For three years Mohammad Yunus, his wife Babli, five children and a handful of relatives have made their home on a patch of concrete in the middle of an eight-lane motorway, near a stretch of road named after Mahatma Gandhi. As lorries, buses and motorcycles roar past on either side of the family’s cramped bit of south Delhi pavement, and a modern, concrete flyover carries several more lanes of traffic overhead, their evening routine unfolds.

A young woman in a pink dress squats to kindle a fire from wooden sticks and plastic bottles, then breaks a lump of dough into pieces to bake into chapatti bread. A three-month-old baby and a toddler sleep on a dirty mat just a few feet from the curb. The incessant blaring of horns makes conversation difficult, but does not rouse the children.

Delhi’s horrendous air pollution – the worst in the world, according to the World Health Organisation – is far from the only worry facing the Yunus family. But the poisonous fumes are doing them serious harm, and the children are particularly vulnerable to its health-wrecking effects.

Babli Yunus has a thick, phlegmy, hacking cough that she says never goes away. Her son Toufiq, who looks about nine (no one at this encampment knows their precise ages), has had a fever for several days. “I keep thinking that my health is so bad, and my children are also suffering, lying around here. But what can I do?” Babli asks as one daughter, aged about three, leans back in her lap, playing with a dirty yellow comb. “This child has a cough and cold. So does this one.”

Babli doesn’t know whether the family’s latest round of illnesses was caused by the diesel and petrol exhaust that gives the air an acrid smell and a heavy feel, or by a soaking rain that left their clothes and blankets wet. “It all goes into our systems,” she says. “We end up eating a lot of dust.”

“Dust, dirt, everything,” her husband echoes. “This is a main road. It never closes.”

It is hard to imagine an existence more wretched than that of the Yunus family, huddled together on this dirty rectangle of concrete about 10 x 4 metres, surrounded by traffic on three sides and pressed on the fourth against a tall metal fence. Mohammad brings home what money he can, working on construction sites and selling vegetables in the market. His children, who do not attend school, earn a few coins collecting discarded plastic bottles from around a nearby bus stop.

Mohammad says a total of 18–20 people live in the space. Their belongings hang from the fence’s spikes: a plastic bucket, bundles wrapped in patterned cloth, a few blankets, a shirt. Pedestrians stream past just inches away, dodging traffic and carrying goods on their heads or under their arms. A few months ago, a girl of around eight living on the same stretch of road was struck by a car and killed, nearby families say.

Delhi’s foul air impacts all sections of its society. A 2010 study of more than 11,000 schoolchildren found 43% had poorly developing lungs, compared with 25% of a rural control group. The Delhi pupils were three times more likely to have severe impairment, and such deficits are believed to last for life.