London Underground employees working in areas exposed to higher levels of fine dust also tend to report more episodes of sickness absence, research has suggested.
However, the study has stopped short of suggesting there is a direct causal link between exposure to PM2.5 particulate matter and heightened sickness absence.
The study has been published in the journal Environment International. It was carried out by researchers at Imperial College London’s School of Public Health and National Heart & Lung Institute and is believed to have been the largest and most comprehensive of its kind ever conducted to link air quality and health effects in a subway system. As a result, it could have implications for subway networks around the world.
There are various sources of PM2.5 emissions within the London Underground, mainly from the wear of rails, wheels and particles released when trains brake, which tend to accumulate in the network.
In a previous study, the research team found that levels of PM2.5 in parts of the network can be up to 15 times higher than in the air outside. These particles are different from those that make up outdoor PM2.5 and are likely to have different health impacts, they argued.
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To investigate further, Dr David Green, from Imperial’s School of Public Health, and his team examined a pseudonymised dataset from Transport for London (TfL).
This included 29,744 staff employed between 1 January 2014 and 31 December 2019.
Roles included drivers, customer service staff and fleet staff (which includes train engineers, depot staff and technical support). Some staff, such as track engineers, were excluded because they used breathing apparatus for their job.
Information about exposure to PM2.5 was available from personal monitors as well as a series of network-wide measurements at underground platforms and stations.
Data on sickness absences was then obtained securely and under agreement from TfL’s HR department, using a method that protected individuals’ privacy.
Analysing the data, the researchers found that exposure to PM2.5 differed significantly among staff and was dependent on the nature and location of their work.
Drivers had the highest exposure over a work shift, but concentrations varied according to the line on which they worked – from an average of 72µg/m3 on the District line to 787µg/m3 on the Piccadilly line, for instance.
As a general rule, drivers with lower PM2.5 exposure worked on lines with shallow tunnels where there was a higher proportion of the track above ground and on trains introduced after 2010.
On the other hand, drivers who experienced the highest exposure to PM2.5 worked on older trains, operating on lines with deeper tunnels with more of their time spent underground.
Exposure among customer service staff also varied, with customer service assistants experiencing higher exposure to PM2.5 (ranging from 17-310µg/m3) than customer service managers (9-154µg/m3), largely because of duration of time spent on station platforms.
Levels of exposure also varied by station. Underground stations and interchanges for multiple lines had higher PM2.5 concentrations, compared to quieter, above-ground stations. This was likely because of higher train emissions and reduced ventilation, the research team suggested.
Looking at reported sickness, the researchers found fleet staff, customer service staff and drivers all had higher rates of sickness absence due to any cause compared with non-exposed office workers.
The researchers looked more closely at sickness absence among drivers, using the District line (which had the lowest recorded PM exposure) as a baseline. They found that drivers on five out of eight lines showed elevated rates of all-cause sickness absence.
However, there was no clear ‘exposure-response relationship’, where researchers would expect to see increasing exposure associated with an increased sickness absence.
Therefore, the researchers concluded they could not say for certain that PM2.5 exposure is directly contributing to sickness absence among London Underground staff.
Nevertheless, they recommended targeted measures, such as new trains with reduced braking emissions, platform screen doors, increased ventilation, air cleaning and surface cleaning.
Dr Green said: “We found that exposure to dust varied widely among staff, even among those doing the same job role, and depended largely on where they were working across the Tube network.
“For example, drivers who worked on older trains and on lines with deeper tunnels were exposed to more PM2.5 – as were customer service staff at stations with interchanges for multiple lines.
“There was some evidence to suggest that staff exposed to higher levels of PM2.5 took more days off sick, but this link wasn’t clear cut,” he added.
Separately, a study by the Karolinska Institutet in Sweden and Imperial College has concluded that using a mobile phone for extended periods is not linked with an increased risk of brain cancer.
Starting in 2007, researchers for the COSMOS study analysed more than 250,000 mobile phone users. They answered detailed questions about their mobile phone use, and were then followed in cancer registries to track the onset of any brain tumours.
The study, also published in the journal Environment International, found no link between mobile phone use and the risk of brain tumours.
The prevalence of brain tumours among the 10% of people who spent the most hours on a mobile phone did not differ significantly from those who used the mobile phone much less, the study concluded.
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