[10] The air quality index (AQI) in Delhi generally falls within the Satisfactory (51–100) and Moderate (101–200) ranges between March and September, and then drastically deteriorates to Poor (201–300), Severe (301–400), or Hazardous (401–500+) levels between October and February due to various factors including the burning of effigies during Vijayadashami, the bursting of firecrackers during Diwali, thermal power plants in the National Capital Region,[11] stubble burning,[12][13] road dust, vehicle pollution and cold weather.

[14][15][16] In November 2016, in an event known as the "Great Smog of Delhi", the air pollution spiked far beyond acceptable levels.

According to data released by the Ministry of Environment, Forest and Climate Change in 2022, the Air Quality Index in Delhi stood at over 200 for at least half the year.

[18] Animal agriculture also contributes to Delhi's pollution problem, as smog and other harmful particles have been produced by farmers burning their crops in other states since the 1980s.

[22] The Supreme Court asked authorities in the NCR region to consider remote work policies for employees.

[24] On November 18, 2024, Delhi recorded its worst air quality of the season, with a 24-hour AQI reading of 491, classified as "severe plus."

This level, as reported by India's pollution control authority, indicates hazardous conditions with significant health impacts, particularly for vulnerable populations.

[26] PM10 levels, for the period 2008 and 2013, based on data of 1600 cities in 91 countries, range from 26 to 208 micrograms per cubic meter of air (μg/m3), with the world average being 71 μg/m3.

[27] As of November 2017, experts in several monitoring stations have reportedly measured an air quality index of 999.

Of NOx emissions, 52% came from industrial point sources (mostly from power plants and 36% from vehicles (but the 36% was potentially more damaging due to being emitted close to people)[citation needed].

In addition, research shows that pollution can lower children's immune system and increase the risks of cancer, epilepsy, diabetes and even adult-onset diseases like multiple sclerosis.

[30][48] In 2016, the Chief Minister of Delhi proposed these measures to reduce air pollution but didn't get any success due to the lack of implementation in reality.

[49] In October 2020, Delhi authorities established a 10-member air pollution control team working in a dedicated conference room.

[50][51] On 25 November 2017, the Supreme Court of India banned the sale of firecrackers in Delhi to alleviate pollution.

[53][54] Recently in October, 2020 union environment ministry has formed a commission for Air Quality Management in National capital region and Adjoining Area Ordinance.

It allows the crop to be sown only during the monsoon months and as a result encourages stubble burning due to restricted cultivation period.

[60] "The Great Smog" also led to cancellation and delay of public transport, primarily trains and flights.

The primary sources of smoke are power plants,[62] stubble burning, lit garbage, road dust, factories, and vehicles.

[65] A Health Emergency was declared in the capital by the Central Government of India to cope with the extrusive amount of polluted air.

The Commission for Air Quality Monitoring (CAQM) directed that all schools, colleges and educational institutions will be closed until further notice, in Delhi and in NCR.

[79][80] In December 2017 during a test match between Sri Lankan and Indian cricket teams in New Delhi, Sri Lanka players began to feel breathing problems and several players vomited both in the restrooms and in the field and had to use face masks until the match was stopped.

[82] In November 2024, pollution levels in Delhi reached drastic measures following increased fire activity in India and Pakistan's Punjab region as farmers burn off excess straw, caused a significant smog on the morning of 8 November, causing it to momentarily become the most polluted city in the world.