Underground.

You have been down in the tunnels

with this amazing tray to measure the air quality.

Tell me what exactly you've been doing

and what you've been measuring.

That's right.

I've spent the last two weeks going

to more than 100 stations in Zone 1 of central London

with these two monitors.

These are two different types of air pollution sensors.

So we focused on PM2.5, which is about a 30th of the width

of a human hair.

And PM2.5 is so small that it can pass through your lungs

into your bloodstream.

And it's linked to all types of inflammation, heart disease,

stroke, lung cancer.

Let's go down and see what the readings are like underground.

So do you notice a personal difference in your own health

doing this project?

I definitely felt very dehydrated

after spending a long time underground.

So we're just going down the escalator.

And the reading is really jumping.

So tell me where you took these measurements.

And what did you find?

So I went to every station within this yellow circle

line in Zone 1.

What I found was that this red line, the Central line,

was the worst.

The light blue one, the Victoria, was also pretty bad.

And these black bits, which are the Northern line,

were also very polluted.

And all three of those are the kind

of very deep, old Tube systems.

Does the construction of the tunnels

make a big difference to the quality of the air?

The tunnels that are very old and very deep

weren't built with very many ventilation shafts.

And that makes it hard for fresh air

to get down into the system.

The tunnels that were actually the first that were built

were built for steam trains.

They were the first trains in the underground.

And so they actually did have quite a lot of ventilation

because the smoke from the train had to escape.

And there's two types of tunnels in the Underground system.

Correct?

So there's the deep Tube.

And those are the lines, like the Northern line, the Victoria

line, that are up to 50 metres underground.

Then there are also the cut-and-cover lines.

And they're much, much shallower.

And parts of those are overground.

It's things like Metropolitan, Circle, Hammersmith & City.

And what's the reading right now?

Right now, it's fluctuating around about 80.

So does it get worse as the train comes by?

Yeah.

It tends to get worse because all the dust and dirt

from the tunnel gets brought into the station.

So what do we think is in the air that is contributing

to this pollution?

There's a high concentration of metals,

things like iron and nickel.

And that's because the friction of the wheels on the rails

and the brakes on the wheels splinters off

little tiny, tiny invisible particles

and gets into the air.

Oh, my goodness.

But then there are also things like skin cells,

little bits of hair, bits of fibre from people's clothing.

So the readings have risen quite a bit

since we entered the carriage.

Yeah.

I don't know if that's because we're near an open window,

and we're going through the tunnel,

and all the dust is getting churned up

and coming into the carriage.

So the safe limit that the WHO has set

is about 25 micrograms per cubic metre.

And the readings that we're seeing here

are consistently three or four times that level.

So what did TFL say when you told them about your findings?

TFL say that they think the air underground is safe.

It fits within the UK's health and safety guidelines,

which they have to abide to by law.

But they did also say that they know

there's a problem in terms of the cleanliness underground.

And they're trying a lot of new things,

piloting cleaning projects.

And they actually invited us underground

to see what these cleaning operations look like.

We went with one of their cleaning teams.

And they work between about midnight and 5 in the morning.

And it's teams of about 10 people.

They have hoovers, scrapers, brushes.

And they literally walk along the tracks

when the electricity is turned off, cleaning up the dust,

scrubbing away, trying to get rid

of all of the dirt and the matter that's around

and that gets into the air.

And does cleaning the tunnels have a big impact

on the air quality?

TFL hopes that it will.

There have been a couple of different trials.

Some have found that, actually, it made the pollution worse,

because it kind of stirred up all of this dust that

was trapped in little grooves under the rails and little bits

in the tunnel.

But they're hoping that in the long run,

if they do enough cleaning and they remove enough

of the dirt and the dust, that it will

have a good long-term effect.

So what we found is that the air on the Underground

really is very polluted.

There's a lot of particulate matter down here.

What is less clear, though, is what the impact of that

is on people's health.

The particulate matter found underground

is very different from what you find in the street.

There's a lot more iron.

There's a lot more metallic stuff in it.

And there hasn't been so much research

done about what the health implications of breathing

in that matter are.

So we know the air is really bad.

And we don't know exactly what that

means for the health impacts.

And that's one of the questions that scientists

are racing to find out.