Delhi air quality | WWF

Delhi air quality

Posted on 01 March 2012    
© Dave Morris

Air quality improved by civil society

By the 1990s, Delhi had among the world's worst urban air quality. Air pollution's heavy impacts on human health, including infant mortality and asthma, led to sustained efforts for improvement by Indian civil society. Ultimately, India's Supreme Court intervened. A range of measures created major gains in air quality and health. However, Delhi's booming growth in vehicles and diesel are threatening to eat up all its air quality gains.

Delhi was awarded the title National Earth Hour Capital in Earth Hour City Challenge 2013

Keywords: air pollution, compressed natural gas (CNG), respirable suspended particulate matter (RSPM)

Delhi used to be one of the world’s most air-polluted megacities. At the turn of the millennium, Delhi was ranked among the world's 10 most polluted cities in the world, together with cities like Karachi, Kathmandu, Beijing, Lima, Arequipa, and Cairo, for total respirable suspended particulate matter (RSPM).

Rapid growth is cited as the main driver. India's economy had grown by nearly 300% between 1975-1995. At the same time, the country's industrial pollution load grew by 350% and the vehicular pollution load by 750%.

Damaging to human health
Globally, an estimated 2 million people die every year from air pollution, more than 50% in developing countries. Urban areas are key hot spots for air pollution. Vehicle use, electricity generation (e.g. coal-fired), industrial production, and domestic cooking and heating are the four main sources of urban air pollution. Major pollutants are SPM, PM10, CO, SO2, NOx, benzene, toluene, xylene, and polycyclic aromatic hydrocarbons (PAHs) (see also Toronto, Stuttgart and Vitoria-Gasteiz).

The health effects of air pollution are the focus of a large body of scientific research. Particulate matter (PM), for example, is now understood to damage health and shorten life span, raising risks for cardiovascular disease, respiratory disease (e.g. asthma and bronchitis) and lung cancer. Infant mortality is a key concern. PM has been demonstrated to lead to large increases in infant mortality for infants with high exposure during gestation. A 2003 study found that infant mortality in the US declined significantly in counties that were compliant with the 1970 Clean Air Act. A study on China also found that when regulations on power plants reduced total suspended particular (TSP) pollution, infant mortality dropped by nearly precisely the same proportion: a 1% reduction in TSP pollution reduced infant mortality by 0.95%.

Activists spearhead turnaround
Dangerous air pollution in Delhi and other Indian cities led to sustained action by public interest groups like the Centre for Science and Environment (CSE). The most successful strategy was to focus efforts on India's judicial system, where a wide range of vital changes for air pollution were achieved. India's Supreme Court was able to order:
  • conversion of all Delhi's commercial passenger vehicles – buses, three-wheelers, and taxis – to compressed natural gas (CNG)
  • catalytic converters for new cars in India's four largest cities
  • closing of industries categorised as hazardous or noxious in Delhi
  • reduced sulphur content of diesel and petrol
  • large increases in the number of buses in Delhi

Better air = better health
Significant gains in Delhi's air quality have been found in a series of independent studies. A 2007 study found that the conversion of buses from diesel to CNG helped reduce PM10, CO, and SO2, while lowering the sulphur content of fuels cut both SO2 and – due to SO2 converting to sulphates, fine particles (PM10). Another study only identified lower CO as a result of the change to CNG in Delhi. Yet another study found a significant decrease in PAH concentrations. Converting buses to CNG is argued to have been key to air quality improvement, due to buses relatively large share of total transport in Delhi.

Better air quality has had “a substantial effect on respiratory health” according to a 2011 study by Foster and Kumar. These benefits were highest for low-income households, possibly because Delhi's poor spend much larger amounts of time out-of-doors. The poorest men spend 7 hours per day outside and the poorest women spend 3 hours per day, compared to 1 hour per day for men and women living above the median in terms of per capita expenditures. In the highest income households, adults spend almost no time outside.

