PM2.5 on the London Underground

Abstract

Introduction: 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.

Methods: 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.

Results: The PM_2.5 mass in the LU (mean 88 μg m^-3, median 28 μg m^-3) was greater than at ambient background locations (mean 19 μg m^-3, median 14 μg m^-3) and roadside environments in central London (mean 22 μg m^-3, median 14 μg m^-3). Concentrations varied between lines and locations, with the deepest and shallowest submerged lines being the District (median 4 μg m^-3) and Victoria (median 361 μg m^-3 but up to 885 μg 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.

Conclusion: 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.

Keywords: Composition; Exposure; Metro; PM(2.5); Subway.

Fig. 1

(a)–(c): Boxplot summary statistics for PM_2.5, particle number, and particle diameter in each of the environments sampled. The lower and upper hinges correspond to the 25th and 75th percentiles, the horizontal line to the median, and the whiskers to 1.5 × the IQR (approx. 95% percentile). The red circle shows the mean. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 2

PM_2.5 concentrations recorded on the Jubilee Line. Stations are marked with either red or blue circles depending on depth, and a background PM_2.5 concentration taken from the ’Kensington and Chelsea – North Kensington’ background monitoring site is shown with a red horizontal line. Areas of discussion in the text are demarked A to D. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 3

PM_2.5 concentrations in μg m⁻³ recorded at each station of the Central Line. Station icons are colour-coded by depth (metres). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 4

PM_2.5 concentrations by line, ordered by median concentrations. The lower and upper hinges correspond to the 25th and 75th percentiles, the horizontal line to the median, and the whiskers to 1.5 × the IQR (approx. 95% percentile). Mean line depth shown in brackets beneath, means shown as white circles.

Fig. 5

Top 30 LU stations ranked by passenger numbers (red), PM_2.5 concentrations (green) and passenger population-weighted exposure (blue). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 6

(a) High time resolution measurements of PM_2.5 (b) The number of trains on the southbound platform at Hampstead Station.

Fig. 7

Chemical composition of PM_2.5 as A – Hampstead Station at four-hour time resolution, B – bar chart of mean surface London background, C – pie chart for surface London background, D – pie chart for Hampstead Station.