Environmental problems in Delhi, India, are a threat to the well-being of the city's and area's inhabitants as well as the flora and fauna. Delhi, the ninth-most populated metropolis in the world (second largest if the entire NCR includes especially Faridabad and Gurugram– Haryana, is one of the most heavily polluted cities in India,[1] having for instance one of the country's highest volumes of particulate matter pollution.[2] The air quality index of Delhi is generally Good (0–50), Satisfactory (51–100) and Moderate (101–200) levels between March and September, and then it drastically deteriorates to Poor (201–300), Severe (301–400), or Hazardous (401–500+) levels in five months between October and February, due to various factors including stubble burning, burning of effigies during Vijayadashami, bursting of firecrackers burning during Diwali and cold weather.[3][4][5] In May 2014 the World Health Organization announced New Delhi as the most polluted city in the world.[6]
Overcrowding and the ensuing overuse of scarce resources such as water put heavy pressure on the environment. The city suffers from air pollution caused by road dust and industry,[7] with comparatively smaller contributions from unclean engines in transportation, especially diesel-powered city buses and trucks, and 2-wheelers and 3-wheelers with two-stroke engines.[8] Another known cause of pollution is slow moving traffic due to frequent pedestrian crossings.[9]