During cold winter days, Anumita Choudhury dare not leave her small second-floor apartment in New Delhi's northern suburbs.
Elderly now, she has developed asthma. The last time she ventured into the streets of the world's second most populous city she began gasping for breath and had to be helped home by her neighbours.
From Kabul in Afghanistan to Hong Kong and Shijiazhuang in China, and from Lima to Sao Paulo in Latin America, people are increasingly suffering in severe toxic smogs - leaving hospitals and health clinics flooded with people with respiratory and heart problems.
Foul air has blanketed much of urban Asia for many weeks already in the northern winter. In Delhi, where there are nearly nine million vehicles, the high court has compared conditions to "living in a gas chamber"; Beijing and 10 other Chinese cities have issued red alert warnings; in Tehran, where mayor Mehdi Chamran says air pollution kills up to 180 people a day, the smog has been so bad that schools have been closed and sports banned.
According to the World Health Organisation, the toxic fumes of growing numbers of diesel-powered vehicles are combining with ammonia emissions from farming, wood and coal fires, tyre burning, open rubbish dumps, and dust from construction sites and brick kilns.
The result is a global crisis that threatens to overwhelm countries' economies as people succumb to heart and respiratory diseases, blood vessel conditions, strokes, lung cancers and other long-term illnesses.
The toxic haze blanketing cities was observed last week from the international space station.
"It's bad now, but we just don't know what will happen in future," says Maria Neira, the WHO public health chief.
"This is the first generation in human experience exposed to such high levels of pollution. In the 19th century pollution was bad, but it was concentrated in just a few places.
"Now there are huge numbers of people living with high levels of pollution. Nearly 70 per cent of people in cities are exposed to pollution above recommended levels."
The problem is most acute in Asia, but many industrialised countries have been hit by smog this winter. Milan, Naples, Barcelona and some other cities in Spain declared an emergency and banned traffic for several days over Christmas; Poland's most popular mountain resort, Zakopane, was choked in fumes; and several London streets breached their annual limits for nitrogen dioxide just days into 2016.
According to a recent study in Nature, led by Johannes Lelieveld, director of the Max Planck Institute for chemistry in Germany, more people now die from air pollution than from malaria and HIV combined. They include 1.4 million people a year in China and 650,000 in India, compared with about 180,000 a year in Europe.
Pending new WHO figures on 2000 cities will show pollution worsening in many countries.
In 2014, 15 out of the 20 most polluted places were in India and China. The others were in Pakistan, Iran and Bangladesh. Of the worst 100, nearly 70 were in Asia and only a handful in Europe or the US.
But the WHO figures include only those cities that measure air pollution, and many of the worst offenders do not. "As the world urbanises, the pollution grows," says Frank Kelly, director of environmental health research at King's College London. "We suspect that many African cities have terrible pollution problems, but there is very little data.
"We know places like Tehran are very polluted. In Europe the pollution is relatively clear in places like Germany, France and Britain, except for the diesel, but in eastern Europe, where they still have old industries, it is still very bad."
Air pollution is rising up the political and economic agendas, as developing countries grasp that the crisis threatens to cripple their economies.
According to a recent WHO study, the cost of disease and the premature deaths caused in Europe every year by air pollution was more than US$1.6 trillion ($2.47 trillion) in 2010, nearly 10 per cent of the gross domestic product of the EU in 2013. Britain was estimated to have suffered US$83 billion in costs associated with air pollution. The figures were Germany US$145 billion, and France US$53 billion. The highest was in Bulgaria, which spent an estimated 29.5 per cent of its GDP on the costs of air pollution fatalities.
More than 90 per cent of citizens in Europe are exposed to annual levels of outdoor fine particulate matter above WHO's air quality guidelines. This accounted for 482,000 premature deaths in 2012, from heart and respiratory diseases, blood vessel conditions and strokes, and lung cancer.
As concern grows, cities have begun to take action. Last week, New Delhi ended a two-week trial that took a third of the city's three million private cars off the road. City authorities said the trial resulted in a 50 per cent drop in air pollution "primarily caused by vehicular traffic", but this is disputed.
China has recognised the problem and moved power stations out of cities. This, along with years of heavy investment in renewable energy and increased fines for polluting industries, has improved air quality in some areas.
If emissions continue to rise at current rates, the number of smog-induced deaths could double to more than six million a year by 2050, says Lelieveld.
The Chinese capital has long suffered from serious air pollution from coal burning and industry, but embarrassment at the Olympic Games, the closure of many factories and the removal of power plants from the centre have drastically cut pollution levels. There is a new willingness to deal with it.
The fast-growing Afghan capital city is one of the unhealthiest in the world, surrounded by hills that trap pollution for days on end and full of cars and buses burning low-quality fuel and millions of houses being warmed with wood and paraffin. According to doctors in the main hospital, air pollution is now the greatest cause of death, killing more than 3000 people a year - more than from war, terrorism, road accidents or HIV and Aids.
Iranian cities are among the most polluted. Last month, the air in Tehran was so bad the city shut all schools, put emergency services on full alert and advised people with cardiac and respiratory problems, as well as pregnant women, to remain indoors. Some of the pollution in the city of five million cars has been blamed on sanctions on imports of refined petrol. This, it is said, has led to the use of low-quality alternatives.
Bangladeshi cities such as Dhaka, Narayanganj and Gazipur are some of the most polluted and fastest growing in the world. Brick kilns, tyre burning, old cars and heavy traffic combine to make them dangerous to live in.
Air pollution in the fast-growing Nigerian city, the largest in Africa, is reaching dangerous levels. But, as in many cities on the continent, there's little monitoring of pollution and no accurate information on fuel consumption.
Los Angeles used to be notorious for its smogs but cleaner cars and better regulation has improved air quality. Some of the worst air in the US is now found in nearby Bakersfield in the San Joaquin Valley, a farming area surrounded by mountains that trap pollution from traffic and agricultural emissions. Warm weather often leads to dangerous photo-chemical smogs.
Peru's capital has the worst pollution in Latin America, according to a 2014 study by the World Health Organisation. Poor fuel, old buses and cars and the city's geography between the Andes to the east and the Pacific Ocean to the west all make the air foul at times. Nearly 80 per cent of the estimated 5000 deaths a year from pollution may be caused by the city's fleet of old minivans and buses. The many poor areas are the most polluted and are often sited close to open waste dumps and other sources of pollution.
Air quality in the Egyptian capital city of more than seven million people is often 100 times above World Health Organisation safe limits. The toxic fumes of a million mostly old cars mixes with smoke from farmers burning rice straw, industrial pollutants and desert and construction work dust. Heart disease, stroke and respiratory infections are now among the top four causes of premature death in the capital.
With 9 million vehicles and 25 million people, Delhi is possibly the most polluted mega-city in the world, often covered in a foul-smelling toxic haze. The pollution was so bad last month that the city authorities imposed an on- and off-traffic system to try to reduce the number of cars on the roads.
Once known for its smogs because of massive coal burning, it is now plagued with noxious NO2 gas from hundreds of thousands of diesel engines and the particulates emitted by taxis, central heating systems and construction sites. The Government does not expect the city to meet legal pollution levels until 2025. Last week, London breached its own legal limit on air pollution for all of 2016 in just over seven days.