Finding hidden pollution on the tube.

Summary: Researchers from Cambridge University have found a method for measuring previously hard to detect pollution particles using magnetism. Previous studies had found that concentrations of particulate matter (as small as one twentieth the width of a human hair) on the London Underground are higher than those in London more broadly (above ground). However, those studies may have even been underestimating overall pollution levels given that some of the smallest particles have been historically hard to measure. Those tiniest particles are so small you would need 7,000 of them piled on top of each other to equal the width of a human hair! The researchers conclude that removing dust from the underground periodically, and using magnetic monitoring, could provide a cost effective strategy for reducing exposure.

Why this is important: Even in an increasingly remote-working friendly environment, public transportation will remain an important component and the health of workers and impact on productivity remains a key risk and concern for all.

The big theme: Achieving acceptable air quality has various significant investment and decision-making considerations. The most obvious are the implications on human health from a purely altruistic point of view. Decreasing pollution with the aim of decreasing human illness and suffering is (should be) a goal unto itself. In practice however, the focus is primarily on how such initiatives (that more often than not increase costs and dig into profits), will be beneficial to business in the long run, whether it be through decreased healthcare costs, increased health and productivity of the workforce, and reputational gains for such decisions.

Summary of a story from the University of Cambridge:

Previous studies in London and Seoul have concluded that most of the particulate matter on the Underground is created as wheels, tracks and brakes grind against each other and very small metallic particles are thrown up. Given that they are metallic researchers from Cambridge University suspected that they could use magnetism to identify and monitor those particles which are often too small to be caught by standard air filters and even then are not individually identified.

Combining disciplines from the Department of Earth Sciences and the Department of Materials Science and Metallurgy, the team led by Hassan Sheikh and Professor Richard Harrison used high-resolution electron microscopy and electron tomography (think of it as '3D magnetic fingerprinting') to analyse dust samples collected from the London Underground across a number of different lines from ticket halls, platforms and train driver cabins. This enabled them to identify the structure of the different particles that made up the dust.  They found that there was a large amount of maghemite particles (a type of iron oxide) with an average diameter of only 10 nanometres. Theses individual particles were found to often form larger clusters of up to two micrometres or microns meaning that traditional monitors were overestimating the average 'particle size.'

This study and prior studies had concluded that because of poor ventilation in the Underground, the maghemite rich dust could be re-suspended in the air when trains pull into platforms making the air quality worse there than in either driver cabins or ticket halls. Possible remedies suggested include placing screen doors between platforms and trains, as there are on the Jubilee Line and Elizabeth Line, regular cleaning of the tracks and tunnel walls and including magnetic filters in ventilation systems.

Why this is important

Anyone who has ridden the Jubilee Line between Canary Wharf and London Bridge knows that there is a point where talking with your companions is a fruitless exercise. The sound of grinding not only destroys the art of conversation, but as we now know throws up a lot of potentially harmful pollution. How harmful? There is plenty of evidence pointing to the harmful health impacts of ultrafine particles and in particular magnetite particulate matter. A 2008 study made a link between increased levels of magnetic iron compounds in the brain and Alzheimer's Disease. However historical studies had not really supported a differentiated risk between exposure on the Underground and ambient air pollution. A study in Stockholm for example failed to find an increased risk of myocardial infarction (heart attack) in tube train drivers compared with other manual workers. That doesn't mean the risk isn't there! I wrote in a previous Perspective about findings from a study that concluded that there was a short term association between out-of-hospital cardiac arrest and increased concentration of PM2.5 particles (particles smaller than 2.5 microns). There are more recent, albeit in vitro studies (in a lab using samples in petri dishes) that have found some increased risks in, for example pneumococcal infections and in broader oxidative stress on underground rail transport systems compared with above ground.

Back in 2017, the Mayor of London introduced a series of measures to reduce dust levels on the Underground including the sampling analysed by the team from Cambridge, requiring new trains and lines to have regenerative/rheostatic braking which reduces friction on the rails, sealed windows and platform safety doors. Industrial vacuum cleaners and magnetic 'wands' were brought in to do cleans over the summer. This new research and its recommendations suggest that cleaning regimen be enhanced and/or some of the other measures extended across the network.

In addition to the iron-rich particles found on the tube, PFAS or 'forever chemicals' are another example of particulate matter that can be found in air, water and food accumulating in humans, animals and the environment. For example a study in Germany found that 20% of samples from children between the ages of three and seventeen had levels of certain PFAs above safe levels as determined by the Human Biomonitoring Commission. They have been found to be harmful to health in a number of ways from neurological disorders to birth defects to certain cancers and reduced kidney function.

Please read: important legal stuff.