NEW DELHI—On the roof of an office building in India's capital, the world's smoggiest city, Kamal Meattle has a unique tactic for cleaning the air: a greenhouse with 400 common plants, including mother-in-law's tongue.

Meattle, the CEO of Paharpur Business Centre, has 800 other plants spread throughout its lower six floors, greening each room and hallway. Their job: remove soot and other chemicals from the often charcoal-colored outdoor air.

In India, where almost no one wears filter masks on the streets as many do in China, Meattle is seen as a radical. He says he's even been dubbed the Mad Hatter of Nehru Place, a high-tech hub that's home to his leafy building and an adjacent lot he converted from a slum into an oasis of 2,000 trees.

He uses rainwater collected in cisterns to spray the trees so they can grow faster and absorb more pollutants. He's urging India's new government to require rainwater harvesting and to paint roofs, and buses, white. And he's pushing to build one of the world's largest energy-efficient office parks, complete with greenhouses.

Meattle hardly seems a firebrand. A soft-spoken grandfather, he's a scion of India's elite who attended school with Rajiv Gandhi and later earned a chemical engineering degree from the Massachusetts Institute of Technology. He sees his efforts, touted in a 2009 TED talk that's attracted more than two million views, as common sense.

"Sustainability is good business, and energy efficiency is low-hanging fruit," says Meattle, whose 25-year-old building was India's first to earn the top rating (in 2010) for a retrofit from the U.S. Green Building Council. He says it uses one-fifth as much energy per square meter as the average office building in India. At least 10 percent of its energy savings is due to plants, which obviate the need to pump in ambient air.

His horticulture is also a practical nod to ancient tradition. "Why did Buddha sit under the peepul [or bodhi] tree?" he asks, adding that the sacred fig with heart-shaped leaves releases oxygen even at night, allowing those beneath a light sleep.

Good Business—and a Health Imperative

As Meattle tells it, he really had no choice but to try something new. "My doctors told me to leave" India in 1992, he says, citing his reduced lung capacity because of the city's air pollution. He decided to stay, seeking instead to solve a society-wide problem that's become increasingly dire. (See related story: "In Climate Talks, Spotlight Turns to India.")

"Delhi's unfit for living between October and March," Meattle says, noting how often its air pollution veers into the "very unhealthy" or even "hazardous" category. The U.S. Embassy, which posts its air quality data on outdoor monitors, has issued warnings against letting kids play outside.

View Images Smog envelops the landmark India Gate in New Delhi, the world's smoggiest city. Photograph by Altaf Qadri, AP

In fact, New Delhi's smog is now nearly three times worse than Beijing's, the World Health Organization reported in May, based on measurements of fine particulates, or PM 2.5. The WHO found that the city had the world's dirtiest air, and the cities ranking second through fourth are also in India. (See related story: "What Step Is Crucial in Fighting Cities' Air Pollution?")

India's indoor air pollution is even worse, exacerbated by the use of woodstoves and by chemical emissions from appliances. The WHO estimates it's India's second biggest killer, after high blood pressure, and contributes to 1.3 million annual deaths. (See related story: "WHO Report: Indoor Air Pollution Is Greatest Environmental Health Risk.")

India's press has only recently begun to focus on the issue, but the people are still largely in denial, says Barun Aggarwal, director of the Paharpur Business Centre's Breathe Easy program and Meattle's son-in-law.

"We are Indians. We have iron lungs," Aggarwal says is a common attitude, adding that many have adopted Nietzsche's "what-doesn't-kill-me-makes-me-stronger" mentality.

Meattle, after reviewing research by NASA and others, drew up a plan that focuses largely on three common houseplants because of their complementary abilities to detoxify indoor air and enrich it with oxygen: the areca palm, mother-in-law's tongue, and the money plant.

Before outside air is allowed indoors, his 50,000-square-foot facility uses a scrubber to wash it with water to reduce the levels of chemical compounds such as nitrogen oxide and sulfur dioxide. The air then passes through his greenhouse to remove formaldehyde, benzene, and carbon monoxide and through a filter to remove bacteria. No smoking is allowed inside.

Since January 2013, Meattle's company has created plant-based air-filtering systems for more than 700 homes in India's capital. It's also working to clean the air inside the embassy schools of the United States and Germany.

But Can Plants Really Clean the Air?

"It's a phenomenon that's growing now," says Bill C. Wolverton, one of the original NASA researchers and author of the 2010 book Plants: Why You Can't Live Without Them. Wolverton has been working in Japan, where plants have been used to make 50 to 60 "ecological gardens" in hospitals. He also says there's budding interest in South Korea and China.

View Images The Paharpur Business Centre's greenhouse focuses largely on three plants: mother-in-law's tongue, the areca palm, and the money plant. Photograph by Wendy Koch

NASA published several studies in the 1980s showing interior plants could purify the air in sealed test chambers akin to a space station. It later tested the value of plants to both clean air and recycle waste in a tightly sealed building known as the BioHome.

Subsequent studies have also suggested that plants could help clean indoor air. In 2009, via greenhouse tests, a Pennsylvania State University research team found plants could reduce indoor ozone, which can be emitted by copy machines and laser printers.

Not everyone is convinced. "I certainly would not rely on plants to clean indoor air...To get them to work, you'd need too many plants," says John Girman, former senior science adviser at the U.S. Environmental Protection Agency's Indoor Air Division. He says a 1,500-square-foot house would need 680 plants to duplicate NASA-like benefits, and the result would be "an indoor jungle" with moisture problems.

Girman co-wrote a study criticizing research such as NASA's that tested plants only in sealed chambers, which don't replicate the actual conditions of buildings with ventilation systems that can bring in fresh outside air. He says increasing ventilation is far more effective than using plants to clean the air.

But what if the outdoor air is filthy? "I don't have a good solution for India," Girman says.

Meattle's building has been proved to alleviate health problems, according to a 2008 study by the Chittaranjan National Cancer Institute and the Central Pollution Control Board. The study compared 94 nonsmokers working there with nonsmokers employed elsewhere in Delhi. It found the former had fewer cases of eye irritation, headaches, hypertension, and respiratory problems.

Another study raised questions. It said the building's indoor air quality is akin to that in the United States. "Given the extreme air pollution levels in the outdoor environment of New Delhi, this is an achievement," says the 2010 research co-authored by Joshua Apte, now an engineering professor at the University of Texas-Austin. It noted the "air-washer and filtration array" but said there's no "strong evidence" that the success is due to the greenery.

"We have an air treatment plant," Meattle says of his innovative combination of scrubbers, filters, and greenery. The result, he adds, is fewer employee sick days, greater productivity, and air as clean as that in Davos, Switzerland.

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