With economic development and urban expansion, people’s commuting distances are increasing, so the time spent on public transportation is increasing. The expansion of global air travel has overcome geographic barriers to disease vectors [1]. Passenger health is influenced by the cabin environment [2]. Airplane cabins provide a conducive environment for the transmission of COVID-19 [3]. Crowded economy-class cabin seats increase the risk of airborne disease transmission between sick and healthy passengers [4]. Of particular note is the emergence of COVID-19, which is transmitted mainly through droplets and aerosols [5,6]. Due to the high occupant density in an airplane cabin, the required total air exchange ratio is much higher than that in a building.

In typical offices, outdoor particulate matter <2.5 µm (PM 2.5) is the main source of indoor PM 2.5 [7]. Sangiorgi et al. reported that more than 80% of indoor PM 2.5 in office buildings comes from outdoors [8]. Mechanical ventilation filtration systems in office buildings are used to reduce indoor particulate matter exposure [9]. In the United States and Singapore, 90% of the indoor air in office buildings is recycled and filtered [10]. In China, a small number of office buildings directly use fresh air systems to provide filtered fresh air [11]. Previous research has found that crowded classrooms, apart from PM 2.5, are also associated with high levels of several chemicals, such as methanol and benzene [12,13]. Moreover, poor ventilation rates may lead to high levels of fungal particles in overcrowded places. Researchers have found that CO and PM levels were positively associated with students’ absence rates [14,15]. Indoor pollutants, including particulate pollutants, can affect student attendance and learning efficiency [16,17]. Many research teams have measured the concentration of pollutants in the classroom, and natural ventilation by opening windows is the main method [18,19]. A significant decrease in CO was found, which led to a reduction in respiratory illnesses in students [20]. But that would only be correct if the windows were actually opened and not left closed (to conserve heat). In addition to CO, NO₂ emitted by motor vehicles in inner cities may cause excess morbidity, especially related to asthma symptoms [21]. The levels of various pollutants were found to be higher indoors than outdoors [22]. Additionally, suggestive evidence links the classroom-level ventilation rate with students’ test results in math [17,23]. Therefore, the health of occupants is extremely susceptible to the influence of outdoor pollutants, and due to the crowded nature of classrooms, the contaminants cause higher levels of exposure for occupants.

The effectiveness of filtration systems for PM 2.5 purification in the area of human activity and therefore respiration has received widespread attention. The PM produced by commercial aircraft not only has a significant impact on the outdoor environment of terminal buildings [24], but also easily transfers from outdoors to indoors through mechanical ventilation systems [25].

The literature on airborne transmission for different modes of transportation and transportation hubs is relatively limited. Due to their unique physical structure and high passenger flow within the transportation infrastructure, subway lines have a high risk of epidemic transmission [26,27]. The CO₂ level was found to be linearly related to the number of passengers according to a correlation analysis [28]. It is important to consider commuter route choice in exposure assessment studies. The external environment has the strongest influence on air quality in subway cabins. The piston wind from the tunnel makes it easier to transport particulate matter into a subway station for a subway without platform screen doors.

2. Overview of Highly Occupied Public Environments