Delhi is the most polluted city in the world, according to a WHO study released on Wednesday.

The 2014 version of the Ambient Air Pollution (AAP) database contains results of outdoor air pollution monitoring from almost 1600 cities in 91 countries.

The national capital has the highest concentration of PM2.5 -- particulate matters less than 2.5 microns-- form of air pollution, which is considered most serious.

This form of concentration consists of tiny particles that puts people at additional risk of respiratory diseases and other health problems, the World Health Organisation said.

The situation is so bad in Delhi that its air has PM2.5 concentrations of 153 micrograms and PM10 concentrations of 286 micrograms--much more than the permissible limits.

In comparison, Beijing, which was once considered one of the most polluted cities, has PM2.5 concentration of 56 micrograms and PM10 concentration of 121 micrograms.

Air quality is represented by annual mean concentration of fine particulate matter (PM10 and PM2.5,-- particles smaller than 10 or 2.5 microns).

The database covers the period from 2008 to 2013, with the majority of values for the years 2011 and 2012.

Reacting to the report, Anumita Roychowdhury of the Centre for Science and Environment (CSE) said the new WHO data base only confirms the health concerns in India.

"According to global burden of disease estimates, air pollution is the fifth largest killer in India. Tiny particles (PM10 and PM2.5) go deep inside our lungs and trigger respiratory and cardiac problems as well as lung cancer," she said.

WHO said that in most cities where there is enough data to compare the situation today with previous years, air pollution is getting worse.

"Many factors contribute to this increase, including reliance on fossil fuels such as coal fired power plants, dependence on private transport motor vehicles, inefficient use of energy in buildings, and the use of biomass for cooking and heating," it said.