Reported by BBC Science
People exercise in a park in Delhi on Thursday, breathing in hazy, polluted air
There are two cardboard boxes in Meenu Rana's living room in Delhi. One is filled with Diwali decorations and twinkling lights. The other holds air purifiers.
"The mornings have already started feeling stuffy. It will get worse in the coming days, so it's better to be prepared," says the 43-year-old teacher.
More from the original report
She is not alone. Every year, as winter and the festive season approach, residents of India's capital begin preparing for something far less celebratory: months of breathing polluted air.
Delhi and its satellite cities record some of the world's highest pollution levels through the year. But as winter sets in, the air often becomes unbearable.
Falling temperatures and slower winds trap pollutants close to the ground instead of allowing them to disperse. Smoke from crop residue burning in neighbouring states adds to emissions from vehicles and factories, as well as construction dust. Firecrackers used during festivals often worsen air quality.
The result is noxious air and a familiar question: why does pollution return year after year, and why hasn't more been done to tackle it?
This year, however, the Delhi government announced its Winter Action Plan months before the worst of the smog arrived. The plan is in place from 1 November to 28 February, covering the months when pollution usually spikes.
Experts have called this a step in the right direction, noting that in previous years measures were often rolled out only after the air quality got too bad to ignore. However, they say this by itself won't be enough, calling for more holistic, long-lasting measures.
Just days after the plan came into effect, the city is slipping back into a depressingly familiar pattern .
This week, a grey haze hangs over Delhi, along with a distinct smoky smell that residents unwillingly recognise. People complain of irritation in their eyes and difficulty breathing, and coughs and sneezes are heard everywhere.
For those who can afford them, air purifiers have become a household necessity. Outdoors, people wear masks or cover their faces with cloth, trying…