NEW DELHI — The truth of New Delhi’s toxic air finally hit home for Rakhi Singh when her 3-year-old son began to cough constantly early this year. She bought air purifiers for her home. When a thick, gray haze turned the view outside her home into a scene from a bad science fiction film last month, she bought pollution masks.

“Having a kid made the reality of the city’s pollution hit me harder,” she said.

The news that the Indian capital is one of the dirtiest cities in the world is three years old. But the awareness that it’s toxic enough to leave its citizens chronically ill and requires long-term lifestyle changes is relatively nascent.

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The first week of November, when a thick blanket of toxic haze covered the city, did much to hasten that awareness. And with the awareness came a brisk uptick in the sale of air purifiers and pollution masks.

As the noise and smoke from millions of firecrackers from the Hindu festival of Diwali died down, the city woke up Nov. 1 to soaring levels of PM2.5 — pollution particles so tiny they can get deeply embedded in the lungs. Levels in the Indian capital averaged well over 900 micrograms per cubic meter, more than 36 times the level the WHO considers acceptable and 15 times the Indian norm.

Manufacturers and importers of air purifiers and pollution masks say late 2016 is the first time they’ve had a serious number of Indian families inquiring about and buying their products.

“It’s only in 2016 that we’ve started to get through to the middle class,” said Barun Aggarwal, who heads Breathe Easy, a company that assesses homes and offices in New Delhi for pollution and provides solutions such as air purifiers and indoor plants.

When Aggarwal started his business in 2013, he had no customers for months. Later that year the World Health Organization report describing Delhi as the world’s dirtiest city was released.

“In the last four months (of 2013) we finally managed maybe 50 customers,” he said, adding that those first customers were almost entirely foreigners living in the Indian capital who came from cities with much cleaner air.

This year he expects to hit close to 5,000 customers, with more local residents looking for solutions, Aggarwal said.

Other businesses show similar numbers.

SmartAir, a company that started selling low-cost air purifiers in China in 2013, set up shop in India in early 2015 and sold 1,000 pieces of its basic do-it-yourself model that year. This year it sold about 500 pieces in the first week of November alone.

Hundreds of people queued up outside the Vogmask store in a posh shopping area to spend as much as 2,000 rupees ($29) on high-end pollution masks manufactured by the California-based company.

When the Indian capital topped the pollution charts in 2013, the city’s first response was almost defiant. There was a belief that pollution sickened only the city’s expatriate population.

“There was a strong defiance. ‘I’m born and brought up in Delhi. This doesn’t affect me,’” was how people saw the pollution problem, said Jay Kannaiyan, head of SmartAir in India.

“This year, that’s gone out the door. Middle-class, even lower-middle-class Indians are looking at air purifiers,” he added.

But while awareness is growing, New Delhi’s response to air pollution isn’t very consistent.

Air pollution is a year-round problem in the Indian capital, but it’s only when the cold winter makes toxic air visible that people respond.

On days when the air so awful that one can see and taste it, the parks are empty and those who cannot afford pollution masks tie handkerchiefs or scarves across their faces.

The week after Diwali, when the city saw its most shocking pollution spike in years, the SmartAir office and shop in south Delhi was open all night, with long lines outside the store.

But on other days, when the sun shines and the haze lifts, most people forget about pollution. The masks vanish and the face coverings come off, even though pollution is often still way above government and WHO norms.

“When the media stops talking about it and people see sunshine they start thinking it’s OK. It’s really not. It’s terrible,” says Singh.

“Right now it’s a completely a panic-driven market,” says Kannaiyan. “People are only buying when it’s so horrendous that they can’t see their own hand in front of them.”