Why respiratory viruses or bacteria have the highest probability to be deposited in the respiratory tract in flu seasons

Objective:

In this study the main aspects of influenza transmission via fine and ultrafine bioaerosols were considered. Here, we aimed to estimate the impact of the different environment conditions on the deposition rate of the infectious bioaerosols in the respiratory tract.

Background:

The latest researches show the infected people generate the fine and ultrafine infectious bioaerosols with submicron particles/droplets (size below 1 µm). The airborne transmission of these particles/droplets in the environment is effective. It is considered the deposition of submicron particles in the respiratory tract (RT) has very low probability. But most studies examined the aerosol deposition in RT under normal environmental conditions and did not paid attention to the affecting the different environmental factors.

Methods:

We review the problems of epidemiology of respiratory infections and aspects of airborne transmission/spread of infectious agents. We contrast these approaches with known data from next areas: inhalation toxicology, respiratory drug delivery and physics of heat and mass transfer in the airways.

Results:

Based on the conducted analysis, we propose the next main concepts:

1 Breathing cool air leads to the supersaturation of air in RT;

2 the air supersaturation leads to the intensive condensational growth(CG) of inhaled viruses or bacteria in RT;

3 CG leads to the intensive and dramatically growth of deposition rate of viruses or bacteria in RT.

We have shown:

a) Under normal conditions of inhaled air (T>20˚C; Relatively Humidity, RH=60%) there is no transition in supersaturated condition in RT and CG is insignificant and probability of virus deposition on epithelium of RT is low – no more than 20%.

b) Breathing cool/cold air of T<+15˚C and RH of [30..60]% leads to the supersaturation in the airways and it can dramatically increase the deposition rate of inhaled bioaerosols in RT(up to 96%).

c) With an increase in RH of inhaled air the supersaturation in RT occurs even at warm temperature of inhaled air (for inhaled air of T<20°C and RH>70% ; T<25°C and RH>90%). Which also indicates the deposition rate of bioaerosols in RT under these conditions is high.

Conclusion:

Under specific environmental conditions (when flu seasons) the processes of supersaturation in the RT can be observed. These results indicate the high probability of virus deposition on epithelium of RT and correspond to influenza and seasonal respiratory infections in temperate and tropical climates.

We believe the effect of supersaturation in the airways can be the key to understanding of ‘the age-old epidemiologic mystery of influenza seasonality in the different climatic conditions’.