Background The subway environment, characterized by dense crowds, relatively enclosed architectural structures, and insufficient ventilation, poses various health hazards. Population health risk attributable to subway hygiene have become an important public health issue. Objective To evaluate the current environmental hygiene of metro line X in Nanjing, and to provide a scientific basis for the prevention and control of harmful factors and the establishment of relevant standards. Methods Station halls, platforms, and carriages of Nanjing metro line X were monitored in March 2023. Six stations were selected by stratified sampling, and monitored once during the morning peak hours (9:00—11:00). Carriage monitoring was conducted after a full day of operation (21:00—22:00). The monitoring indicators included physical factors such as temperature, relative humidity, wind speed, illuminance, and noise (A-weighted); air quality factors such as carbon dioxide (CO₂), carbon monoxide (CO), respirable particulate matter (PM₁₀), fine particulate matter (PM_2.5), ammonia (NH₃), formaldehyde, benzene, toluene, xylene, total volatile organic compounds (TVOC), ozone (O₃), total bacterial count, and total fungi count; hygiene status of the inner surface of the central air conditioning ventilation system, such as dust accumulation, total bacterial count, and total fungi count; supply air, such as PM_10, PM_2.5, total bacterial count, and total fungi count; and surface microorganisms on objects, such as total bacterial count, total fungi count, coliform bacteria, and Staphylococcus aureus. Results were evaluated and discussed according to Hygienic indicators and limits for public places (GB 37488-2019) and Hygienic specification of central air conditioning ventilation systems in public buildings (WS 394-2012). Results The environmental temperature medians (P₂₅, P₂₅) in the platforms and carriages of Nanjing metro line X were 20.7 (19.95, 21.65) ℃ and 21.8 (19.80, 35.80) °C, respectively, which exceeded the standard limits. The relative humidity and noise level in the carriages were 38.80% (24.6%, 46.3%) and 79.50 (76.25, 82.00) dB, respectively, failing to meet the national health standards. The total fungi count in supply air in 22.2% of the monitoring points exceeded the standard value. Coliform bacteria and Staphylococcus aureus were detected on the surfaces of some high-touch objects, with the bathroom faucet being the most severely contaminated. The medians (P₂₅, P₂₅) of temperature [21.8 (19.8, 35.8) ℃], wind speed [0.48 (0.39, 1.02) m·s⁻¹], noise [79.5 (76.25, 82.00) dB, and CO₂ content [0.079% (0.070%, 0.091%)] in the carriages were higher than those in the station halls and platforms (P<0.05). There were significant differences in microclimate, air particulate matter, and air microbial levels between urban and suburban stations (P<0.05); the concentrations of PM₁₀ [0.076 (0.046, 0.079) mg·m⁻³)] and PM_2.5 [0.063 (0.044, 0.068) mg·m⁻³)] in suburban stations were 2 times higher than those of urban stations. The temperature [21.60 (20.45, 21.80) ℃], humidity [45.20% (40.95%, 50.10%)], CO₂ [0.100% (0.100%, 0.825%)], and PM₁₀ concentration [0.070 (0.041, 0.080) mg·m⁻³] in transfer stations were all significantly higher than those in non-transfer stations (P<0.05). The temperature at stations was significantly correlated with the CO₂ content (r_s=0.6107, P<0.001), the concentration of air particulate matter and the content of air microorganisms were positively correlated with the relative humidity (P<0.05), and the air PM₁₀ and PM_2.5 concentrations were positively correlated with the total bacterial count and the total fungi count (P<0.05). There was a significant positive correlation between the total bacterial count and the total fungi count on the inner surface of the air duct (r_s=0.5877, P<0.05). The concentrations of PM₁₀ and PM_2.5 in supply air were positively correlated (r_s=0.9635, P<0.001), and both were also positively correlated with the total bacterial count in supply air (r_s=0.6848 and 0.6511, P=0.002 and 0.003). Conclusion All chemical factors of metro line X in Nanjing meet the national standards, while physical factors (temperature, relative humidity, wind speed, and noise) exceed the standards, and coliform bacteria and Staphylococcus aureus are detected on the surfaces of bathroom faucets and carriage poles. It is advisable to reinforce microclimate management and noise reduction measures in carriages, while ensuring regular cleaning and disinfection of public facilities and equipment.