The good news: Indian cities pump out far less nitrogen dioxide (NO2), a bellwether of urban air pollution, than their counterparts in the West and China, a new study by the US space agency Nasa has found.

The study measured how urban pollution in a city affects its population. Scientists at Nasa’s Goddard Space Flight Center studied four major global air-pollution areas: China, the US, Europe and India.

“Current NO2 levels in India are still low compared to China, US and Europe," the lead author of the study, Lok Lamsal, said in an email interview.

Using satellite observations, Nasa scientists directly measured air pollution’s dependence on population and found that a European city of 1 million people had six times higher NO2 pollution than a similar-sized Indian city. When cities grew from 1 million to 10 million people, a common enough phenomenon in India over the last half century, pollution from surface-level NO2 doubled. In China, which has industrialized and urbanized at a rate faster than any country in history, NO2 pollution in similar-sized cities increased five times.

The bad news: Overall NO2 levels in Indian cities are soaring, even as they head southwards in the West, and—as a World Bank study notes—13 of the world’s 20 most polluted cities are in India.

“NO2 emissions/concentrations (changed) rapidly in India from 2005 to 2012 due to increased industrial activities," Lamsal acknowledged.

At climate change negotiations, the Indian message to the West has always been this: You started the problem, and each of you pollutes the air more than each of us.

This is not an unreasonable position, but it obscures the fact that only three countries now exceed India’s total emissions of carbon dioxide, the main gas implicated in warming the earth.

Global warming is now a reality, even if its effects are not as clear. One component of climate change is clearer: urban air pollution because it is something you can feel and smell.

Much as carbon dioxide levels play the role of global warming harbinger, nitrogen dioxide does the same for air pollution.

NO2 is a smelly, toxic gas, creating ozone on the ground—a major component of smog—and acid in the air. It is a precursor to nitrates, which add to respirable and poisonous particles in urban air. The gas irritates the eyes, nose, throat, causes shortness of breath and aggravates respiratory illnesses, such as asthma, especially in young children.

NO2 comes chiefly from burning oil, petrol, gas, diesel or coal, whether in combustion engines and industrial processes. Although there are other components to dirty urban air, chiefly sulphur dioxide and particulates, NO2 levels are a good indicator of the overall health of a city.

Indian cities of a million people, the Nasa study found, had NO2 surface concentrations of 0.23 parts per billion (ppb). That is significantly less than similar cities in the US (0.98 ppb), Europe (1.33 ppb) and China (0.68 ppb).

Indian cities of 10 million people averaged 0.53 ppb, compared with 2.55 ppb in the US, 3.13 ppb in China and 3.86 ppb in Europe.

These levels—measured from space using an ozone-measuring Nasa satellite—might indicate that India has a good opportunity to change track, but in all likelihood the country will find it hard to use that opportunity.

A study published in the journal Atmospheric Chemistry and Physics in April revealed spiralling concentrations of NO2 in columns of air, “strongly increasing over China, the Middle East, and India, with values over east-central China (its new industrial zones), tripling from 1996 to 2011". Most megacities in China, India and the Middle East indicated a doubling or tripling of NO2 levels within the study period, increasing in some cases by up to 20% in a year. During the same period, atmospheric NO2 levels “significantly" decreased over Western Europe, the US and Japan, said the study, by physicists at the University of Bremen, Germany.

Lamsal and his colleagues reached similar conclusions, though their data pertains to a smaller period (2005 to 2012). “If the trend continues, Indian cities will be more polluted than US or European cities in the near future," said Lamsal.

Indeed, a recent World Bank report warned that over the last decade, the vulnerability of Indians to pollution-related sickness has greatly increased. India ranked 126 of 132 countries in an environment survey, lagging China, Bangladesh and Pakistan. In the effects of air pollution on human health, India ranked last.

Air pollution might be an unavoidable result of economic growth, but it comes at a rising environmental cost, presently estimated by the bank at 5.7% of GDP.

The report asked: “Does growth, so essential for development, have to come at the price of worsened air quality and other environmental impacts?"

The time to answer that question is fast running out.

Samar Halarnkar is a Bangalore-based journalist. This is a fortnightly column that explores the cutting edge of science and technology. Comments are welcome at frontiermail@livemint.com. To read Samar Halarnkar’s previous columns, go to www.livemint.com/frontiermail

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