Sign Up

Choked — Delhi’s Pollution Crisis

Unpacking the commonly used term “air pollution,” what it really means and the associated health risks.

December 27, 2016

Unpacking the commonly used term "air pollution," what it really means and the associated health risks.

In December 2003, I boarded an Air India plane from Frankfurt, Germany, for New Delhi. It was meant to be a direct eight-hour flight.

In the event it took us 52 hours to reach Delhi, including hours of circling the Indian capital’s airport, further hours of cooling our heels on the tarmac at Ahmedabad airport and an overnight stop in Mumbai.

Most of the Indians aboard the plane were equanimous. “It’s the fog,” they said about the spongy, brown fug that had prevented us from landing on schedule. “You know, the winter fog?”

Since the mid-1990s, every winter the air in Delhi turned opaque, with visibility often dropping to a few feet. I remember driving home from New Year’s Eve celebrations with the headlights on full, unable to see anything beyond the windshield. It was frightening.

Drawing parallels between Beijing and Delhi

I did not spend much time brooding about the quality of the air, beyond regarding the smog as an inconvenience. Somewhere in my mind, the conversion of Delhi’s public transport to compressed natural gas (CNG) in the early 2000s had calmed the anxieties that the evidence of my own senses provided.

It was around the time of the CNG conversion that I moved to live in the Chinese capital Beijing, where I spent seven years writing for various Indian newspapers.

Only after I began to engage with China’s degraded environment in my reporting did the parallel fact of Delhi’s smoggy skies become harder for me to ignore.

Every Christmas holiday I returned home to putrid air that left my eyes and throat aching as much as anything I had experienced in China.

Yet, whenever I raised the issue with friends, I was met with blank stares or eye-rolling intended to indicate how much of a “foreigner” I had become.

A close look at polluted air

And then came the stunner. In 2014, a WHO study ranked Delhi as the world’s most polluted city. Neither Beijing nor any other Chinese city even figured in the top 20, but 13 Indian cities did.

These were Patna, Gwalior, Raipur, Ahmedabad, Lucknow, Kanpur, Firozabad, Amritsar, Ludhiana, Allahabad, Agra, Khanna and, of course, Delhi. This ranking was based on PM2.5 levels.

What is Particulate Matter?

Air pollution, a term commonly bandied about, is in fact a confounding complex of particulates and gases, and needs some unpacking.

The abbreviation “PM” refers to particulate matter, which is designated as either 10 or 2.5. These numbers refer to the size of the particles.

Dust particles from windblown soil and construction activity tend to be between 2.5 and 10 micrometres in diameter and are categorized as PM10. To put this in context, a single strand of human hair is usually between 50 and 70 micrometres in diameter.

PM2.5 refers to even finer particulates, less than 2.5 micrometres in diameter, which are only visible through an electron microscope.

Produced by the combustion of motor vehicles, power plants, burning of biomass (leaves, twigs, stalks, dung cakes, etc.), and so on, these particulates are minuscule enough to be absorbed deep into the lungs.

The lungs keep the human body breathing by sucking in life-giving oxygen from the air so that it can pass into the blood, and by expelling carbon dioxide.

The health risks of PM2.5

PM2.5 is so fine that it bypasses the natural filters of the body in the nose and the airways, settling deep into the microscopic air sacs or alveoli of the lungs where oxygen and carbon dioxide are exchanged.

This causes potential damage to these vital organs and, worse, poisoning the body by passing into the bloodstream along with oxygen molecules.

Of all the components of air pollution, the health effects of PM2.5 are the best researched and the news is terrible. Exposure to fine particulate matter is linked to illness, hospitalization and premature death.

High levels of PM2.5 in the air cause not only respiratory disorders like bronchitis, asthma, inflammation of the lungs and persistent coughing, but can also lead to life-threatening heart attacks and strokes.

In late 2013, the International Agency for Research on Cancer, part of the World Health Organization (WHO), concluded that particulate pollution can cause lung cancer.

The various poisonous gases

Noxious gases of various types, collections of molecules that are tinier than the smallest particulates (only 2–3 atoms wide), are the other component of the air pollution mix.

These include sulphur dioxide (SO2), nitrogen oxides (NOx) and ozone. SO2 and NOx result from various kinds of combustion and also produce PM 2.5 indirectly under the right atmospheric conditions.

Effects of Sulphur Dioxide

Coal and oil-fired power plants, steel mills, refineries, pulp mills and smelters are among the largest releasers of SO2 since they operate by burning fuels like coal, oil and diesel that contain sulphur.

Sulphur dioxide easily and rapidly enters the bloodstream through the lungs and even short-term exposures to high levels of SO2 can kill.

High levels of SO2 in the air lead to a burning sensation in the nose and throat, breathing difficulties and severe airway obstructions.

Over time SO2 can also affect lung function permanently. Asthmatics are sensitive to the respiratory effects of even low concentrations of SO2.

In one study, guinea pigs were unable to breathe as deeply or take in as much air per breath when exposed to even less than one part of SO2 per million parts of air.

SO2 levels in vehicular fuel

More severe symptoms seen in animals exposed to high concentrations of SO2 include decreased respiration, inflammation or infection of the airways, and destruction of areas of the lung.

Exposure to 400–500 parts per million (ppm) of SO2 is considered immediately dangerous to life and health, but this is in fact the level of SO2 present in low-quality diesel still used in some parts of India.

Diesel cars have been making rapid inroads in India with their share in new sales doubling in the decade 2002 to 2012, from 27% to 55%.

Fortunately, latest emissions standards mean that SO2 levels in vehicular fuel (both petrol and diesel) have been restricted to 50 ppm in many cities. However, there are parts of the country where older standards with far higher levels of SO2 continue unchecked.

Effects of Nitrogen Oxides

Diesel-based vehicles pose another pollution-related problem. They emit around twenty times more nitrogen oxides (NOx) than petrol cars.

Like SO2 and PM2.5, NOx is ruinous for the respiratory system, inflaming the lining of the lungs and reducing the body’s immunity to lung infections.

Wheezing, coughing, colds and bronchitis are all linked to breathing in high levels of nitrogen oxides. Nitrogen Oxides can also aggravate existing heart conditions, leading to hospitalization and, on occasion, death.

This is an excerpt from Choked by Pallavi Aiyar (Juggernaut app)


In 2014, a WHO study ranked Delhi as the world’s most polluted city. No Chinese city even figured in the top 20.

High levels of PM2.5 in the air cause not only respiratory disorders but also life-threatening heart attacks and strokes.

Nitrogen Oxides can aggravate existing heart conditions, leading to, on occasion, death.

High levels of SO2 in the air lead to breathing difficulties and severe airway obstructions.