Pollutants in underground spaces mainly include TVOC, CO, PM
10, radioactive Rn, and so on. Concentrations of formaldehyde and TVOC in underground malls are higher than those in outdoor environments. Tao et al.
[4] investigated concentration levels of formaldehyde and TVOC in 9 underground malls in Xi’an, China. Mean mass concentrations of formaldehyde and TVOC range from 0.05 mg/m
3 to 0.26 mg/m
3 and from 0.34 mg/m
3 to 3.56 mg/m
3, respectively. Liu et al.
[5] investigated patterns of pollutant concentrations and their sensitivities to traffic volume in naturally ventilated underground parking garages. They found that the PM
2.5 concentrations in underground parking garages were slightly higher than in ambient environments, but PM
10 concentrations were significantly higher. Chow et al.
[6] investigated indoor CO concentrations in a large underground parking garage in Hong Kong. CO concentration does not necessarily rise when the total number of cars increases. Oh et al.
[7] investigated the exposure of particulate matter and TVOC in underground parking garages under various ventilation modes and their impact on health. The levels of inhalation exposure to PM and TVOC as well as the health effects associated with them, are affected by the ventilation types used in underground parking garages. For most residential underground parking garages in China, natural ventilation is the most popular ventilating mode, when construction and operating costs are considered. Therefore, more effective ventilation systems are required depending on the traffic–volume and indoor air quality (IAQ). Yang et al.
[8] investigated IAQ in an underground shopping mall. Although the CO
2 concentration can meet the requirement stipulated in the standard, people also recommended that IAQ, the maximum acceptable CO
2 concentration stipulated in the standard, should be investigated further. Li et al.
[9] investigated the combined effect of elevated temperature, RH and CO
2 concentration on human responses. They found that high RH and CO
2 concentration degrade air quality, and that the combined effects of high air temperature, RH and CO
2 concentrations result in a variety of human responses. Braniš
[10] compared PM concentrations measured on streets, in underground spaces and within subway trains. The highest PM
10 concentrations were found inside Metro trains (113.7 mg/m
3 and 1.44 mg/m
3), followed by underground station spaces (102.7 mg/m
3 and 1.29 mg/m
3), and outdoor environments (74.3 mg/m
3 and 0.85 mg/m
3). In Europe, related agreements stipulate that subway air-conditioning filters should be replaced regularly every month
[11]. However, Moreno et al. found that filters could maintain air quality for at least 3 months, and if pathogens can also be maintained within the standard range during this period, then the current agreement needs to be changed. They also pointed out that the WHO should give additional and equally important attention to improving underground air quality
[11]. “Construction specifications on underground excavation engineering of hydraulic structures” stipulates that the minimum air velocity used when calculating the ventilation of large underground spaces is 0.15 m/s
[12]. Li et al. found that the air velocity could control the dust level, provide the required oxygen concentration and control the temperature within an acceptable range, but it was not enough to reduce the CO concentration below the acceptable safety limit
[13].
Radon is a major environmental carcinogen identified by the World Health Organization, and it is primarily emitted by soil and building materials. In reality, the radon risk is primarily caused by short-lived radon progeny. The increased exposure caused by short-lived radon progeny in underground mines is primarily due to inadequate ventilation and increased radon exhalation from surface materials
[14]. Li et al.
[15] investigated the daily and seasonal variations of radon concentrations in underground buildings in major cities of China. Radon concentrations in the underground buildings fluctuate in two cycles per day. In winter, radon concentrations in the underground buildings are lower than in summer, which is the opposite of the situation above ground level. The primary external factor causing this phenomenon is seasonal variation in outdoor temperature
[16]. Jin et al.
[17] summarized the characteristics of underground indoor radon, its progeny, and the source to those of unequilibrium radon and its progeny. Underground indoor radon levels are influenced not only by building materials, but also by geological structure, air tightness of houses, indoor ventilation rate, people’s living habits, and other factors.
During the construction phase, common harmful components in underground spaces are CO, CO2, NO2, SO2, dust and exhaust gas from construction equipment. Ventilation during the construction phase, which is used for comprehensive control of underground space construction environments, has a direct impact on the body health and construction efficiency of tunnel construction workers.