Spend an afternoon outside in the centre of Delhi and you’ll find it increasingly difficult to breathe, your eyes will redden and begin to water and, if you stick around too long, you’ll develop a sore throat.
During rush hour at one of the most polluted spots - at the junction of the historic Red Fort and Chandni Chowk, one of the city’s main bazaars in the heart of old Delhi - it takes just a few minutes to feel those effects.
Around the time I was there, scientists at the Centre for Science and Environment (CSE), a Delhi think tank, had measured that the air around Chandni Chowk contained around 689 micrograms of PM2.5 particles per cubic metre, which is almost 28 times the World Health Organisation’s (WHO) "healthy" limit of 25 micrograms per cubic metre.
The tiny PM2.5 particles, hundredths the width of a human hair and invisible to the naked eye, come from diesel exhausts, farms and factories.
They can reach deep into people’s lungs and trigger severe lung and heart problems.
On average across Delhi, the WHO has measured the air containing around 153 micrograms of these particles per cubic metre, which is still six times the healthy limit (and 10 times worse than a typical day in London).
The pollution in Delhi has a double-whammy effect. Three people in Delhi die every hour as a result of it, and the city is only just beginning to get to grips with the health effects.
In addition, the pollutants also have effects for the rest of the world - among the PM2.5 particles coming from vehicle exhausts are also greenhouse gases such as ozone and carbon dioxide, which will contribute to future climate change.
Cleaning up cities such as Delhi will have a direct impact on the future environmental health of our planet.
So it is time to act, to clean up the air, Indian Nobel laureate and child-rights campaigner Kailash Satyarthi told ITV News.
“We will make suicide if we are not able to do it now - are we not going on the path of suicide if we are not able to protect air and water and Earth?
“We have to do it now, we are already late, we have become so late due to our greediness, due to our hippocracy, due to the gap on the promises we made for our earth and we deliver on the ground. That lacks morality and that's why I call for an urgency with a sense of morality.”
Satyarthi called for world leaders at the forthcoming COP21 climate summit in Paris to take environmental protection seriously and be “accountable to our children not just for their safety but for the safety of the planet".
The impact of the pollution on Delhi citizens was clear the moment we stepped into the Vallabhbhai Patel Chest Institute (VCPI).
Prof Raj Kumar, a leading respiratory doctor at the hospital, showed me corridors overflowing with patients, and he told me he now kept his clinic open for longer every day to try and keep up with the growing cases of asthma, lung disease and other breathing difficulties among Delhi citizens.
Nearby at the Sir Ganga Ram Hospital, neonatologist Professor Neelam Kler took me to see an intensive care unit for vulnerable babies.
There I saw premature babies, those born drastically underweight and some with birth defects.
Working with the Public Health Foundation of India, Prof Kler’s team has tracked a steady increase in these problems and she convinced that Delhi’s terrible air is playing a role.
When pregnant women breathe in pollutants, they can reduce the women's lung function, which means less oxygen gets to their baby’s organs as they develop.
Prof Kler calls these the “hidden effects” of pollution. “I think we already see the spectrum of health problems that are related to the bad quality of air which we are breathing - ranging from respiratory problems, respiratory allergies, increasing number of children and adults with asthma, increasing number of cancers which could be partly related to increasing pollution and an epidemic of babies who are born growth restricted who are exposed to the problems related to being small at birth,” she said.
“I think putting together we are heading towards health disasters.”
The pollution itself comes from many sources, including factories, construction sites, power stations and farms around the city.
Chief among the culprits, though, is the rapid increase in the number of vehicles in the city, particularly those burning diesel.
There are more than 8.5 million vehicles in Delhi, with around 1,400 new ones added every day.
In 2000, only 4% of the new car sales were diesel, according to Anumita Roy Chowdhury of the CSE. Today diesel cars account for half of new sales.
Coupled with relatively low emissions standards, the city’s air has become steadily worse over the past 15 years.
The sad thing, said Chowdhury, is that this rise in diesel cars has managed to dwarf a previous effort by the city to improve its air.
“If you look at about 15 years ago that was a time when the city was extremely polluted, but what we [saw] is there was a sharp public opinion, judiciary stepped in, the media took this up,” she said.
“We got rid of the diesel vehicles, we capped the number of commercial vehicles in the city, we could improve the emission standard, we shifted polluting industry out of the city, we shut down two coal-based power plants.”
In addition, around 10 years ago, all public transport in the city centre was forced to use cleaner-burning compressed natural gas.
As a result, between 2003 and 2007, the air in Delhi improved. But the concurrent rise in diesel cars eventually overtook that good progress.
“And that’s the scary story unfolding today. What has gone wrong is that the momentum that we had built to clean up, we could not keep that momentum going,” said Chowdhury. “Somewhere along the line the pollution source has overwhelmed the action.”
Every industrialised country has been through the same issue, said Prof Kler, so Delhi should learn lessons about how to grow and develop without also poisoning its citizens. “I think its a time that we take a note of it and do something about the quality of air, the quality of water, our waste disposal, so that we don’t become a heap of garbage.”
Delhi (and India in general) needs to move quickly towards better emissions standards, said Sumit Sharma of The Energy and Resources Institute (Teri) in Delhi, and improve the maintenance and inspection of older vehicles.
Further along, growing cities such as Delhi need to decide how to build out their cities, preferably investing in cleaner public transport rather than just building more roads and increasing the number of cars further.
The authorities in Delhi are certainly aware of the pollution problem (it’s hard to imagine how they wouldn’t be) but concrete action has so far has been slow.
Even so, Sharma said he was hopeful about Prime Minister Narendra Modi’s ambition to tackle problems such as pollution, citing a recent trip he made to California, where engineers and policymakers faced similar problems decades ago.
“[Modi] has learnt about how Californian people improved air quality,” he said. “They had similar conditions 40 years back, [but] they now have more vehicles but 80% reduction in the pollution emissions.”
The promising thing is that, if the city did act, the impacts of a clean-up action could be felt quickly.
In October, while we were filming in Delhi, the city held its very first car-free day one morning in the area around Chandni Chowk, during which only public transport was allowed in the area for a few hours.
Nearby, a group of protestors marched around with placards, chanting slogans decrying the usual choking congestion in that area.
It was still a chaotic and busy place that day, to be honest, as taxis and buses filled the roads.
In front of the Red Fort, we met Greenpeace activists who had been talking to passers-by about the need for clean-up action while monitoring the change in air quality after just a few hours without cars - they were already seeing impressive results.
The next day we got measurements from the CSE - the levels of PM2.5s in the air had dropped by 60% (to 310 micrograms per cubic metre) on car-free day, compared to the day before. The pollution was still well above safe limits, but it was a good start.
Those figures, and the lessons of the previous decade in Delhi, give people such as Chowdhury confidence that pollution could be brought under control in the city, given the right amount of public and political will.
And we should all watch with interest - developing, growing cities such as Delhi hold the future of our world’s climate in their hands.