Preparing to choke India’s annual pollution threat explained

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India is again preparing for its worst weeks of air pollution. Each year, from late October, a thick blanket of smog settles over vast swathes of northern India, including the capital, New Delhi, pushing air pollution levels off the charts. Here’s why.

As winter approaches each year, a combination of factors such as theburning of crop residues, fouling of the air by industry, vehicle exhaust, and dust from construction work, lead to a sharp spike in pollution levels. As wind speeds drop after the monsoon season, smog and other pollutants tend to hang in the air. The problem is exacerbated as people let off fireworks to celebrate the Hindu festival of Diwali in late October or early November.

1.The “Gas Chamber”

As wind speeds slow, the Himalayas tend to trap the colder, low-hanging winter air across northern India and Pakistan, with air pollution accumulating there, unable to escape. The image below, captured by NASA’s Terra satellite, shows a thick layer of pollution last winter.

The MODIS Corrected Reflectance True Colour imagery of India as seen on Nov 8, 2017. Source: NASA Worldview

Last year, a dangerous toxic smog forced authorities to shut schools, ban diesel-run generators, construction, burning of garbage and non-essential truck deliveries.

As pollution levels climbed to 12 times the recommended limit and the Indian Medical Association declared a public health emergency, Delhi Chief Minister Arvind Kejriwal called the city a “gas chamber”. Last week, he warned the city may face the same fate this year because of the unrestrained stubble burning.

How bad is it?

North Indian cities, including Delhi, top a list of places with the worst air in the world, according to the World Health Organization (WHO). The database of more than 2,600 cities showed that 14 of the 18 most polluted cities on the planet are in northern India, based on the amount of particulate matter under 2.5 micrograms found in every cubic metre of air. These tiny particles known as PM2.5 include dust, dirt, soot and smoke. They can penetrate deep into the lungs and even enter the blood. They’ve been linked to heart disease, strokes and cancer.

Delhi’s annual mean PM2.5 reading is 143 micrograms per cubic metre, almost 14 times more than the WHO guideline. Another north Indian city, Kanpur, tops the database at more than 17 times the WHO guideline level of 10 micrograms. India’s own annual PM2.5 guideline is set at 40 micrograms per cubic metre.

Air quality by city
Each small square on the chart below represents a city. Cities are positioned horizontally according to annual mean PM2.5 levels. Where cities have the same reading, they are stacked on top of each other.

The WHO says globally about 7 million people die every year from breathing polluted air. Most of the deaths happen in poor Asian and African countries. In India, the WHO estimates ambient air pollution could have been responsible for anywhere from 960,000 to more than 1.2 million deaths in 2016.

Breaking down the cause

Man-made emissions combine with a number of meteorological factors to aggravate the pollution problem every winter. Here are the main contributors to the yearly phenomenon.

2.Stubble burning

Between end-September and mid-November, farmers from Punjab and Haryana states burn an estimated 35 million tonnes of crop residue after they harvest their rice crop.

A farmer burns the stubble in a rice field in Zirakpur in the northern state of Punjab, India, 

The satellite image below shows fires burning in the north Indian states of Punjab and Haryana. It was captured by the VIIRS instrument onboard the Suomi National Polar-Orbiting Partnership (Suomi NPP) spacecraft on October 30, 2017.


Satellite image: NASA Worldview

After the introduction of mechanised harvesters, burning crop residues became common because the machine leaves stalks that are about one foot tall. Burning is the quickest and cheapest way to clear the stalks before planting winter crops such as wheat and rapeseed.

Authorities in Delhi have in the past blamed the governments of Punjab and Haryana for allowing the farmers to burn the paddy stubble. India’s federal environmental court has banned the practice of burning crop residues in five states, including Punjab and Haryana.

A farmer burns the stubble in a rice field in the northern state of Haryana, India, October 9, 2018. 

