After Neeno Kaur received chemotherapy and a bone marrow transplant to treat her leukemia, doctors advised her to stay out of New Delhi during the winter months to protect her lungs.

The Indian capital has some of the worst quality air in the world, and almost never dips below the level that scientists consider hazardous to human health.

During Diwali, when residents routinely flout a ban on firecrackers, the smog is almost unbearable.

Ms Kaur's solution, with her husband of 49 years, Peter Satwant Singh, was to fill their home with 15,000 plants, going to extreme lengths to protect their health from harmful air.

Their three-storey home in Sainik Farm, an affluent settlement in South Delhi, contains trees of figs, oranges, pomegranates, lemons and vegetables including lettuce, rocket, spinach, tomatoes, mint and bok choy. Ornamental plants adorn every surface.

The couple, aged 70 and 80, also have 120kg of fish in two tanks in the garden, using recycled water and rainwater harvesting.

Although the smog outside mostly scores more than 300 on the international air quality index, inside their home the level is just 13 and contains almost no particulate matter.

"It feels like we are living in the mountains," Ms Kaur told The Telegraph. "The air is so clean and breezy here all the time."

Their home is covered entirely with transparent polythene sheets, and there is only one inlet for air from the outside world.

Polluted air is drawn into the house by two fans fitted to a mesh cooler filter, with water trickling through it round the clock.

It is then filled with oxygen by the couple's thousands of plants, which use 1,000 litres of water a day in the summer and are mainly grown using aquaponics, a farming method that combines the aquaculture practice of raising fish with hydroponics, a method of growing plants in a soil-less environment.

The method converts the waste produced by fish into nutrients that plants can absorb.

It is highly efficient, uses significantly less water than traditional farming methods and eliminates the need for chemical fertilisers.

"Polluted Delhi air passes through the cooler filter at the entrance, which filters and cools the air," Mr Singh said.

"The plants in the greenhouse oxygenate the air, and the clean cooled oxygenated air passes through our greenhouse and our home.

"An exhaust fan in the kitchen throws the fresh clean air out into the polluted Delhi air."

Just a year after marrying in 1973, Ms Kaur and Mr Singh moved back to their home state of Punjab, and started work on their ancestral farm, where they kept bees.

Ms Kaur was recognised twice by the state government for her development and expansion of honey production in the state.

They then moved to New Delhi, a city of 33 million people, for their children's education, and noticed a marked difference in the quality of the air they breathed.

In 1998, Ms Kaur was diagnosed with chronic lymphocytic leukemia, which required her to undergo rounds of chemotherapy. In 2009, she received a bone marrow transplant.

"This cured the leukemia but damaged her lungs," Mr Singh said, adding that the couple had originally moved to Goa in the winters to live in a house bought by their son and daughter-in-law.

For years, they experimented with growing organic vegetables, which they could not produce in Goa, from their home in Delhi and transporting them south each year.

But just before the Covid lockdown began in 2020, they decided to settle permanently in the capital, and launched a drastic home improvement programme to make the air safer.

"Everyone called us mad people," Mr Singh said, recalling his neighbours' surprise at their home covered with polythene sheets. "But when people would come inside the house, they would be shocked."

"To achieve something like this you have to be a little mad and passionate," Ms Kaur added, laughing.

The couple believe their model can be replicated for flats and small houses, and Mr Singh is training others to do the same with online Zoom classes.

Ms Kaur says that they work as a team, with Mr Singh, who has a degree in maths, designing the systems and her growing the food.

"We both are passionate about living sustainably, having an organic healthy lifestyle," she said, adding that they supply the extra produce to a group of people in South Delhi that helps them to cover the cost of production and taking care of the plants.

Her lung condition is "long over", she said. "I would say I've the energy of a 40-year-old now and I take no medication."