As India's north continues to struggle with extreme pollution levels, the story has put a fresh spotlight on air quality in cities across Asia.Beijing has long been notorious for its smog - but statistics show that India, Pakistan and Bangladesh have worse air by far.So why is South Asia so much more polluted?Of the world's most polluted 30 cities, 22 are in India, according to research by IQ AirVisual, a Swiss-based group that gathers air-quality data globally, and Greenpeace.The remaining eight cities are all in Pakistan, Bangladesh and China - but the list doesn't include Beijing, which comes in at number 122.Just looking at global capitals, it's also Asian cities that top the ranking.Looking at overall countries, it's Bangladesh that has the worst air, followed by Pakistan and then India.All these rankings are based on average air quality per year.As these countries have very different densities of measuring stations and transparency of data, the statistics have to be read with a degree of caution. But they certainly indicate an overall trend.
Pollution in urban areas is usually a mix of different factors - mostly traffic, fossil fuel burning power plants and heavy industries.What differentiates China from India is that in the latter, there is still a lot of burning of agricultural stubble when farmers want to clear their fields. The burning usually takes place in autumn.Media captionThe BBC's Vikas Pandey shows what it's like driving through the high levels of smog in Delhi"In this episode, the big problem really seems to have been the agricultural burning," assistant professor Thomas Smith of the London School of Economics told the BBC."That's one thing that China has tackled. All agricultural burning has been banned, full stop."A global overview for fires and thermal abnormalities is made available by Nasa, and allows users to track developments over past days and weeks.The area north-west of Delhi shows a highly unusual concentration of fires, Prof Smith points out."And you can't underestimate how important agricultural burning is - even though people often think only of cars and heavy industry as the causes."In the wake of the pollution spike, India's Supreme Court ordered a stop to stubble burning in the states around Delhi.But the city's situation is made worse by the colder winter air which is more stagnant, trapping the pollutants in place.Prof Smith also points out that "while India is largely reactive, Beijing tends to be more proactive and preventative to try to stop the problems from happening in the first place".
Pollution levels are categorised by measuring the levels of dangerous particles in the air. The result is then classified on a scale from good to hazardous."The effect of pollution is different for every person," explains Dr Christine Cowie of the University of New South Wales."Some people complain about irritation to the eyes, to the throat, exacerbations of wheeze and asthma symptoms. Coughing is certainly also a very common symptom - even in non-asthmatic people."And of course it's the elderly who suffer, the very young and people with pre-existing respiratory illnesses like or heart problems."She explains that even a short exposure to the unhealthy or worse pollution levels can trigger an asthma attack or increase the risk of a stroke. The longer the exposure, the greater the risks.And the ways to protect oneself are limited.There's the advice to stay indoors, to reduce physical exercise and to wear a mask - but in many poorer parts of the world, none of these options really work for regular people."It is toxic air," says David Taylor, professor of tropical environmental change at the National University of Singapore."It must be very uncomfortable - especially if you're having to work outside and if you're having to do jobs that require quite a lot of energy."
Air quality in Delhi has deteriorated into the "hazardous" category"It's like doing hardcore exercise when you just take a walk outside. And depending on the kind of pollution, you can of course also smell it in the air."How does it compare to Europe or the US?Today, pollution levels in Europe, Australia and the US are significantly lower than the extreme readings that India has experienced in the past few days.But it's not always been like that. London, for instance, was notorious for its pollution during much of the 19th and early 20th centuries.As late as 1952, the so-called Great Smog covered London with a thick toxic layer of pollution, bringing the city almost to a standstill for days, not unlike the situation in Delhi now. It's thought to have resulted in thousands of deaths.While London experienced a different mix of pollutants, "it probably wasn't far off what's currently happening in Delhi," says Mr Taylor.Back then, power stations along the Thames were major polluters of the British capital.In fact - much of what we know today about the health impact of air pollution dates back to the experiences of the London smog during those years.
Air pollution: How three global cities tackle the problem
4 November 2019
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Image copyrightAFP/GETTY
India's capital Delhi is blanketed under a hazardous shroud of air pollution.
City authorities have imposed a car rationing scheme in a bid to bring levels down, but experts believe the real blame lies with crop burning by farmers in neighbouring states.
Delhi is the latest city to try to come up with ways to tackle increasingly dangerous pollutants in the air.
This is what other cities have done in a bid to beat air pollution.
London
Media captionThe Great Smog of London remembered 60 years on
When was pollution at its worst?
