Exposure to particulate matter on the London Underground in healthy subjects and patients with chronic respiratory disease

Exposure to ambient air pollution - notably tiny particles called particulate matter (PM) - increases ill health and in some instances early death. There is no direct evidence of impacts on health from exposure to tunnel dust while travelling on the London Underground. However, this conclusion rests on a somewhat limited evidence base. Given that both long- and short-term exposure to ambient air pollutants is understood to be detrimental to health, it seems reasonable to assume that the same would be true of exposure to tunnel dust, when PM concentrations are much higher than those above ground. However, given the different physio-chemical properties of subway particles, compared with those found in ambient air, means that it is not currently possible to state the nature and extent of any health risk.

In this study we will utilise a tried and tested approach to address the possible health impacts of the air in the London Underground. Using the same protocols as in our 'Oxford Street' studies we will investigate the health impacts of using the London underground network in healthy volunteers and subjects with cardio-respiratory problems.

The outcome of this work will either provide assurance that the London underground air is safe to breathe and indicate that no further action is required to clean up the tube network or, alternatively, if health effects are found then this will support action for further cleaning up the underground environment. In these circumstances an improved underground environment will therefore benefit the health of all underground commuters.

There is evidence of the health effects of breathing atmospheric particulate matter (PM). In particular that traffic-derived particulates have adverse impacts on ischaemic heart disease and chronic obstructive pulmonary disease (COPD). Inhalation of urban particulate matter induces airway oxidative stress with an acute inflammatory response that may upregulate systemic inflammatory responses.

Although the effects of atmospheric PM have been under scrutiny, little is known about the effects of subway PM. A forthcoming report by the Committee on the Medical Effects of Air Pollutants (COMEAP) has highlighted the presence of high concentrations of PM on the London Underground. Many of the PM2.5 measurements made in the London Underground are higher than those measured in other locations worldwide, most probably due to its age, depth, tunnel distance and limited ventilation. PM2.5 mass was found to be approximately 15 times higher in the Underground (mean 302 ugm-3) than roadside environments (mean 26 ugm-3) in central London.

Elevated concentrations of PM in the London Underground are caused by additional sources, principally the wear of train consumables (e.g. wheels, brake blocks and electrical contacts), non-rolling stock sources (e.g. rail grinding and wear, people) and station sources (escalators and refurbishment work). The composition of PM therefore has a different composition to that of atmospheric PM, with underground PM2.5 found mainly to contain iron oxides, with the remaining mass made up of elemental carbon and organic carbon.

We will undertake a randomised crossover study to investigate the health effects of PM on the London Underground in patients with COPD, and will employ healthy persons as a control group. A study of this disease with a focus on common vascular and inflammatory responses will enhance our understanding of potential mechanisms for cardio-respiratory outcomes.

Who might benefit from this research?

Vulnerable populations, including children, the elderly and people with pre-existing conditions, are more susceptible to the adverse health effects of ambient air pollution. The latest London Travel Demand Survey indicates that, between 2015 and 2016, 72% of passengers on the London Underground were below 45 years of age. Although no statistics are available, it is possible that there may be a similar distribution of morbidity in the travelling public as in an age-matched population as a whole. Few experimental studies have assessed the health impacts of PM on subways, and the impact on those with chronic diseases has not been evaluated. Given an average of 4.8 million tube journeys are made on the London Underground every day and a reasonable proportion of those will be by people with cardio-respiratory conditions the findings may be of direct relevance to the health of these commuters.

How might they benefit from this research?

PM2.5 from roadside environments has been shown to affect lung function whilst causing pulmonary inflammation, which in turn is likely to cause systemic inflammation that results in increased arterial stiffness, endothelial dysfunction and autonomic dysfunction in both healthy subjects and those with chronic disease. If London underground PM is shown to elicit similar (or elevated) responses in commuters then this will support the action for further cleaning up the Underground environment. An improved underground environment will therefore benefit the health of all underground commuters.