Numerical Modeling of Face Shield Protection against a Sneeze

Abstract

The protection provided by wearing masks has been a guideline worldwide to prevent the risk of COVID-19 infection. The current work presents an investigation that analyzes the effectiveness of face shields as personal protective equipment. To that end, a multiphase computational fluid dynamic study based on Eulerian–Lagrangian techniques was defined to simulate the spread of the droplets produced by a sneeze. Different scenarios were evaluated where the relative humidity, ambient temperature, evaporation, mass transfer, break up, and turbulent dispersion were taken into account. The saliva that the human body generates was modeled as a saline solution of 8.8 g per 100 mL. In addition, the influence of the wind speed was studied with a soft breeze of 7 km/h and a moderate wind of 14 km/h. The results indicate that the face shield does not provide accurate protection, because only the person who is sneezed on is protected. Moreover, with a wind of 14 km/h, none of the droplets exhaled into the environment hit the face shield, instead, they were deposited onto the neck and face of the wearer. In the presence of an airflow, the droplets exhaled into the environment exceeded the safe distance marked by the WHO. Relative humidity and ambient temperature play an important role in the lifetime of the droplets.

Computational characterization of the behavior of a saliva droplet in a social environment

“…For this case, the temperatures are 14 °C and 35 °C and relative humidity of 0%, 20%, 40%, 60% and 80%, with a starting speed of 0 m/s, so it has only affected the forces of gravity, the weight of the drop, according to Bozic 9 . According to the CFD model of Pendar et al 22 , approximately the experimental average speed that can catch a sneeze is 20 m/s, but when it is emitted to the environment in just a few seconds loses the speed adapting to that of the environment, Ugarte-Anero et al 1 . Figure 3 shows a sketch of the forces to which a particle is subjected.…”

“…Stiti et al 15 shows that a droplet of an initial diameter of 21 µm in 2 s could become an aerosol when a temperature above 20 °C and an RH below 80% occurs. Studies such as Xie et al 8 , Liu et al 16 and Ugarte-Anero et al 1 state that at higher relative humidity the evaporation time of these is longer. These same studies, observe that the larger diameter of size the evaporation time is greater.…”

“…Different types of masks were designed. Notably, mathematical studies such as Ugarte-Anero et al 1 demonstrate through a Computational Fluid Dynamics (CFD) research that the face-shields, which cover the entire surface of the face, do not fully protect a person from sneezing. Later, and currently is developing, the invention of different vaccines.…”

“…Even if you focus on the same person, depending on what you eat during that day, it can vary the composition of saliva. Composed of 99% of water and 1% composed of other elements, notably sodium and chlorine for being more abundant, Almeida PDV et al 7 , CFD studies make an approximation to saliva as a saline solution of 0.9 w/v, Xie et al 8 , being the most accurate approximation of evaporation time to the aerosol, Ugarte-Anero et al 1 . Even so, droplets of saliva have the same method of evaporation as a droplet of pure water.…”

“…The evaporation rate of pure water is higher than the evaporation rate of a drop of saline solution by the high solubility that NaCl has, affecting the vapor pressure of saturated water, Shahidzadeh et al 11 . This phenomenon is marked by various characteristics that occur in the environment, such as, for example, relative humidity (RH) and ambient temperature among others, Ugarte-Anero et al 1 , Wang et al 12 and Sen et al 13 . Gregson et al 14 indicates two important factors in the evaporation rate of an aqueous sodium droplet, the temperature of the gaseous phase and the solute concentration in the starting droplet, deriving that a higher concentration of solute indicates a longer evaporation rate time.…”