Thick smog used to frequently blanket the UK capital in the 19th and 20th centuries, when people burned coal to warm homes and heavy industry in the city centre pumped chemicals into the air.
Referred to as "pea-soupers", the most famous of these events was the so-called Great Smog of London in 1952. It was recently dramatised in the first series of the Netflix drama, The Crown.
Cold weather in the preceding days meant people had burned more coal - often of low quality, which released more sulphur dioxide - while inner-city coal power stations added to the haze. An anticyclone then settled over London, trapping cold air under a layer of warm air.
The smog lowered visibility to a few feet and, over four days, is thought to have killed more than 10,000 people.
What was the solution?
In 1956 the UK passed the Clean Air Act.
It regulated both industrial and domestic smoke, imposing "smoke control areas" in towns and cities where only smokeless fuels could be burned and offering subsidies to households to convert to cleaner fuels.
Image copyrightGETTY IMAGES
Image caption
London used to regularly suffer from thick smog
The act was extended in 1968, and air quality substantially improved in London through the following decades.
What's it like now?
Air pollution remains at hazardous levels in London.
The city recently introduced an Ultra Low Emission Zone, which charges drivers of more polluting vehicles. London City Hall said in October that toxic air pollution had dropped by a third in the six months since the measure came into place.
But the UK capital still has some of the highest pollution levels in Europe. Particulate matter and nitrogen dioxide remain the greatest concern, and thousands are thought to die prematurely every year because of pollutants in the air.
Beijing
Media captionPeople in Beijing talk about smog in 2017
When was pollution at its worst?
China's rapid industrialisation brought a huge rise in air pollution.
Coal-burning power stations and a boom in car ownership from the 1980s onwards filled Beijing's air with hazardous chemicals.
In 2014, a report by the Shanghai Academy of Social Sciences said the city was nearly "uninhabitable for human beings" because of the pollution.
What was the solution?
Years of hard work.
A UN report this year shows that in the space of just four years, between 2013 and 2017, fine particle levels in Beijing dropped by 35%, while levels in surrounding regions dropped by 25%. "No other city of region on the planet has achieved such a feat," the report says.
But this was because of measures introduced and refined over the course of two decades, beginning in 1998.
Image caption
How fine particles known as PM2.5 can affect your health
Since then, the Chinese government has imposed ultra-low emission standards, created an advanced air quality monitoring system, and built more public transport.
What's it like now?
Beijing hasn't entirely fixed its problem. The city still struggles with pollutants - notably the particles known as PM2.5, which are badly affecting Delhi. Commuters in face masks to protect against the poor air remain a common sight.
And numerous cities around China still face pollutant levels far above internationally recommended standards.
Has Beijing's air quality improved?
Air pollution 'harms mental performance'
But the UN data shows the importance of cutting vehicle emissions, government incentives for private businesses, data transparency, and diversifying the economy away from heavy industry to successfully cut pollution levels.
Mexico City
Media captionHector Ruiz is on a one-man mission to make Mexico City cleaner
When was pollution at its worst?
Mexico's capital was infamous in the 1970s and 1980s for its poor air. Just breathing was the equivalent of smoking dozens of cigarettes a day.
Its tens of millions of inhabitants driving across the huge city in hundreds of thousands of cars pushed pollutant levels sky high. The city's position within a high-altitude valley means the poor air is often trapped by a ring of mountains.
In 1992 the UN gave it the dubious honour of being "the most polluted city on the planet".
What was the solution?
In 1989, the city became the first in the world to impose curbs on car usage.
It cut the number of cars on city roads by 20% from Monday to Friday, depending on their number plates. It immediately helped lower pollutants.
Media captionMexico's drastic bid to reverse decades of deadly pollution
This was followed by a package of reforms dubbed ProAire, which expanded public transport and imposed stricter vehicle emissions standards, among other measures.
All this helped improve Mexico City's air quality substantially in the years that followed.
What's it like now?
Deteriorating. Studies show some locals ignore the Hoy No Circula programme, or buy second cars to ensure they can drive every day.
The population keeps growing, and spreading out - meaning longer trips in more cars. And those who don't drive often rely on older, less environmentally friendly buses to travel.
In May this year city officials declared an environmental emergency after PM2.5 particle levels rose to more than six times the World Health Organization daily mean recommended limit.
Just as with other cities, Mexico City's experience show there is no easy solution to tackling air pollution.
