The mask intervention

Public transport, serving as the method of choice for the daily travel of the majority of people, especially as the predominant vehicle for inter-city population movement, has been facing great challenges in preventing SARS-CoV-2 cross-infection due to its limited space and a large number of occupants. Even worse, the high transmission risk of asymptomatic persons during the peak of the outbreak aggravates the challenge. A series of cases of SARS-CoV-2 infection on ships, planes, trains, and buses can be found everywhere through the contact tracing reports of COVID-19 (Moriarty et al. 2020). Direct contact and close contact transmission of SARS-CoV-2 are still the main ways to spread the diseases in closed spaces at present, with not mutually exclusive (Morawska et al. 2020). In these two modes of transmission, a confined space provides a fast channel for the virus from the infected person to the exposed person, so that it can maintain a high virus concentration and vitality in the process of transmission, achieving further reproductive. Due to the coupling effect of multiple factors, such as the different types of virus, virus concentration, environmental conditions, exposure time, and heterogeneous of exposed people, it is difficult to quantify. Thus, the transmission mechanism of SARS-CoV-2 in public transport fluctuates and is still unknown. So passengers should take personal protective measures as much as possible to minimize the risk of infection in a closed vehicle.

The high effectiveness of N95 masks is widely recognized. Whereas, the limited supply at the peak of the outbreak exacerbated the shortage of masks. In the face of the exponentially growing number of new cases, some countries began to encourage the public to use homemade masks that were made of common materials for self-protection (Davies et al. 2013). However, except for N95 masks, the public have always doubt the efficacy of surgical and homemade masks, because the National Institute for Occupational Safety and Health (NOISH) regulated the 0.30 μm particle diameter filtering efficiency as the basis to evaluate the performance of masks; thus, some people pointed out the ordinary masks cannot block exhaled virus droplets of small particle size, and thus wearing masks cannot reduce the risk of infection. Hence, our objective is to quantitatively distinguish the working efficiency of the three kinds of masks through the Wells-Riley model and give scientific mask-wearing suggestions under different conditions.

In order to detect the protection levels of several common masks, we reviewed and analyzed the experimental research results of N95, surgical, and homemade masks according to the four papers (Davies et al. 2013; Lee et al. 2008; Noti et al. 2012; Weber et al. 1993), and used filtration efficiency as the indicator, obtaining the efficiency distribution characteristics along with different particle sizes of the masks, as depicted in Fig. 3.

Filtration efficiency of several masks for 0–4 μm aerodynamic diameter

It can be seen that the five different types of N95 masks do show excellent filtration efficiency for all particle sizes. The filtration efficiency of three surgical masks and one homemade mask are obviously lower than that of N95 masks, especially for small droplets less than 1 μm in diameter. However, it is undeniable that various types of masks can achieve the blocking effect of exhaled droplets to different degrees. The filtering efficiency of N95 masks remained stable above 98% (Lee et al. 2008; Noti et al. 2012), surgical masks ranged from 75 to 93% (Davies et al. 2013; Lee et al. 2008; Noti et al. 2012). Even homemade masks can have a filtering efficiency of 71–82%. Hence, according to the papers, we assumed that the average filtration efficiency of N95, surgical, and homemade masks were 98, 80, 70%, and could reduce the amount of contaminated air inhaled to 0.03, 0.2, and 0.3p, respectively.