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As you read this sentence, your brain is telling your diaphragm to contract, your ribs to expand, air is passing through your trachea — you are breathing. Air is essential to human life, and yet the air we breathe can also be a source of great harm. 

Globally, some seven million deaths each year are linked to the effects of air pollution, according to the World Health Organization, making it the biggest environmental killer. Pollution kills more people than car accidents, diabetes or dementia. 

The effects are particularly pronounced for children, who can experience stunted lungs and lifelong cognitive impacts.

But how bad, really, is the air we breathe? What does air pollution, often presented in abstract statistics, feel like in daily life? Can people in cities wrest any control over the level of pollution they experience? 

To answer these questions, the FT asked correspondents in five cities — Beijing, Lagos, London, New York and São Paulo — to carry a personal pollution monitor for one to two weeks and record the results. 

The device, the Flow pollution monitor made by Plume Labs, takes measurements once a minute for four types of pollutants and uses location data to create a map of the user’s journey.

The device also identifies which types of pollution are worst at any given moment, recording the levels of small particulate matter (PM2.5), larger particulate matter (PM10), nitrogen dioxide and volatile organic compounds (VOCs). 

Air pollution is not a single substance — many types of molecules and particles combine to create unhealthy air, and generate new pollution through chemical reactions.

All five correspondents experienced “very high” pollution levels more than once. Our Beijing writer recorded the most severe pollution on average; São Paulo was the cleanest city in the set. 

Sometimes pollution came from unexpected places — our New York reporter discovered that restaurant kitchens and indoor chemicals were a big source of contaminants.

Of course, people have lived with some degree of air pollution for a long time. Even indoor fires for cooking or warmth create hazardous pollution. But during the 20th century the rise of the car and the use of fossil fuels made outdoor pollution in cities an increasing concern. 

In the 21st century, new technologies enable us to measure pollution more accurately than ever before, while a growing body of research shows the effects on the human body to be more extensive than previously believed.

The situation is not hopeless. Increasing awareness and better data have spurred governments to take action. Large cities such as New York, Beijing and London have embarked on clean-up acts. More monitoring stations, air-quality apps and public-health alerts are empowering us to dodge the worst pollution.

As our team learnt more about their own individual exposure, the results were at times depressing, often surprising — and highlighted how difficult it is to avoid the unseen contaminants in the air around us.

Leslie Hook

Beijing by Lucy Hornby
Lucy Hornby cycling near the CCTV headquarters in Beijing’s Central Business District
Lucy Hornby cycling near the CCTV headquarters in Beijing’s Central Business District © Gilles Sabrie

Hiking with my children along the Great Wall in late June, I was startled to see the tall towers of Beijing’s business district, poking like needles into a crystal blue sky. 

It was a rare and magnificent sight, even after several years of improvement in Beijing’s air quality. When the kids were younger, 90km visibility would have been unthinkable.

Those were the “airpocalypse” days that turned the Chinese capital into a global poster child for polluted air. 

Beijing’s air quality has improved but is much worse than other cities. This chart shows average PM10 pollution reading from each writer’s Flow monitor over the duration of the experiment

Pollution policies were a litmus test for parenting style, just like “screen time” in some other places. “What’s your air-quality limit for allowing the children outside?” “Do you have air purifiers in each room of your home?” “Would you consider a school that doesn’t allow remote monitoring of the air quality in the classroom?” 

These were the questions that concerned me and my parenting friends.

But I also believed that the mental-health benefits of not being cooped up with toddlers outweighed whatever grime was working its way into their lungs. My kids went outside, even when we couldn’t see the building across the street. At the age of three, my son could accurately judge whether it was a red-flag day, yellow-flag day or green-flag day. 

On “orange alert” days, my daughter’s pre-school closed, leaving parents to wonder if the school air was really worse than the air at home. We finally caved in and bought an air purifier. I hated the whirring noise and the strangely sweet smell it emitted, so rarely turned it on.

And then Beijing’s air noticeably improved. Should we credit the determination of a one-party state to address middle-class griping? Or did the officially unacknowledged economic slowdown of 2014-16 wipe out the factories with the dirtiest smokestacks? To be fair, it was probably both.

Beijing's air quality has improved over the past ten years but is still above the safe level
Comparison of Lucy's data from the Flow monitor compared with the ten year long term average over the corresponding weeks

The improvement in family life is subtle but real. People don’t feel imprisoned in their homes any more. Children play outside in the golden evening light. Moods are better. Masks are fewer. Air has gone back to just being air.

When my colleague Leslie Hook came up with the idea for this project, I was certain it would show that air quality is no longer much of a concern in Beijing. The city might be the poster child for clean-up instead, I told her.

