# How Human Mobility Models Can Help to Deal with COVID-19
## 6 Analysis of COVID-19 Transmission
### Background  
In the case of SARS-CoV-2, it has been proven that its transmission is fundamentally airborne [9], mainly by droplets and aerosols. Viruses in droplets (larger than 100 \(\upmu\)m) typically fall to the ground in seconds within 2 m of the source. Because of their limited travel range, physical distancing reduces exposure to these droplets. Nevertheless, the SARS-CoV-2 virus is able to travel more than 2 m through aerosols (smaller than 100 \(\upmu\)m) emitted by infected people when breathing and can remain suspended in air for many seconds to hours. In both cases, droplets and aerosols, they are highly concentrated near an infected person, so they can infect people more easily in close proximity. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline  & \multicolumn{3}{c}{**2 m**} & \multicolumn{3}{c}{**5 m**} & \multicolumn{3}{c}{**10 m**} \\ \cline{2-10}
**Scenario** & \(C\) & \(P\) & \(E[D]\) & \(C\) & \(P\) & \(E[D]\) & \(C\) & \(P\) & \(E[D]\) \\ \hline RWP & 9660 & 2784 & 7.6 & 24,222 & 4340 & 31.61 & 47,104 & 4883 & 108.9 \\ SWIM & 12,368 & 3436 & 7.8 & 62,624 & 4934 & 27.7 & 213,222 & 4950 & 102.2 \\ RWPa & 31,778 & 4739 & 12.4 & 78,692 & 4948 & 73.9 & 143,874 & 4950 & 277.7 \\ Plaza & 44,714 & 14,108 & 2.8 & 90,514 & 33,990 & 8.2 & 154,930 & 60,375 & 13.9 \\ Station & 5941 & 2632 & 21.9 & 7212 & 3682 & 43.1 & 14,324 & 5961 & 49.2 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The number of contacts, pair of individuals contacted and the accumulated average duration for the different scenarios and considering different distances. In order for the virus to establish and infect a new host, the receiving person must be exposed to this virus. The ease with which the virus can install itself in the receiving host depends on common and differentiated biological factors, as there are more genetically protected individuals than others. Not all exposed people are infected. However, the number of viral molecules that can land on the mucosa of another individual, and, therefore, the degree of success of the invasion is important and increases the chances that the virus will infect a new individual. Therefore, in this paper, we focus on evaluating only the exposure risk to the virus, without considering the receiver's biological factors that can finally infect her/him, which can be defined as her/his immunity to the virus. Note that the final risk of transmission will be proportional to the exposure risk and inversely proportional to the immunity of the person. To mathematically model the exposure risk (\(ER\)) we consider three main factors: the exposure time, the exposure intensity, and the quality of the medium. The goal is to have a simple and easy to understand expression to evaluate how the exposure risk depends on these factors. This way, we could easily communicate the main factors associated with the spread of the SARS-CoV-2 virus, including nonscientists such as policymakers and people in general. We are not pretending to obtain more realistic and complex expressions, like the ones developed in [28; 29; 30; 31; 32], which would require complex modelling and a lot of computational resources. The exposure time (\(t\)) is the first important factor since it measures the time a person is exposed to the virus. That is, the longer the exposure time, the greater the number of viral molecules that reach the exposed person, and thus, the risk that some of them may prosper increases. The second factor, the Exposure intensity (\(I_{x}\)), can be defined as the number of viral molecules reaching an individual per unit of time. Some authors use the concept of a quantum, as the dose of airborne droplet nuclei required to cause infection in 63% of susceptible persons) [29]. The exposure intensity factor depends on the intensity of the emitter, the proximity between individuals, and the quality of the medium between them.