Delhi is inching closer to that time of the year when it becomes virtually uninhabitable because of the abysmal quality of its air. As the month in which Diwali is celebrated almost always overlaps with the crop burning season in Punjab and Haryana in October-November, the city’s residents could yet again choke on its polluted air.

According to several research studies, the sources of pollution are many and come from within and outside the city. For instance, the annual October-November spike in air pollution can be blamed on the burning of crop residue in fields in the neighbouring states of Punjab and Haryana. But other causes of pollution can be found within the city. These include vehicular pollution, including from privately-owned diesel cars, and emissions from coal-fired power plants and tandoors in hotels and restaurants.

It is clear that air pollution takes a toll on public health. For instance, PM 2.5 – particulate matter of size up to 2.5 microns – pollutants are small enough to lodge themselves into human lungs. Several diseases such as strokes, lung cancer and upper respiratory tract illnesses are related to particulate matter exposure. Last year, a study by the University of Chicago said if New Delhi adhered to World Health Organisation standards on the permissible levels of particulate matter in the air, its residents could gain as much as nine years in life expectancy.

Here is an explainer on Delhi’s pollution and how prepared the city is to face the smog this year.

What are the major causes of pollution in Delhi?

Several studies have cited various reasons for Delhi’s dipping air quality. For instance, a study by the Indian Institute of Technology, Kanpur, released in January 2016, suggested that vehicular emissions mainly from trucks were one of the largest and most consistent sources of pollution in the city. The study also pointed out that coal and fly ash contribute to 30% of PM 10 – particulate matter of size up to 10 microns – in the city in the summer. These come from coal-fired power plants as well as from tandoors. Soil and road dust contributed 26% to PM 10 and PM 2.5 in the summer. The study says: “The silt load on some of the Delhi’s road is very high and silt can become airborne with the movement of vehicles. The estimated PM10 emission from road dust is over 65 tons per day. Similarly soil from the open fields gets airborne in summer.”

In 2012, independent research group Urban Emissions pointed out in a study that the transport sector was one of the highest sources of all emissions. “Most of these, except for some light duty vehicles, operate on diesel,” the study said. It added that though Delhi’s buses, taxis and auto-rickshaws switched to CNG in the early 2000s, the benefits of this shift had been lost “due to an increase in the sales of diesel based passenger vehicles”.

How do external factors affect the city’s air quality?

A study released in August by The Energy Resources Institute or TERI and Automotive Research Institute of India found that in the winter Delhi contributes no more than 36% to PM 2.5 or fine-particulate matter pollution. In the summer, it contributes only 26% of PM 2.5. The remaining sources of this pollution come from areas surrounding the National Capital Region, the study said.

The air quality in Delhi sees a steep decline three times in a year, said officials in the Delhi Pollution Control Committee. The first dip, between October and November, comes during the burning of crop residue in fields in Punjab and Haryana, a phenomenon that has occurred since the 1980s. As the fields burn, winds blowing southwards push the smoke in the direction of Delhi.

The second dip takes place between December and January, when cold weather conditions cause a variation in the mixing height of the air. This is the height at which air turbulence can disperse pollutants suspended in the air. Without vertical mixing through convection, the pollutants get trapped at surface level, leading to smog.

The third dip in Delhi’s air quality occurs during the summer, between April and June, when dust storms lead to the level of particulate matter shooting up beyond prescribed limits.

How prepared is Delhi to face the smoggy situation this year?

With the 2017 ban of petroleum coke, a dirtier alternative to coal, and the closure of the Badarpur Thermal Power Plant scheduled for October, Delhi could witness better air quality this year, officials said.

Delhi has already seen better air quality days this year than it has since 2016, claimed a study. The Indian Express reported in August that between January 1 and August 26, the Central Pollution Control Board, had recorded 118 days in Delhi when the Air Quality Index was moderate to satisfactory. This is five more than 113 days of good to moderate air quality in 2017 and 74 days in 2016.

The report added that pollution control board officials said that the city’s air quality had improved after measures such as the Graded Response Action Plan and the Comprehensive Action Plan were implemented. The Graded Response Action Plan was put in place in 2017 on the directions of the Supreme Court. Under it, different government agencies are required to draw up a step-by-step action plan to control air pollution at an early stage. This includes an emergency response plan in case pollution breaches a certain level. For instance, construction would be halted, diesel generators will be shut and vehicular movement would be restricted. The Comprehensive Action Plan was finalised by the Union environment ministry in March specifically to manage air pollution in the Delhi-National Capital Region. This plan has listed a number of steps to reduce vehicular emissions, the number of vehicles on the road and construction dust, and has indicated agencies responsible for implementation.

Delhi’s transport commissioner Varsha Joshi said that the government was better equipped to handle the situation this year as it has taken measures to scrap old vehicles and increase last-mile connectivity in the transport sector. “The last lot of the old yellow and green Delhi Transport Corporation buses have been removed,” said Joshi. “We’re also shifting to more legal e-rickshaws. We have stopped the use of diesel generator sets.”

In August, the Central Pollution Control Board started introducing dust suppressing and other air purifying machines in parts of the city to curb air pollution and trap particulate matter. Apart from this, officials in the Delhi Pollution Control Committee said that initiatives to line roads with greenery were underway. “The focus has shifted from power and transport to dust control, garbage collection and greening pavements,” said the official. The Delhi government also announced subsidies for restaurants and hotels to encourage them to switch from using coal tandoors to electric ones.

Should Delhi act alone to fight pollution?

Experts say the impact of industrial and agricultural activities in other states on the quality of Delhi’s air cannot be ignored. “If you try to control the city’s pollution in isolation, it is impossible,” said an official from the Delhi Pollution Control Committee, who also called this an issue that the entire Indo-Gangetic belt has to deal with. This belt includes the states of Punjab, Haryana, Uttar Pradesh, Bihar and Jharkhand.

Experts also say that the Centre needs to get more strict with regulation and monitoring. “We know that changes are happening from the bottom up because there is pressure,” said Anumita Roy Chowdhary of the Centre for Science and Environment. “The National Clean Air Program is still in its draft stage but it is supposed to target cities to meet certain goals on air quality. There needs to be a standard setting for polluting sources all over the country. So, the Centre needs to tighten standards and check monitoring. State governments have to work with local area sources. We need to work immensely on waste management. As winter approaches, people will burn garbage if there is no other alternative. The public is now more aware.”