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Infection via inhalation of pathogens is termed airborne transmission. It occurs by spreading of either airborne droplet nuclei (typically 0.5 to 12 µm in diameter) or dust particles containing the infectious agents. Pathogens carried in this way can be widely dispersed by air flows and may become inhaled by a person close to the patient or may move a longer distance from the source patient. The distance that pathogens can move depends on factors such as their concentration, viability, diameter and aerodynamic behavior; as well as, environmental or physical conditions such as temperature, airflow and humidity [23]. Airborne transmission can cause nosocomial infections like tuberculosis, measles, chickenpox, severe acute respiratory syndrome (SARS) and flu (H1N1, bird flu, etc.). The cost related to the treatment of persons infected by airborne diseases represents a significant part of the health care system budget [24].
A large amount of droplets can enter the indoor air from a patient mouth by sneezing or coughing. First, these droplets are humid. After the release they start to dry-up and their diameter drop to 0.5-12 µm. Droplets with this new range of size are small enough to remain airborne and move passively through the air for a long period of time [9, 24]. According to Xie et al. (2007) [25] exhaled droplet nuclei are carried more than 6 m by sneezing while their initial velocity is around 50 m/s. Based on their results, coughing makes the droplet nuclei pass a distance more than 2 m while having an initial velocity equal to 10 m/s. Bolashikov et al. (2012) [8] reported that the initial peak of flow velocity generated by cough varies from 6 m/s up to 30 m/s.
The size and number of infectious airborne particle depend on the mechanism of generation, the age and the healthiness of the person who produced it. Different literatures present various amount for the size, number and the initial velocity of airborne particles. For example, Cole et al. (1998) [26] reported that coughing generates approximately 10⁴ to 10⁵ droplets. According to Marko Hyttinen et al. [33], sneezing generates up to 4×10⁴ droplets, while a cough or talking for 5 min produce 3×10³ droplet nuclei. Chao et al. (2009) [28] reported that the number of expelled droplets is 947 to 2085 per cough and 112 to 6720 during speaking.
Generally, the diameter of a virus is usually between 0.02-0.3 μm, a bacteria diameter is between 0.3-10 μm and fungal spore is 2-5 μm [24, 26]. Multiple pathogens are needed to create a sufficient level of infection for disease to be expressed. These pathogens need some larger aerosols like skin flakes, which are 13-17 μm in diameter or droplet with typical size of 20-40 μm in diameter, to be transported. About 95% of the respiratory generated droplets are smaller than 100 μm, while most of them are between 4-8 μm [27, 28, and 29]. Also, Yang et al. (2007) [30] reported that the coughing droplets size average is 8.35 μm. Papineni et al. (1997) [31], by means of improved diagnostics optical particle counting, showed that 80-90% of exhaled droplets in breathing or coughing are less than 1μm in diameter. Edwards et al. 2004 [32] confirmed Papineni et al. [31] results by a similar experiment on 11 healthy human subjects. They suggested that inhaled particles during normal breathing are smaller than 1μm. Hyttinen et al. [33] in a review in 2011 reported that the size of coughed droplets is in the range of 0.6-16 μm and the average size is 8.4 μm. They mentioned that the size of droplet nuclei is between 0.6-5.4 μm, and 84% of them are smaller than 2.1 μm. Chao et al. (2009) [28] reported that the geometric mean diameter of the droplets exhaled during coughing is 13.5 μm and for speaking, it is around 16.0 μm.
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