Large cities like living entities, release wastes, which include carbon dioxide, the main greenhouse gas, air pollutants, sewage and other water pollutants and even excess heat that collects in vast expanses of concrete pavement and buildings.

Humans directly produce a significant share of this waste, but emissions from industrial, power generation and transportation systems respire the largest quantities of greenhouse gases and other air pollutants.

Other urban metabolisers include sewage systems, landfills, domestic pets and pests like rats, which in some cities outnumber people, Kolb said.

These findings were presented at the 238th National Meeting of the American Chemical Society, said a release.