 1?m (3.3 feet) according to the WHO) [15–17]. Noteworthy, deposition of influenza virus into the lungs (as small particles) versus the upper respiratory tract (as large droplets) may increase infection risk and illness severity [3, 9, 18–20]. Assessments of microbiological air quality are useful for studies of the airborne transmission of pathogens and of other biological particles in the air that may cause noninfectious diseases (such as allergy to pollen). The two principle methods of assessing microbiological air quality are passive monitoring and active
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