According to the World Health Organization, Delhi is currently ranked as the city with the worst air pollution in the world. A study of 1,600 cities across 91 countries showed Delhi had the world’s highest annual average concentration of small airborne particles (known as PM2.5) of 153 micrograms per cubic meter. For perspective, the WHO has found no “safe” threshold for PM2.5 exposure but recommends countries aim for an annual mean measurement of 10 micrograms per cubic meter.
WHO fact sheet

To plumb the data or check out other cities, visit the WHO ambient air pollution database (summary of findings and raw data). The graph below, taken from this WHO report compares PM 10 levels for selected cities and shows Delhi as the tallest red bar.

Notice in the data and reports on air pollution that two different sizes of particulate matter are measured: PM 10, or particles of 10 microns diameter or less and the relatively more concerning PM 2.5 or particles of 2.5 microns diameter or less.   Both measures are important to health as particulate matter affects the health of more people than any other pollutant, but they can’t be used interchangeably in comparisons. Below is a WHO chart illustrating the annual PM 10 data for various cities.

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Source: The World Health Organization

Where is this data coming from?
The WHO maintains a worldwide, public database on urban outdoor air pollution in its Global Health Observatory. The database contains measured outdoor air pollution levels of PM 10 and PM 2.5 from 1100 cities in 92 countries for the years 2003-2010. These are used for estimating mean annual exposures of the urban population to fine particulate matter. In 2013, the WHO began collaborating with major institutions and agencies worldwide in the development of a global air pollution platform that includes data on air pollution concentrations based on satellite monitoring, chemical transport models and ground measurements, inventories of pollution emissions from key sources, and models of air pollution drift – permitting estimates of air pollution exposures even in areas where there are no ground level monitoring stations.

Links to the database and further information on how the data is collected.

Country Data

Beyond the findings for Delhi, 13 of the 20 cities found to have the highest annual averages of PM2.5 in the WHO study above are Indian cities.

Satellite Data
Using satellite-based remote sensing technology and ground level data from the Central Pollution Control Board, Dr. Michael Greenstone  and his team produced an influential study (Greenstone et al, 2015) showing that 660 million people (more than half of India’s population) live in areas that exceed India’s standards for fine particulate pollution. Large portions of India, in particular in the north along the Indo-Gangetic Plain can been seen to be heavily polluted and in non-compliance with India’s National Air Quality Standards (NAAQS.)

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Greenstone et al, February 21, 2015 Vol 1 no 8, Economic and Political Weekly

The Environmental Performance Index (EPI)

The Environmental Performance Index, produced by Yale and Columbia Universities, ranks how well countries perform on high-priority environmental issues in two broad policy areas: protection of human health from environmental harm and protection of ecosystems.

The 2014 report ranked India 155 out of 178, noting “India’s air quality is among the worst in the world, tying China in terms of the proportion of the population exposed to average air pollution levels exceeding World Health Organization thresholds,”
Press release
The India country profile on the issue of air quality
EPI methods and data