We gradually learned that Delhi’s true menace came from its air, water, food and flies. These perils sicken, disable and kill millions in India annually, making for one of the worst public health disasters in the world. Delhi, we discovered, is quietly suffering from a dire pediatric respiratory crisis, with a recent study showing that nearly half of the city’s 4.4 million schoolchildren have irreversible lung damage from the poisonous air.

For most Indians, these are inescapable horrors. But there are thousands of others who have chosen to live here, including some trying to save the world, others hoping to describe it and still others intent on getting their own small piece of it. It is an eclectic community of expatriates and millionaires, including car executives from Detroit, tech geeks from the Bay Area, cancer researchers from Maryland and diplomats from Dublin. Over the last year, often over chai and samosas at local dhabas or whiskey and chicken tikka at glittering embassy parties, we have obsessively discussed whether we are pursuing our careers at our children’s expense.

Foreigners have lived in Delhi for centuries, of course, but the air and the mounting research into its effects have become so frightening that some feel it is unethical for those who have a choice to willingly raise children here. Similar discussions are doubtless underway in Beijing and other Asian megacities, but it is in Delhi — among the most populous, polluted, unsanitary and bacterially unsafe cities on earth — where the new calculus seems most urgent. The city’s air is more than twice as polluted as Beijing’s, according to the World Health Organization. (India, in fact, has 13 of the world’s 25 most polluted cities, while Lanzhou is the only Chinese city among the worst 50; Beijing ranks 79th.)

So many of our friends have decided to leave that the American Embassy School — this city’s great expat institution — is facing a steep drop in admissions next fall. My pastor, who ministers to a largely expat parish here, told me he feared he would lose 60 percent of his congregants this summer.

We nearly left two years ago, after Bram’s first hospitalization. Even after his breathing stabilized, tests showed that he had lost half his lung function. On our doctor’s advice, we placed him on routine steroid therapy and decided that as long as his breathing did not worsen again, we could stay in Delhi.