Characteristics, sources and risk assessment of volatile organic compounds in a subway environment - A case study in Xi’an, China

Volatile organic compounds (VOCs) are ubiquitous, with toxicity that pose risks to human health and the environment. The adverse effects of VOCs could be higher in the subway environment due to possible accumulation because of inadequate ventilation. However, systematic and comprehensive studies on VOCs pollution in subway environments are lacking. This paper investigated the types and pollution characteristics of VOCs in 4 different lines of the Xi'an Subways including carriages, platforms and concourses, aiming to identify the pollution sources, to assess the influencing factors for VOC concentrations, and to evaluate the health risks of VOC exposure. A total of 113 VOCs were detected with propionaldehyde having the highest concentration (39±17 µg/m³), followed by acetone (29±13 µg/m³), acetaldehyde (23±15 µg/m³), methylene chloride (17±24 µg/m³), and formaldehyde (12±40 µg/m³). Several factors, including the number of passengers, subway operation period, and the track position, have been identified to affect the VOC concentrations in carriages. VOC concentrations are decreasing from underground platform > elevated platform > underground concourse > ground platform. Decorative materials, outdoor sources, human body emissions, and cleaning supplies were the main sources of VOCs in subway carriages with human body emissions as the largest contributor (35.42 %). The carcinogenic risk to passengers of formaldehyde (3.86 × 10^–5), methylene chloride (1.76 × 10^–5), acetaldehyde (1.23 × 10^–5), trichloromethane (8.17 × 10^–6), 1, 2-dichloroethane (4.82 × 10^–6), 1, 2-dichloropropane (3.05 × 10^–6) and 1, 3-butadiene (1.08 × 10^–6) were above the acceptable risk level (≤10^–6). These results provide scientific basis for planning the improvement of the air quality of in subway system now and in the future.