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][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][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][14,15]. Indoor pollutants, including particulate pollutants, can affect student attendance and learning efficiency ^[16][17][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][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][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][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

A minimum of 30 m³/h per person is required in common environments. Highly occupied public environments gather a large number of people in some time periods, and an air supply of 20–30 m³/h per person is needed. In addition, the highly occupied public environments have larger spaces, the personnel activities are more complex, and the social distance is shorter. Personnel activities may increase the pollutant transmission distance. During the COVID-19 pandemic, public safety in crowded places became a great concern. People’s work and studies are closely related to densely crowded environments, as shown in Figure 1. Means of transportation are usually boarded in transportation hubs, which contain dense and highly mobile populations. This complex environment increases the risk of personal exposure. Commuters can easily carry viruses and bacteria from transportation hubs to public transportation, where crowds are dense and the social distance is short, which increases the risk of cross-infection. Finally, controlling the level of pollutants and reducing the risk of personal exposure in the air environment of densely crowded spaces such as classrooms and offices is urgently needed. The relevant review articles on pollutant exposure in highly occupied public environments are shown in Table 1. Droplets are large heavy particles that transfer from person to person in close proximity and are the main reason for the need to socially distance (say 1–2 m). They usually fall to the ground. The other main and more insidious mode of transfer of virus particles is small and lighter-weight aerosols that can drift within enclosed interior spaces of airflow and cause infection to others.

Classification	Highly Occupied Public Environments	Research Object	Main Conclusions
Crowded places	Classroom	NO2, O3, PM, VOCs, bacteria	Occupancy activities caused a large fraction of virus transmission due to the resuspension of previously deposited matter [29].
	Hospitals, Schools, Offices	Infectious diseases	Ventilation is positively associated with airflow direction control in buildings [30].
	Homes, Schools, and Hospitals	Humidity, influenza virus, VOCs	Humidity and temperature can be adjusted to achieve a satisfactory work environment [31].
	Public Spaces	Aerosol	The range of typical indoor aerosols can be used as a reference for biosensors designed to improve public safety [32].
Public transport	Pedestrians, Car, Bus, Massive Motorized Transport	Black carbon, carbon monoxide, coarse particles, fine particles, NO2	Pedestrians have higher levels of inhaled pollutants than commuters using motor cabins [33].
	Taxi, Bus, Subway, Busy Street	PM, CO, NO2, volatile organic compounds (VOCs), polycyclic aromatic hydrocarbons (PAHs)	There are differences in measurement methods and a lack of uniform measurement standards [34].
	Walk, Cycle, Car, and Bus	PM 2.5, ultrafine particles (UFPs), and black carbon	The concentrations of PM 2.5, ultrafine particles (UFPs), and black carbon (BC) in Asian cities are higher than those in the USA and Europe [35].
	Cycle, Car, Subway, and Bus	PM, BC	The levels of a bus passenger’s exposure to PM and BC largely depend on the bus route chosen [36].
	Walk, Bicycle, Car, Bus	PM 2.5, BC, UFPs, CO	Car drivers are exposed more than pedestrians [37].
	Walking, Cycling, Bus, Car, and Taxi	PM 2.5, BC, UFPs, CO	Pedestrians are exposed to lower levels of UFPs and CO than people inside vehicles [38].
	Bicycle, Bus, Automobile, Rail, Walking, and Ferry Modes	UFPs	Exposure to UFPs can have acute effects on health-compromised individuals [39].
	Subway	PM	Underground PM is more toxic than urban PM [40].
	Subway	PM	The dust in a subway system is more toxic than ambient airborne particulates [41].
	Subway	CO, PMs, VOCs	The ventilation mode, passenger numbers, and surrounding pollution level outside of a metro station could have important effects on the metro air quality [42].
	Aircraft	Airborne expiratory contaminants	Most researchers have applied Lagrangian models to analyze transient phenomena [43].
	Aircraft	Droplets, SARS	Most diseases have an incubation period that is longer than the period of air travel [44].
Transport hub	Transportation and Transportation Hubs	Respiratory viruses	Air transportation appears to be important for accelerating influenza propagation [45].
	Subway Stations	High temperature, high humidity, PM, VOCs	Ventilation can spread air pollutants through a complex airflow [46].
This study	Classroom, Office, Subway station, Airport, Bus, Subway, Aircraft	CO, PM, droplets	The pollutant concentration detection system should be combined with the air-conditioning system in crowded places. Passenger flow had a great influence on pollutant transport in transport hubs.

The average concentrations of PM 2.5 and total suspended PM that people taking subways are exposed to are 8 and 12 times those to which taxi drivers are exposed, respectively [41]. Moreover, uniform measurement standards are rarely applied to Asia ^[34][35][34,35]. Future work should place an emphasis on confirming the contribution of ultrafine particle (UFP) exposure to total PM exposure during transportation [39]. Exhaled droplets are transmitted in indoor environments, either directly or through air distribution [47]. PM, black carbon (BC), CO, fungal particles, bacteria, and viruses are frequently the main pollutants identified in subways. Occupant density and occupant activities have a high impact on indoor air quality. The level of indoor aerosols is mainly affected by indoor population density, outdoor air level, and ventilation type [48]. Considering the coupled relationship between the indoor environment and ventilation, internal ventilation strategies should be determined according to outdoor conditions [46]. Currently, there are insufficient data to assess the relationship between the ventilation rates of highly occupied public environments and airborne infectious diseases, and there is a lack of quantitative research on the impact of occupant activities on pollutant transmission. Therefore, future work should focus on investigating the health risks posed by air pollutants and further developing advanced ventilation strategies to improve air quality in crowded places. The main indoor air pollutants in classrooms include viruses, PM, and volatile organic compounds (VOCs). Poor ventilation and crowding are environmental factors that affect the risk of airborne infection in ground public transportation ^[49][50][49,50]. In the absence of sufficient air renewal, high transmission of influenza may occur in confined spaces. Therefore, the concentration in underground or closed stations may be several times higher than the concentration in the surrounding air ^[51][52][53][51,52,53]. Due to the metal wear of wheels and brake shoes in subway tunnels, the level of Fe-containing particles and black carbon level (PM, BC) are high in subway stations ^{[54][55][56][57]}[54,55,56,57].