Objective This review aims to investigate (i) the concentrations of air pollutants/bacteria in the underground mass-transportation in comparison to the above-ground environment; and (ii) the health implications upon exposure to the underground mass-transportation environment. Methods A systematic review was conducted using keyword searches in Medline, PubMed and ScienceDirect. After screening for relevance, 25 papers are extracted and critically reviewed. Results A number of air pollutants/bacteria/fungi were found having higher concentrations in the underground mass-transportation as compared with those in the above-ground environment. Air quality in the underground mass-transportation is influenced by factors like the service years of subway since operation, train specifications, sampling time, and sampling location. No acute implications were identified, but irritations, human cell damages and anti-inflammatory responses were identified. Conclusion Air quality in underground is worse than in the above-ground environment except for the more recently built subway systems. Research for health implications upon long-term exposure to underground environment is recommended. / published_or_final_version / Public Health / Master / Master of Public Health