From penalising cars to planting trees, dozens of mayors from every continent pledged cleaner air on Friday in a bid to improve urban health and tackle climate change.
More than 90% of people breathe dirty air, causing death and disease, according to the World Health Organization (WHO).
"This dirty air kills 7 million people a year, largely in cities, and contributes to the global climate emergency," United Nations Secretary-General Antonio Guterres told the C40 summit of world mayors in Copenhagen.
Leaders of more than 90 cities representing more than 700 million people and a quarter of the global economy met in the Danish capital this week to push climate action.
As activists from Swedish teenager Greta Thunberg to the "Extinction Rebellion" protest movement take an increasingly visible stand on climate change, officialdom is catching up.
Among the mayoral initiatives are incentives to 'do good' - cheap bus fares - and penalties for doing harm - such as spewing excess emissions - as cities test tactics to coax change.
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From Los Angeles to Tokyo, 35 cities committed on Friday to meet WHO minimum air quality levels by 2030 - something they said could save 40,000 lives a year - and report on progress.
"Toxic air pollution is a global crisis, and as mayors, it is our fundamental responsibility to protect the public from this invisible killer," said Mayor of London Sadiq Khan.
The mayors met in the Danish capital- a Viking village turned city of wind turbines - days after climate-change protesters took to the streets from Austria to New Zealand pledging two weeks of peaceful civil disobedience.
Delegates at the summit opening were met on Wednesday by demonstrators from local group Klima Aktion DK, armed with fake binoculars made from toilet rolls.
"Our message to the C40 Mayors is: The people are watching you! We want to see action!" the group wrote on Facebook.
London to Lisbon
More than 2 million people live in areas of London that have illegally high levels of air toxins, according to official data.
To change that, the city this year introduced the world's first Ultra Low Emission Zone, requiring all vehicles to meet strict standards or pay a fee to enter the centre, Khan said.
Others offered carrots instead of sticks.
The mayor of Lisbon, Fernando Medina, said the Portuguese capital launched a discounted single fare ticket in April that had already resulted in a 30% increase in public transport use.
It has also increased green spaces, he said, with the equivalent of some 200 football pitches newly planted.
Milan's Mayor Giuseppe Sala said the Italian city was implementing a large low-emission zone and planned to completely switch its bus fleet to electric within seven years.
"Politically it is not easy to tell people they can't use their car but at this moment it is necessary," Sala told the Thomson Reuters Foundation on the sidelines of the conference.
Paris Mayor Anne Hidalgo said the city was installing sensors - including in schools and on buses - to measure pollution levels and planned to ban all diesel vehicles from Paris by 2024 and all fuel cars by 2040.
"The era of toxic emissions that poison the air we all breathe is coming to an end," she said.
Cities are vital to limit global warming as they account for about three-quarters of carbon emissions and consume more than two-thirds of global energy, according to the United Nations.
Known technologies such as electric buses could deliver more than half the cuts needed to keep global temperature rises below 2 degrees Celsius (3.6 degrees Fahrenheit), the U.N. says.
Los Angeles Mayor Eric Garcetti said he had experienced first hand the consequences of toxic air, having grown up in a home located between two freeways in California.
"Because of the emissions that were there, we had a cancer cluster on our street. My mother, my father and my sister have all had cancer, (and) luckily survived it," he said.
"Our residents deserve to know that future generations will inherit a liveable planet - and that our air, water, and natural resources will be protected and preserved," he added.