Air quality gains eaten up?
Unfortunately, Delhi's air quality improvements are being undermined. The huge growth in vehicles and diesel use are linked to increases in PM10, PM2.5, NOx, and ozone. The average concentration of RSPM in Delhi is about 120 μg/m3, to be compared with the World Health Organization (WHO) guidelines of 20 μg/m3.

Another cause, cited by the Indian Foundation of Transport Research and Training, is that the loading and air pollution norms for heavy and commercial vehicles are not being enforced. A truck that is overloaded by 30% is associated with huge increases in pollutants, according to a study by the Central Road Research Institute: from 3.17 gm/km of NOx under standard loading to 119.2 gm/km, and from 104.13 gm/km of PM to 611.75 gm/km. Mass transport vehicles are also skirting the rules by registering outside Delhi, thus continuing to run on diesel instead of CNG.

Asha B. Chelani, Sukumar Devotta, 2007, “Air Quality Assessment in Delhi: Before and After CNG as Fuel”, Environmental Monitoring and Assessment, 125:257–263

Express News Service, 2011, “CNG or not, Delhi remains pollution hotspot”, The Sunday Standard, 5 June,

A. Foster, N. Kumar, 2011,“Health effects of air quality regulations in Delhi, India”, Atmospheric Environment, 45 (9)

S.K. Goyal, S.V. Ghatge, P. Nema, S.M. Tamhane, 2006, “Understanding Urban Vehicular Pollution Problem Vis-a-Vis Ambient Air Quality–Case Study of a Megacity (Delhi, India)”, Environmental Monitoring and Assessment 119(1): 557–569

Michael Greenstone, Rema Hanna, 2011, “Environmental Regulations, Air and Water Pollution, and Infant Mortality in India”, Working Paper 17210, National Bureau of Economic Research,

Michael Greenstone, Aparna Krishnan, Rohini Pande, Nicholas Ryan, Anant Sudarshan, 2011, “Improving human health through a market-friendly emissions scheme”, Paper presented at seminar on Global Environment and Disaster Management: Law and Society organized by the Supreme Court of India, July 23, Delhi, India,

P. S. Khillare, Tripti Agarwal, Vijay Shridhar, 2008, “Impact of CNG implementation on PAHs concentration in the ambient air of Delhi: A comparative assessment of pre- and post-CNG scenario”, Environmental Monitoring and Assessment, 147:223–233

Neha Lalchandani, 2011, “10 yrs on, CNG advantage lost”, The Times of India, November 9,

Apoorva Mandavilli, 2007, “India's watchdog: A breath of fresh air”, Nature, 445 (7129), 706-708
Urvashi Narain and Ruth Greenspan Bell , 2005, “Who Changed Delhi’s Air? The Roles of the Court and the Executive in Environmental Policymaking”, Discussion Paper 05-48, Resources for the Future

Urvashi Narain, Alan Krupnic, “The Impact of Delhi’s CNG Program on Air Quality”, Discussion Paper 07-06, Resources for the Future,

Khaiwal Ravindra, Eric Wauters, Sushil K. Tyagi, Suman Mor, René Van Grieken, 2006, “Assessment of Air Quality After the Implementation of Compressed Natural Gas (CNG) as Fuel in Public Transport in Delhi, India”, Environmental Monitoring and Assessment, 115: 405–417

Sangeeth Sebastian, 2011, “MIT study says high RSPMs in Delhi's air poses threat to residents”, India Today, July 24,

Shinsuke Tanaka, 2010, “Environmental Regulations in China and Their Impact on Air Pollution and Infant Mortality”, Job Market Paper, Boston University,

World Health Organization (WHO), 2008, “Air Quality & Health: Questions and Answer”,

Savita Verma, 2011, “Diesel powered vehicles polluting Delhi's air”, India Today, 13 November,

Key data are retrieved from the UN World Urbanization Prospects: The 2011 Revision,

Text by: Aaron Thomas

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