Beijing’s air quality has improved noticeably since the city’s ‘airpocalyse’ years – though Lucy’s Flow monitor still recorded some ‘very high’ pollution levels
Beijing’s air quality has improved noticeably since the city’s ‘airpocalyse’ years – though Lucy’s Flow monitor still recorded some ‘very high’ pollution levels © Gilles Sabrie

The first surprise was something I should have remembered: what passes for a good day here (AQI — air quality index — below 100) is still considered polluted by anyone else’s standards (in Los Angeles, a good day is AQI below 50). On days when I was congratulating Beijing on its renewed liveability, the Flow was tut-tutting over “moderate” or even “very high” pollution levels.

The second surprise was how much of a difference it makes to be inside, rather than out. The Flow’s monitor spiked when I biked to work or walked the kids to summer camp. My rough reckoning is that our exposure to air contaminants doubled when we were outside, on good days and grey days alike.

Chart showing total hours of exposure to PM2.5 pollution over a three-week period

Was I wrong to have ignored my boy’s pesky cough, back in the days when pollution was palpable? The Flow makes me feel a bit guilty. But I am glad the kids were happy ice skating, playing soccer and riding their bikes, even on those grey-white days when you could see, and smell, and feel the sky.

How the Flow pollution monitor works

The device measures four of the most common pollutants:

Small particulate matter (PM2.5) can penetrate far into the lungs because it is less than 2.5 micrometres in diameter (one-thirtieth the size of a human hair)
Larger particulate matter (PM10) includes soot and other particles produced by burning fossil fuels, and natural particles such as fine dust
Nitrogen dioxide (NO2) mainly results from burning fuel, such as in diesel vehicles. It can aggravate respiratory diseases, particularly asthma
Volatile organic compounds (VOCs) refers to carbon-based chemicals that evaporate at room temperature — for example, chemicals in paint or dry-cleaning

Lagos by Neil Munshi
Neil Munshi in the Marina district, Lagos Island
Neil Munshi in the Marina district, Lagos Island © Yagazie Emezi

When my wife Sarah and I lived in Mumbai a decade ago, we didn’t think much about air quality. But then we were young and invincible and smoked a pack a day.

Now we’ve got young kids and live in Lagos — another great, ­polluted megacity — and we’ve traded in the cigarettes for an addiction to AirVisual, a crowdsourced, live air-quality app that we check obsessively on our phones.

There’s usually just one air-monitoring station connected to the AirVisual app in the entire city of 21 million people — compared with scores in cities such as London or New York, where apparently many more people have bought the sensors that connect to the service — so we are lucky the monitoring station is near our house.

During the Harmattan, the period in winter when the Saharan winds pick up half a billion tonnes of dust from the Sahel and scatter it southward across west Africa, we tend to keep the kids inside. 

I was using the Flow to test the air quality in June, as the rainy season ramped up. I wanted to see how much that great staple of Lagos life — diesel generators — contributed to high ­pollution levels.

Nigeria’s grid is so decrepit that it delivers a daily average of just 4,000MW of power to its 200 million citizens, about an eighth of what the UK’s 66 million people receive.

So Nigerians — as they must in almost every facet of civic life — find a way around an absent ­government, with generators that come in all sizes: from the little roll-on suitcase-sized ones you see outside market stalls to the tractor-trailer behemoths double-stacked next to fancy apartment buildings.

One day, I drove in a loop through the city, from old-money Ikoyi to fast-growing Lekki, then up to the densely populated mainland and back to downtown Lagos Island. I popped out from time to time to test the exhaust coming out of various generators. 

Map showing how traffic affects exposure to pollution in Lagos

But the highest readings that I had during the journey were from dumper trucks, rust-bucket minibuses that looked as if they were held together with twine and chewing gum, and cars with broken tailpipes — all belching out acrid clouds of exhaust that would cripple the air-conditioning filtration system of even the most sophisticated Japanese-made car (or, in my case, a 12-year-old Toyota).

Lagos is famous for its “go-slows” — a pidginism of perfect literal clarity — the sort of traffic that snarls up tens of thousands of commuters for hours from a single car accident. Lagosians treat traffic as both a burden and a badge of honour, and it is where most residents — including me — spend most of our time outside the house or office.

Those of us who live in southern Lagos — “the island”, as it’s known, where most people work — have it much easier than our neighbours on the mainland, where the vast majority live. The average Lagos commute is a staggering four hours a day, among the highest in the world.

Neil found that the highest levels of pollution came from old trucks, minibuses and cars – this in a city where the average daily commute is four hours
Neil found that the highest levels of pollution came from old trucks, minibuses and cars – this in a city where the average daily commute is four hours © Yagazie Emezi

Things are only likely to get worse. Lagos welcomes hundreds of thousands of new residents each year. But infrastructure improvement sorely lags behind population growth, state-provided public transit is all but non-existent and travel is almost entirely road-based. That means more cars, more trucks, more minibuses, more motorbikes — more pollution.

