One of the world's most populous cities has recruited an unusual ally in the fight against air pollution: pigeons.

Paris-based Plume Labs has teamed up with Twitter to equip former racing pigeons with tiny, lightweight backpacks containing pollution sensors. As the pigeons fly through the streets of London, England, they collect air quality data. Users who tweet their location to the pigeons' account will receive a report on the air quality in their area of the city.

The pigeons will only be flown for three days, then returned to normal life with their caretaker. The campaign is intended to raise awareness about the serious issue of outdoor air pollution, which results in an estimated 3.7 million premature deaths each year, according to the World Health Organization.

Plume Labs, which operates a global network of pollution sensors to provide realtime air quality reports, is also recruiting humans in London to beta test a new wearable sensor that will help runners, cyclists and pedestrians understand what they're breathing in.

Related: Interactive map shows global air pollution in realtime