Air Pollution in Public Transport Microenvironments: A Global Scopus-Based Review of Exposure, Methods, and Gaps

Air pollution associated with public transport systems constitutes a critical yet highly heterogeneous component of urban exposure, in which commuters and transport workers are frequently subjected to pollutant concentrations exceeding those reported by ambient background monitoring networks. This review provides a comprehensive and integrative synthesis of the global scientific literature on air quality in public transport microenvironments—including buses, bus stops, terminals, and underground stations—through a multidimensional analytical framework that explicitly accounts for climatic classification, socio-economic context, meteorological drivers, transport microenvironment typology, sampling strategies, analytical techniques, and exposure metrics. A large body of peer-reviewed studies published worldwide was examined to identify dominant patterns, methodological trends, and persistent knowledge gaps. Across regions, the evidence consistently reports elevated concentrations of particulate matter (PM₂.₅, PM₁₀, and ultrafine particles) and traffic-related gaseous pollutants, particularly within confined or poorly ventilated environments and during peak traffic periods. Marked geographical, climatic, and socio-economic imbalances are evident, with a strong concentration of studies conducted in temperate and tropical climates and in countries with very high or high Human Development Index, while arid, continental, and low-HDI regions remain substantially underrepresented. From a methodological standpoint, the literature is dominated by short- to intermediate-term monitoring campaigns relying on active sampling, mobile measurements, and increasingly calibrated low-cost sensors, whereas long-term stationary observations and standardized integrative monitoring frameworks remain scarce. Although advanced analytical approaches—such as chemical characterization, environmental magnetism, receptor modeling, computational fluid dynamics, and inhaled dose assessment—are progressively being applied, their combined and systematic implementation remains limited. Overall, this review reveals persistent methodological, geographical, and conceptual gaps and underscores the urgent need for standardized, interdisciplinary, and long-term monitoring strategies to improve exposure assessment and support evidence-based mitigation policies aimed at reducing health risks associated with public transport-related air pollution.