There is some hope that the city is beginning to take air quality seriously. In June, the incoming Lagos state government announced plans to set up six new air-quality monitoring ­stations across the city.

That will provide helpful data to policymakers — and to residents like us, obsessively checking our apps. 

Using the Flow monitor was in some ways reassuring. The air quality at our flat, in our neighbourhood and at my daughter’s school was generally better than I had expected.

But we’re already among the luckiest Lagosians. On days when the air is particularly bad, we can travel from a sealed, air-conditioned flat in a sealed, air-conditioned car to our sealed, air-conditioned destination.

What cities like Lagos must figure out is how to expand the bubbles that exist around the better-off so that they are no longer necessary for anyone, so everyone can breathe more easily.

London by Leslie Hook
Leslie Hook on London’s Southwark Bridge
Leslie Hook on London’s Southwark Bridge © Benjamin McMahon

When I first moved to London in 2018, I was surprised that friends complained about the air pollution. I could see blue skies from my window and admire the clear views over Southwark Bridge as I commuted home. After seven years reporting from China, including three in Beijing during the worst of its smog, I thought I knew what pollution looked like.

But my glowing assessment has changed. Thanks to the prevalence of diesel vehicles, London’s air contains dangerous levels of nitrogen dioxide, mostly from cars and lorries; it has remained stubbornly above legal limits since 2010. 

As I carried the Flow device around the streets, it regularly reported “very high” pollution. One of the worst spots was on Southwark Bridge — despite its nice views — where I discovered that trucks waiting at traffic lights were churning out nitrogen dioxide.

Chart showing daily pollution levels on different modes of transport on a London commute

The problem went beyond just vehicles: there were episodes of moderate air pollution all across southern England on several days when I carried the device, due to agricultural pollution that is worst during spring. When farmers spread slurry on their fields, it generates ammonia that mixes with other chemicals to form harmful particles.

The device also revealed that most of my exposure to air ­pollution occurred during my commute. I usually walk to work, but the device showed that the air pollution was “high” on the narrow and congested streets of central London. 

So I tried riding my bike instead — not much difference. A bus ride also had high levels of pollution, due to the exhaust of other vehicles nearby.

The biggest surprise came when I hopped on the underground: the air on the Tube was terrible, and the five underground trips I made while carrying the Flow device generated the worst pollution readings I experienced anywhere. Although nitrogen dioxide was low, the presence of dust, soot and particles produced by train brakes all contributed to high levels of particulate matter.

Pollution on the London Tube rarely makes headlines, but I discovered that the issue has been studied before. A research paper published in January found that the Northern Line, which I had been using, is the most polluted, partly because it is so deep underground. 

At Hampstead station, the deepest on the network, levels of small particulate matter were 30 times worse than on the average roadside. The depth of the tunnels makes ventilation difficult, while dust and soot have built up over decades.

The approach to London Bridge Station. Pollution levels were worst on the Tube, due to dust, soot and particles produced by train brakes
The approach to London Bridge Station. Pollution levels were worst on the Tube, due to dust, soot and particles produced by train brakes © Benjamin McMahon

London has been trying to improve its air by introducing cleaner buses and taxing old diesel vehicles in the city centre. And the levels of nitrogen dioxide have fallen slightly over the past decade, suggesting that these policies are starting to work.

In 2021, London’s tax on polluting vehicles will expand to cover the areas within the North and South Circular, which is expected to have a big impact on cutting smog.

However, none of my commuting options significantly reduced my air-pollution exposure. I still walk to work, trusting that exercise will at least help boost my overall health — though I have changed my route, to less crowded streets. Despite the blue skies, the air here is still a long way from clean.

New York by Hannah Kuchler
Hannah Kuchler in Lower Manhattan
Hannah Kuchler in Lower Manhattan © Julia Gillard

Sitting in a hipster restaurant in my new neighbourhood in Brooklyn, the main thing in the air seemed to be millennial money and too many podcast ideas. But my Flow device showed otherwise: invisible large particulate matter was drifting over the rosé and small plates.

We were indoors and yet the device was flashing orange, warning me of high levels of pollution just one step lower than the “purple” reading I was getting on busy roads and in the subway. There were copious amounts of large particulate matter, most likely cooking fumes floating in from the kitchen, and “volatile organic compounds” — often chemicals from paint or cleaners.

Perplexed about the invisible threat, I set out to investigate pollution in indoor environments. A shrimp and grits restaurant scored “very high”, with more large particulate matter than the subway station. 