The fire from paddy fields releases gases like carbon monoxide and nitrogen dioxide, both known for forming smog. Crop burning is one of the major reasons behind a sharp rise in pollution levels every winter. From Punjab and Haryana, the hazardous air makes its way across the entire northern part of the country.

3.Wind reversal

With the retreat of the monsoon season, winds change direction. Instead of strong winds blowing from the east and helping clear the air, softer winds arrive from the northwest. This allows a large amount of the pollutants to drift slowly towards Delhi and other cities in northern India.


Detected by satellite


Aerosol optical thickness



Clear sky

Hard to see the Sun

March 2017

October 2017

Very few fires in the Punjab region. Winds blow from the southeast, helping to clear the air.

Thousands of fires in Punjab.

Slower northwest winds allow the smoke to accumulate and drift over northern India.














4.The winter inversion

During the winter, cold air descends from the Himalayan mountains onto the plains below. This cools the air near the ground and over the low-lying cities, such as Delhi. The smoke from stubble burning lingers near the base of the mountains, trapped in the cold air.

Cold air

In normal conditions, the warm air near the ground rises and carries the pollution away. During the winter, this layer is reversed due to the cold Himalayan air. This is known as temperature inversion. Pollution from vehicle exhausts and swirling construction dust in cities further contribute to the smog trapped in the pocket of cold air, making the situation worse.

Warmer air layer

The polluted city air cannot escape as it is colder and denser than the warm air layer above

Weak winds prevent air mixing near the surface

5.Diwali festivities

Another contributing factor to smog is the huge number of fireworks set off as part of the Diwali festivities. During this annual festival of lights, Hindus celebrate the triumph of good over evil with firecrackers and small oil-filled clay lamps.

In New Delhi, the morning after Diwali always brings a blanket of thick white smog — and the situation will likely be the same this year. Hourly air data from the U.S. Embassy in Delhi shows the spike in air pollution over the night of Diwali and the morning after.

Last year India’s top court temporarily banned the sale of firecrackers in and around the capital ahead of Diwali. But those who had already bought the crackers were allowed to burn them. The court is yet to make any pronouncement this year.

A man tries to protect himself from firecrackers while celebrating the Hindu festival of Diwali, the annual festival of lights in New Delhi, India, November 11, 2015. 

Ten years after the Supreme Court said fireworks must be evaluated on the basis of their chemical composition instead of the noise they produce, the Petroleum and Explosives Safety Organisation (PESO), formerly known as the Department of Explosives, of the Commerce and Industry ministry set some guidelines but they only cover 4 of 40 notified categories of explosives. It took the organisation three years to come up with guidelines for the four categories.

A worker fills small paper rolls with gunpowder mixture to make firecrackers at a factory ahead of Diwali, the Hindu festival of lights, on the outskirts of Ahmedabad, India October 21, 2016

A worker checks the dryness of firecrackers at a factory ahead of Diwali, the Hindu festival of lights, on the outskirts of Ahmedabad, India October 6, 2017. 

Firecrackers are made of highly toxic heavy metals such as copper, cadmium, lead, manganese, zinc, sodium and potassium. These metals, if present in the air, can trigger an asthma attack, and cause severe headache and respiratory problems. They can also contribute to a chronic cough.

Hoping for the best

With farmers already setting their harvested rice fields on fire in Haryana and neighbouring Punjab state, India’s frantic efforts to stave off a spike in pollution level this winter could go up in smoke.

“The message from the top office is to take steps to avoid the repeat of 2017. Otherwise heads will roll,” said a senior Indian government official who declined to be identified in line with government policy.

Other measures by the authorities to combat air pollution this year include pressing road sweeping machines and water sprinklers into service in an attempt to reduce dust in Delhi, and the large-scale planting of saplings to eventually act as a shield against pollution, said the official.

Although the National Green Tribunal, India’s main environmental court, has banned crop residue burning, the decree rarely gets enforced.

A lot may depend on whether the winds slow as much as they did last year.

“Let’s hope for the best,” said the government official. “After taking a number of steps, we’re just keeping our fingers crossed.”

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