PM_2.5 on the London Underground

Despite the London Underground (LU) handling on average 2.8 million passenger journeys per day, the characteristics and potential health effects of the elevated concentrations of metal-rich PM_2.5 found in this subway system are not well understood.

Spatial monitoring campaigns were carried out to characterise the health-relevant chemical and physical properties of PM_2.5 across the LU network, including diurnal and day-to-day variability and spatial distribution (above ground, depth below ground and subway line). Population-weighted station PM_2.5 rankings were produced to understand the relative importance of concentrations at different stations and on different lines.

The PM_2.5 mass in the LU (mean 88 microg m^-3, median 28 microg m^-3) was greater than at ambient background locations (mean 19 microg m^-3, median 14 microg m^-3) and roadside environments in central London (mean 22 microg m^-3, median 14 microg m^-3). Concentrations varied between lines and locations, with the deepest and shallowest submerged lines being the District (median 4 microg m^-3) and Victoria (median 361 microg m^-3 but up to 885 microg m^-3). Broadly in agreement with other subway systems around the world, sampled LU PM_2.5 comprised 47% iron oxide, 7% elemental carbon, 11% organic carbon, and 14% metallic and mineral oxides. Although a relationship between line depth and air quality inside the tube trains was evident, there were clear influences relating to the distance from cleaner outside air and the exchange with cabin air when the doors open. The passenger population-weighted exposure analysis demonstrated a method to identify stations that should be prioritised for remediation to improve air quality.

PM_2.5 concentrations in the LU are many times higher than in other London transport Environments. Failure to include this environment in epidemiological studies of the relationship between PM_2.5 and health in London is therefore likely to lead to a large exposure misclassification error. Given the significant contribution of underground PM_2.5 to daily exposure, and the differences in composition compared to urban PM_2.5, there is a clear need for well-designed studies to better understand the health effects of underground exposure.