Total hours exposed to pollutants at an unsafe level, over a three-week period in a Brooklyn apartment

A lobby undergoing building work read “high” for small and large particulate matter — another danger, as small particulate matter penetrates deeper into the lungs. Visiting an office above a spa, I enjoyed the smell of lavender wafting in the lift — but my Flow warned again that there was too much large particulate matter.

The only places that scored “low” were air-conditioned offices, shops and cars. Yet expending energy to clean the air hardly seems the best solution for the environment.

Cooking smells and lavender oil seem quite pleasant additions to the air. How bad could they be?

Joseph Gardner Allen, director of the Healthy Buildings programme at the Harvard TH Chan School of Public Health, convinced me otherwise. 

“The particulates released while you cook can be worse than the worst outdoor air pollution anywhere in the world,” he tells me. And since we spend on average 90 per cent of our time indoors, indoor air quality really matters. “Indoor health is really brain health, heart health, reproductive health.”

Manhattan seen from Brooklyn. Hannah encountered unexpectedly high levels of air pollution in indoor environments such as restaurants
Manhattan seen from Brooklyn. Hannah encountered unexpectedly high levels of air pollution in indoor environments such as restaurants © Julia Gillard

I started to fear that I had received my Flow two weeks too late. I had recently moved into a new apartment, choosing it on the basis of size, location and noise levels. But I had never thought about measuring the air quality inside a prospective property.

According to readings taken over a couple of weeks, the air in my flat was most often a yellow “moderate’’. After cooking, the large particulate matter was high — although, given that I was making only one meal, not as high as in a restaurant.

Volatile organic compounds were also high. An expert from Plume Labs, which makes the Flow device, told me this could be the chemicals emitted from all the new flat-pack furniture I’d purchased. I laughed off his suggestion to leave my furniture in the open air to ventilate it, certain it would be stolen.

As a renter, my options felt restricted to opening the window and choosing my cleaning fluids wisely. Other suggestions — such as installing an air vent above the stove — were clearly impractical.

Local authorities that are trying to take some responsibility for outside air pollution have been slower to act on indoor air pollution. But last year, New York City passed the Asthma-Free Housing Act, requiring landlords to meet standards to minimise indoor allergens.

Depressingly, my future health may rely on a landlord whose name I don’t even know. “The person who manages your ­building has a bigger impact on your health than your doctor,” says Allen.

Sao Paulo by Andres Schipani
Andres Schipani with Luzía da Silva in the Higienópolis district of São Paulo
Andres Schipani with Luzía da Silva in the Higienópolis district of São Paulo © Larissa Zaidan

It all started the day Luzía da Silva came in coughing. “Where I live, there’s bad air,” she told me. 

Luzía works as a cleaner in my apartment building in the leafy area of Higienópolis, close to the centre of São Paulo. She lives in an area not far from the Pinheiros river, which is mainly an open sewer.

So, in sunny June, when I went around São Paulo carrying the Flow device, both of us were shocked to find out that the atmosphere can be quite democratic in this city of 22 million people.

“Our Lady! I always thought the air in my area is worse,” Luzía said, standing outside my apartment block, where the Flow spiked purple, meaning the air pollution was very high. Outside her house, it barely blinked yellow, indicating moderate levels.

Map of Sao Paulo showing pollution readings in different parts of the city

In annual average terms, São Paulo exceeds the WHO’s guidelines for PM2.5 and PM10. Some districts are better than others. Outside the mansion of the millionaire state governor, in the posh residential area of Jardim Europa, the Flow’s reading was green — low. But this was also true in the gritty favela of Paraisópolis. 

Lidinaldo Bastos, a local, told me: “It is better here than in the city centre, it is very polluted there.” He was right. Downtown, where the streets are highly congested, the Flow went red. 

Patricia Iglecias, president of the state environmental agency, told me that fumes from gasoline are “the biggest problem”.

Andres was surprised to discover that air-pollution levels in well-off Higienópolis were higher than in some of the favelas
Andres was surprised to discover that air-pollution levels in well-off Higienópolis were higher than in some of the favelas © Larissa Zaidan

Despite improvements in air quality in recent years, partly due to regulations limiting the amount of sulphur in petrol, the ­metropolitan area still has almost nine million motor vehicles. 

And while workers like Luzía may live in areas where the air is cleaner than others, they still have to spend a lot of time commuting and waiting for buses in traffic, inhaling high levels of toxic particles. 

I have the option of taking air-conditioned cabs or even, if I choose, moving to a less polluted area.

Evangelina Vormittag, director of the Institute of Health and Sustainability, calls this an “environmental injustice”. If the air-pollution levels in São Paulo continue as they are, she estimates that more than 50,000 people may die of related health issues by 2025. 

“People are conscious the air is polluted but they mainly feel the breathing problems, not the systemic ones that will become chronic when the toxic particles enter their bloodstream,” she says.

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