Influenza is one of the most contagious and rapidly spreading infectious diseases and an important global cause of hospital admissions and mortality. There are some amounts of the virus in the air constantly. These amounts is generally not enough to cause disease in people, due to infection prevention by healthy immune systems. However, at a higher concentration of the airborne virus, the risk of human infection increases dramatically. Early detection of the threshold virus concentration is essential for prevention of the spread of influenza infection. This review discusses different approaches for measuring the amount of influenza A virus particles in the air and assessing their infectiousness. Here we also discuss the data describing the relationship between the influenza virus subtypes and virus air transmission, and distribution of viral particles in aerosol drops of different sizes. 1. Introduction Influenza is one of the most contagious and rapidly spreading infectious diseases and an important global cause of hospital admissions and mortality . Influenza virus concentration [2, 3], air circulation time, air temperature, and humidity  play an important role in overcoming the epidemic threshold. Influenza virus particles are constantly circulating in the air (airborne) in different forms (within dust particles or aerosol droplets) [5, 6]. There are some amounts of the virus in the air constantly. These amounts are insufficient to cause disease in people (the immune system of healthy humans prevents infection). However, at a higher concentration of the airborne virus, the risk of human infection increases dramatically. Early detection of the threshold virus concentration is essential for prevention of the spread of influenza infection. Furthermore, manufacturers are going to integrate detectors of virus particle numbers into hospital air control system equipment. This review discusses different approaches for measuring the amount of influenza A virus particles in the air and assessing their infectiousness. One of the fundamental works focused on the definition of the harmful concentration of the influenza A virus in the air is a paper by Alford, with coworkers . It is cited in many recent reports [8–10]. A study was initiated to determine the minimum infectious aerosol dose and the resulting patterns of infection and illness. Observations made during experimental infections with human volunteers are particularly interesting and relevant. In studies conducted by Alford and colleagues , volunteers were exposed to carefully titrated
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