What's Worse for Pollution - London Underground or Overground?

When talk turns to public transport, most people immediately associate it with a greener method of getting around. After all, transitioning from petrol and diesel passenger cars to communal forms of moving from A to B should be good for everyone, right? Well, this might be true in theory, but it’s certainly not less polluted for users of London’s underground network.

According to a study conducted by King’s College London, air quality in the Tube is approximately 15 times more dangerous than that above ground. For the millions of people who use the underground to get to work every day, that’s quite a concerning statistic – and one which must be addressed immediately through measures geared towards cleaning up the foul air.

Bottom of the pile

Over recent years, the rising tide of environmentalism and an increasing emphasis on the healthiness of our lungs has meant that air quality in cities and the vital role of monitoring have become more prominent issues in the mainstream media. Unfortunately, London’s transport system is not only more polluted than the air at surface level – but it’s also worse than other countries, too.

The main offending contaminant is particulate matter 2.5 (PM2.5), an ultra-fine particle that can be inhaled and cause serious damage to the body’s internal organs. The King’s College research found that stretches of the Tube contained far higher concentrations of PM2.5 than similar underground networks in Australia, China, Spain, South Korea and the USA. In particular, London Bridge, Oxford Circus and Waterloo were highly polluted, while the Northern and Victoria lines are those with the most contamination across the board.

Old, deep and long

The reasons for the dismal state of the air on the Tube are manifold. Firstly, the London underground is the oldest system of its kind, with almost 175 years’ worth of dirt and dust having accumulated over time. Each time a train passes, that detritus becomes unsettled and easily inhaled into the lungs of passing commuters.

What’s more, the longer and deeper a rail tunnel is, the harder it becomes to ventilate. The deepest station of the Tube network is Hampstead, which stands at 58.5m below street level, while the longest tunnel is 27.8km between Morden and East Finchley. It’s unsurprising that this stretch of the Northern Line is one of the most polluted in the entire network.

High time for action

Our improved ability to measure air quality via particulate matter sensing means that the government has come under increased pressure to tackle the issue head-on. Fortunately, Transport for London (TfL) is taking the problem seriously, investing an estimated £60 million into cleaning and refurbishing the Tube’s stations and tunnels each year.

As well as general maintenance, that investment also goes towards a number of forward-thinking initiatives, such as the trialling of new techniques of cleaning the tunnels and testing out air filtration systems. They vacuum and deep clean the dirtiest sections of the track, as well as carrying out comprehensive analysis on how the elevated levels of iron oxide found in the underground impact human health.