than another, but two people living in the same city

might experience vastly different levels of pollution.

To measure this we've given our correspondents around the world

a new device that tracks their personal exposure to air

pollution, to see how it might change their behaviour and what

they will learn about their cities.

The device measures four types of air pollution

but we're mainly interested in two; particulate matter, that

can penetrate the lungs leading to cardiovascular and

respiratory diseases; and nitrogen dioxide

emissions from vehicles, which also contributes

to respiratory diseases such as bronchitis and asthma.

The device takes readings every minute

allowing our team to track their exposure as they

go about their everyday lives.

Beijing is a city known for its pollution.

When I first had kids parents were

crazy about not letting them outside, especially on days

when the pollution was so high that you couldn't

see the buildings next door.

But now it's much more common to see blue skies in Beijing again

and I don't like to keep kids to indoors.

I bike to work and I let them play outside as much as I can.

Am I doing the right thing?

Let's test it.

Well, the good news is pollution levels in Beijing

have improved a lot.

But I was surprised to see that my exposure still increased

every time I went outdoors.

Whether I bike to work or walk the kids to camp

the measurements still spiked.

Despite all the improvements we've

got a ways to go before we can breathe easy in Beijing.

We're in Lagos, so that means we're

stuck in traffic, which is not what I planned to measure

this week, but it's become increasingly clear

that it's the leading cause of air

pollution in this great city.

It's a few days later and I'm stuck in traffic again.

I've been pleasantly surprised by the quality here in Lagos.

But when it does get bad even the most fortunate folks

in town like me aren't protected by our air conditioned cars.

They're no match for a dump truck next to us in traffic.

For the average Lagosian it's infinitely worse.

Now that I have the sensor one of the things I want to check

out is my daily commute.

I usually walk to work.

But because of the exhaust from black cabs, and buses,

and trucks, by the time I get to the office

my throat is usually feeling a bit scratchy.

Would the air be better if I took a bus, or the Underground,

or my bike?

Let's find out.

So I've been doing this for a week now

and the air pollution has been high on average.

I switched between cycling, taking the bus,

and taking the Underground.

And while the type of pollution I was exposed to changed

between those modes of transport,

the level of air pollution really didn't.

We had several days where there were

big episodes of particulate matter sweeping

across southern England.

And, disappointingly, my main takeaway

was that when it's polluted outside you just

can't really escape.

Well, this is my neighbourhood in Sao Paulo, it's quite leafy.

And I rarely experienced problems

breathing when I'm around here.

Although it is relatively close to the city centre where air

pollution is supposed to be high.

And sometimes I go to report in the slums

and often I come out with the feeling

that the people who live in those slums, in those favelas,

breathe an air of worst quality - more polluted than the one

I breathe when I'm around here.

But now that I have the sensor I can go out and find out.

What... it transpires that from monitors' readings,

good air and bad air could be quite democratic here in Sao

Paolo.

I've got very high pollution readings around the area where

I live, my leafy neighbourhood so to speak,

and quite low or moderate pollution readings in poor

areas, in slums like this favela - Paraisopolis.

There was some obvious peaks of high pollution in the city

centre of Sao Paulo, for example,

and very low readings of pollution in, let's say,

posher areas, for example, literally right across

the street from where the governor of the state of Sao

Paulo lives.

So it seems that, for me, in order to be able to breathe

cleaner air, I may have to move to a different neighbourhood.

So, what did we find?

If you're looking to avoid pollution altogether

that turns out to be basically impossible.

And if you try to escape one type of pollution,

you might find yourself exposed to another.

You can avoid pollution by staying inside,

but that also doesn't seem like a good answer.

It is possible, though, to identify and avoid localised

risk spots.

Having a device like this, it turns out,

can make a difference.