…Influenza is one of the most contagious and rapidly spreading infectious diseases and an important global cause of hospital admissions and mortality [1]. Influenza virus concentration [2, 3], air circulation time, air temperature, and humidity [4] 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.
… Infectious viruses and viral RNA can be detected in both larger particles of >5 μm and smaller particles of <5 μm [9, 14, 32]. Experimental studies have demonstrated that the influenza virus can remain infectious in small particle aerosols and can transit across rooms [24, 33]. Cowling et al. [33] found that aerosol transmission (particles <5 μm) accounted for approximately half of all transmission events. Infectious influenza was recovered in all aerosol fractions (5.0% in >4 μm aerodynamic diameter, 75.5% in 1–4 μm, and 19.5% in <1 μm; n = 5) [24].
The aerosol fraction that is <4 μm (the “respirable fraction”) is of particular concern because it can remain airborne for an extended time and disperse throughout a room occupied by a patient with influenza. Also, particles containing influenza RNA are small enough to be drawn down into the alveolar region of the lungs. The infectious dose required for inoculation by the aerosol route relative to contact or droplet transmission is unclear, but two reviews of previous studies concluded that the infectious dose by the aerosol route is likely to be considerably lower than the infectious dose by intranasal inoculation [21, 34] and that aerosol inoculation results in more severe symptoms [21], presumably because aerosol particles are able to deposit deeper in the respiratory tract. However, the viability of influenza viruses in particles of different sizes and the persistence of viable airborne viruses in the environment are not yet known.
…Among the temperatures tested, transmission was highly efficient at (5°C/41°F) but was blocked or inefficient at (30°C/86°F). Dry conditions (20% and 35% RH) were also found to be more favourable for spread than either intermediate (50% RH) or humid (80% RH) conditions.
… tested two influenza B viruses transmission at low (5°C/41°F) versus intermediate (20°C/68°F) temperatures. The transmission was more efficient under colder conditions. Thus, transmission of human influenza viruses by a respiratory droplet or aerosol route in the guinea pig model proceeds most readily under cold, dry conditions. These findings suggested two means by which environmental factors could drive the wintertime seasonality of influenza.
… Clear correlation and dependence of the number of diseased subjects on the concentration of the influenza virus in the air were shown in various models. For example, a relationship between the number of infected pigs and the influenza detection in the air was identified in a study on a single H1N1 viral strain [47]. The chance of detection of an influenza positive air sample increased 2.2 times per each additional nasal secretion by a sick pig. This suggests that the risk of aerosolization and perhaps aerosol transmission increases as the number of infected pigs increase.
…Atkinson and Wein [8] created a mathematical model that describes aerosol (i.e. droplet-nuclei) and contact transmission of influenza A virus subtype H5N1 within a household containing one infected. It was demonstrated that in addition to the concentration of particles in the air that a person inhales, time plays a determining role in the influenza virus infection.
