My Commute's Air Quality

TLDR: If you can wear an FFP2 or FFP3 mask on the tube, you'd be doing your lungs a favour. The Northern line has 30 times more PM2.5 particles than the WHO recommended exposure limit

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I recently began a new job which required me to change my commute habits slightly. Previously, to get into central London from my home, I'd take an overground train into Waterloo station. With my new role, I'd be taking an underground train on the Northern Line.

I'm not a big fan of the underground, despite its storied history and impressive (if dated) engineering. Its cramped, hot and most of all, you can see the air quality is poor.

This latter point got me thinking though. How bad is the air quality really? Was this a case of me worrying about something not worth worrying about? It turns out, others, including Transport for London (TfL) have been looking into this already.

Initial studies from Imperial didn't find a link between PM2.5 (particulate matter 2.5 micrometers in diameter) and sick days taken by staff. Personally, I think that might be asking the wrong question though. PM2.5 particles do their damage by penetrating deep into lung tissue and entering the bloodstream. Health issues from long-term exposure to fine particulate matter include cardiovascular disease, respiratory problems, and reduced lung function. The World Health Organization considers no level of PM2.5 exposure to be safe.

Indeed, TfL themselves have a page dedicated purely to air quality on the tube. On it, I noticed the following excerpts:

"COMEAP said in 2018 that there was not enough evidence at that time to determine the health impact of underground particulate matter and that more research is needed. Tube dust vs outside air pollution ... Air quality underground is different from the ambient air on the surface. Above ground, common air pollutants such as nitrogen dioxide and diesel exhaust emissions have been identified as harmful to humans - these are not found inside the Tube network."

This reads as deflection and is irrelevant to the issue at hand. We know London's air quality is poor outside of the tube. It's rightly gotten a lot of attention in recent years. But this doesn't allay my concerns that the tube's air quality is any better than huffing an overburdened vacuum bag.

Fortunately, there is some hard reasonably recent data available to digest/inhale, so I decided to look at the actual numbers.

The Numbers Don't Lie

As one of London's deepest and oldest lines, the Northern Line consistently shows some of the highest particulate matter concentrations across the network. PM2.5 levels averaged 306 μg/m³ on deep lines like the Northern, compared to 68 μg/m³ on shallow lines. To put this in perspective, both figures far exceed WHO guidelines of 10 μg/m³ and EU limits of 25 μg/m³ for outdoor air.

So I'm going to be exposed to air quality that would be considered hazardous if it existed at street level.

The high particulate matter levels come primarily from brake dust and wheel-rail friction. Every time a train brakes or accelerates, microscopic metal particles are released into the tunnel air. The enclosed environment means particles accumulate over decades of operation.

TfL has acknowledged the issue, with monitoring programs showing that the deepest sections of the network consistently record the highest pollution levels.

What This Means for Commuters

For regular commuters like myself, the question becomes: what does this mean for my health? A typical Northern line journey exposes me to PM2.5 levels that are 30 times higher than WHO recommendations. While the exposure time is relatively short (usually 20-30 minutes), the concentration is so high that even brief exposure becomes concerning when multiplied across potentially hundreds of commuting days per year.

The lack of a clear link between PM2.5 exposure and sick days in TfL staff doesn't necessarily mean the air is safe. Cardiovascular and respiratory effects from particulate matter exposure often develop gradually over years, making them difficult to detect in short-term studies.

Personal Mitigation Strategies

Individual commuters have limited options. I can only think of two mitigations:

1. Use alternative travel methods. Research shows buses expose passengers to approximately one-third the air pollution of equivalent tube journeys, even when bus journeys take twice as long. Walking, cycling, and car travel all have significantly lower PM2.5 exposure than the Underground.
2. Wear protection. FFP2 or FFP3 masks can filter out fine particulate matter, and provided you have an effective seal (no facial hair guys), they're extremely effective

From a systemic perspective, TfL is investing in newer rolling stock with regenerative braking systems that should reduce particle generation. However, these improvements will take years to fully implement across the network.

My Northern line commute has made me acutely aware of an environmental health issue that affects millions of Londoners daily. While the convenience of the Underground is undeniable, the health costs are real and largely hidden from public view. Perhaps it's time for more transparent air quality reporting and aggressive investment in cleaner transport technology.

Until then, I'll continue my underground commute with a new appreciation for fresh air at street level - and a well-fitted mask.

References

1. Imperial College London. "Largest study explores impact of dust on Tube." May 2024. https://www.imperial.ac.uk/news/252000/largest-study-explores-impact-dust-tube/

2. Transport for London. "Improving air quality on the Tube." https://tfl.gov.uk/corporate/about-tfl/improving-air-quality-on-the-tube

3. Transport for London. "Station dust monitoring July-August 2024." https://content.tfl.gov.uk/station-dust-monitoring-july-august-2024.pdf

4. World Health Organization. "Ambient (outdoor) air quality and health." September 2021. https://www.who.int/news-room/fact-sheets/detail/ambient-(outdoor)-air-quality-and-health

5. Moreno, T., et al. "PM2.5 on the London Underground." Environment International, Vol. 134, 2020. https://www.sciencedirect.com/science/article/pii/S0160412019313649

6. Aarnio, P., et al. "Spatial variability of fine particulate matter pollution (PM2.5) on the London Underground network." Journal of Transport Geography, Vol. 79, 2019. https://www.sciencedirect.com/science/article/abs/pii/S2212095519301488

7. BBC Science Focus. "Does the air pollution on the London Underground harm your health?" https://www.sciencefocus.com/news/london-underground-air-pollution

8. Penconek, A., et al. "Particle Size-Selective Assessment of Protection of European Standard FFP Respirators and Surgical Masks against Particles." Journal of Occupational and Environmental Hygiene, 2016. https://pmc.ncbi.nlm.nih.gov/articles/PMC5058571/

9. Smart Air Filters. "Masks Ratings, Standards, and Effectiveness Comparison (N95, KN95, FFP1, FFP2)." https://smartairfilters.com/en/blog/comparison-mask-standards-rating-effectiveness/