The world's most polluted cities have been described as deadly gas chambers with millions of people dying around the planet every year after inhaling toxic greenhouse gas emissions.
But some cities are leading the way with India's capital New Delhi this week choking in thick, toxic smog.
"My eyes are stinging, I have a bad cough and I feel unwell," a student said.
Breathing the air has been likened to smoking 50 cigarettes a day.
"The way people stay indoors during rains or a storm - people should take the same precautions now," said Dr Krishan Kumar Aggarwal of the Indian Medical Association.
All schools in India's capital have been closed, a public health emergency declared and from next week residents can only drive on alternate days.
New Delhi is the world's most polluted capital but other clogged cities have also been forced to take action.
There are children in London whose lungs are underdeveloped- Sadiq Khan, London Mayor
Outdoor air pollution mainly from vehicles and industry causes around 4.5 million deaths every year around the globe.
Most of those are in poorer countries such as India, but in London, one of the world's most advanced cities, nearly 9500 people die every year from air pollutants.
"There are children in London whose lungs are underdeveloped. There are adults who suffer a whole host of conditions caused by the poor quality air from asthma to dementia to suffering strokes," Sadiq Khan, London Mayor told 1 NEWS.
Central London is now doubling an existing congestion charge, for the oldest and most polluting vehicles.
"It's actually going to penalise the people who can least afford to pay," said one London driver.
There's been huge growth in transport emissions- Russel Norman, Greenpeace New Zealand
Many cities, including Paris and Los Angeles, are now setting themselves tougher environmental goals than their governments.
Auckland is joining a city collective which has just pledged to buy only zero-emissions buses from 2025.
A recent study showed harmful vehicle emissions in New Zealand cause more than 250 deaths every year.
The country has committed to reducing all greenhouse gas emissions to 30 per cent below 2005 levels by 2030.
"The big problem for our cities has been transport. There's been huge growth in transport emissions over the last decade or so," said Russel Norman of Greenpeace New Zealand.
"And that's obviously linked to the transport strategy of the previous government which was more and more and more roads and motorways."
The former Green Party leader says the new government's push towards more public transport may help its emissions reduction goal.