Delhi
Air pollution

According to the World Health Organization (WHO), Delhi was the most polluted city in the world in 2014. In 2016, WHO downgraded Delhi to eleventh-worst in the urban air quality database. However, as recently as 2022, data from the WHO and IQAir, alongside comprehensive research, ranked Delhi as the fourth most polluted city globally. According to one estimate, air pollution causes the death of about 10,500 people every year. Air quality index is generally moderate (101–200) level between January and September, and then it drastically deteriorates to Very Poor (301–400), Severe (401–500) or Hazardous (500+) levels in three months between October and December, due to various factors including stubble burning (a type of biomass burning), fire crackers burning during Diwali and cold weather. During 2013–14, peak levels of fine particulate matter (PM) increased by about 44%, primarily due to high vehicular and industrial emissions, construction work and crop burning in adjoining states. It has the highest level of the airborne particulate matter, PM2.5 considered most harmful to health, with 153 micrograms.

Rising air pollution level has significantly increased lung-related ailments (especially asthma and lung cancer) among Delhi's children and women. The dense smog and haze during winter results in major air and rail traffic disruptions every year. According to Indian meteorologists, the average maximum temperature during winters has declined notably since 1998 due to rising air pollution.

India's Ministry of Earth Sciences published a research paper in October 2018 attributing almost 41% of PM2.5 air pollution in Delhi to vehicular emissions, 21.5% to dust/fire and 18% to industries. The director of Centre for Science and Environment (CSE) alleged that the Society of Indian Automobile Manufacturers (SIAM) is lobbying "against the report" because it is "inconvenient" to the automobile industry. Environmentalists have also criticised the Delhi government for not doing enough to curb air pollution and to inform people about air quality issues. In 2014, an environmental panel appealed to India's Supreme Court to impose a 30% cess on diesel cars, but till date no action has been taken to penalise the automobile industry.

Most of Delhi's residents are unaware of alarming levels of air pollution in the city and the health risks associated with it. In 2020, annual average PM2.5 in the city stood at 107.6 µg/m, which is almost 21.5 times the WHO's PM2.5 Guideline (5 µg/m; set in September 2021). These pollution levels are estimated to reduce the life expectancy of an average person living in Delhi by almost 10.1 years.

However, as of 2015, awareness, particularly among the foreign diplomatic community and high-income Indians, was noticeably increasing. Since the mid-1990s, Delhi has undertaken some measures to curb air pollution—it has the third-highest quantity of trees among Indian cities; the Delhi Transport Corporation operates the world's largest fleet of environmentally friendly compressed natural gas (CNG) buses. In 1996, the CSE started a public interest litigation in the Supreme Court of India that ordered the conversion of Delhi's fleet of buses and taxis to run on CNG and banned the use of leaded petrol in 1998. In 2003, Delhi won the United States Department of Energy 's first 'Clean Cities International Partner of the Year' award for its "bold efforts to curb air pollution and support alternative fuel initiatives". The Delhi Metro has also been credited for significantly reducing air pollutants in the city.

However, according to several authors, most of these gains have been lost, especially due to stubble burning, a rise in the market share of diesel cars and a considerable decline in bus ridership. According to CSE and System of Air Quality Weather Forecasting and Research (SAFAR), burning of agricultural waste in nearby Punjab, Haryana and Uttar Pradesh regions results in severe intensification of smog over Delhi.
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