For three days in March 2016, 10 London pigeons became famous. Seeing pigeons take to the sky from Primrose Hill in north London was not unusual in itself. But these pigeons were wearing backpacks. And the backpacks were monitoring air pollution.

Once in the air, the backpacks sent live air-quality updates via tweets to the smartphones of the Londoners below. In almost all cases, the readings were not good. London’s air pollution problem has been getting worse for years, and it often rises to more than three times the European Union’s legal limit.

The pigeons and their backpacks were just the latest in a series of increasingly desperate attempts to monitor and control air pollution. According to the World Health Organisation, it is the world’s biggest environmental risk to health, and it “continues to rise at an alarming rate”. It kills over three million people every year and is especially a problem in urban areas: only one in 10 people live in a city that complies with the WHO air quality guidelines. This goes for the developed and developing world alike. Air pollution in Delhi shortens the life expectancy of its residents by 6.3 years and one in 12 deaths in London are linked to its dirty air.