Delhi, Ahmedabad, Dibrugarh, Trivandrum and Hyderabad are polluted by carbon monoxide (CO) which extends to as far as 10 kilometres from the surface of the earth (in the troposphere), reported Deccan Chronicle. The Indian Institute of Technology in Gandhinagar and Physical Research Laboratory in Ahmedabad conducted a study along with researchers from Germany's Max Planck Society over a period of 14 years – 2001 to 2014. Normally, the troposphere is believed to have a CO content of about 50 ppbv.

Pollution from vehicles is a major contributor to high carbon monoxide levels. However, researchers believe winds carry the CO produced by biomass burning in Africa and Southeast Asian countries to the Indian subcontinent, thus adding to the already high levels of the gas in the country's atmosphere. "CO levels over New Delhi is lower than in Hyderabad or Dibrugarh in Assam, because the wind pattern is such that it flows over south India and other regions more than the northern parts in the country," Naveen Chandra, one of the researchers, said.

Five researchers collected values of CO at 200 Hectopascals (hPa), which is around 10 kilometres from the earth's surface, and at 900 hPa, about 800 meters from the surface. At 200 hPa, they found CO at the highest of 118 parts per billion volume, over Ahmedabad followed by 115 ppbv in Hyderabad during the monsoon. During winter, Hyderabad recorded the highest CO content at 228 ppbv at 900 hPa.

"As carbon monoxide due to vehicular pollution and other sources is released, it travels upwards. In monsoon as there is a lot of wind, CO particles reach heights of 10 kilometre in as quickly as two hours. That is why in monsoon CO values are the highest in upper layers of the troposphere. In winter, it is the opposite as there is not much wind and the CO particles are closer to the Earth's surface, at 900 hPa than at 200 hPa," Chandra, who is associated with PRL, said. He added that CO "affects the ability of the atmosphere to cleanse itself of many other polluting gases" and said that it is also responsible for urban smog and lower-atmospheric (“bad”) ozone.