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New research on health impacts of air quality on London Underground staff

A new study by Imperial College London has examined the impact of airborne dust on employees working within the London Underground. 

Over 19,000 people are employed by the  London Underground and many of those are exposed to elevated concentrations of PM2.5 on a daily basis. The study – the largest to date – aimed to quantified that exposure and investigate the possible association with sickness absence.

red and yellow train in train station

In parts of the network PM2.5 levels can be up to 15 times higher than in the air outside. Furthermore the underground particulates – from the wear of rails, wheels and braking have different characteristics than those above ground and thus are likely to have different health impacts.

The team used a dataset  from Transport for London (TfL) which included 29,744 staff employed between January 2014 and  December 2019 who all regularly worked on the London Underground network.

Roles included drivers, customer service staff and fleet staff (train engineers, depot staff and technical support). 

Information about exposure to PM2.5 was available from personal monitors and a series of network-wide measurements at underground platforms and stations.

The data on sickness absences were obtained from TfL’s HR Department, using a method that protected individuals’ privacy.

The study did reveal that staff working in areas with higher PM2.5 levels were more prone to take days off sick but the researchers stress that ‘the findings do not provide enough evidence to establish that exposure to PM2.5 directly leads to sickness absence.’

However, the findings do shine a light on how much exposures can vary across the network, even among those doing the same job. Drivers were exposed to considerably more PM2.5 on lines with deeper tunnels, with more of their time spent underground. The trains themselves also tended to be older.

Drivers were found to be more exposed to PM2.5 than other staff but the levels they experienced ranged hugely, from an average of 72µg/m³ on the District line to 787µg/m³ on the Piccadilly line.

Customer service staff who spend more time on platforms had PM2.5 exposure ranging between 7µg/m³ and 310µg/m³ while the highest levels their managers experienced was 154µg/m³

Looking at reported sickness, it was seen that fleet staff, customer service staff and drivers all had higher rates of sickness absences compared with non-exposed office workers. Drivers on five out of eight lines showed elevated rates of all-cause sickness absence.

However, the team found no clear ‘exposure-response relationship’, where researchers would expect to see increasing exposure associated with an increased sickness absence so they feel they cannot say for certain that PM2.5 exposure is directly contributing to sickness absence.

Imperial PhD candidate and first author of the study Justie Mak said: ‘While this study is one of the largest and most comprehensive of its kind, linking exposure to health effects in the human body is complicated – the effects may be too subtle to detect in this way, or take a long time to develop.

‘Chronic health impacts linked to dust exposure are likely to develop gradually, probably after prolonged inflammation. We are currently looking at the longer-term health effects among London Underground workers and will be reporting that later in the year.’

In conclusion, the report suggests future areas of research: ‘Toxicological studies may provide insight into mechanisms on a cellular or organism level, and comparisons made to PM from different sources. Future research should include long-term prospective epidemiological studies, which provide a better means of assessing the adverse health effects of exposure to PM2.5 and can be used to investigate the health effects of chronic exposure to subway PM.’

The study was funded by Transport for London and the National Institute for Health Research.

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