"A dirtier or more wretched place he had never seen. The street was very narrow and muddy, and the air was impregnated with filthy odours ... Covered ways and yards, which here and there diverged from the main street, disclosed little knots of houses, where drunken men and women were positively wallowing in filth."
The well-heeled residents of 21st century Clerkenwell would not recognise the description of their chic streets in Charles Dickens' Oliver Twist. By locating Fagin's thieves' kitchen in Saffron Hill, he was choosing one of Victorian London's most notorious slums; today, even a small flat on the same street can cost more than £500,000 ($954,000, euro741,000).
But the sort of urban squalor Londoners associate with the 19th century is growing "at an unprecedented rate", according to the United Nations. Next year, the UN estimates, more than 11m people -- one out of every three city residents -- will live in slums.
As Dickens' contemporaries were well aware, the growth of London's slums in the 19th century was fuelled by an influx of people from the countryside. For England, this represented the biggest social shift of the century: the point, reached in 1851, when for the first time there were more people living in English cities than in the countryside. In 2007, such a tipping point will occur for the whole world: before the year is out, the balance of the world's population will change, perhaps forever. For the first time, more people will live in cities than rural areas.
Since most of these people will be in the developing world, the UN predicts in its State of the World's Cities report that urban growth "will become virtually synonymous with slum formation in some regions".
This rapid growth will present enormous environmental problems. Overcrowding and poor housing are the most obvious issues for slumdwellers, but these are compounded by poor sanitation and a lack of clean water. There is often little provision for the disposal of solid waste. For instance, only 25 per cent of daily waste is collected in Nairobi. Slums also often suffer badly from Pollution, because they are built on contaminated land or undesirable areas near large industrial installations.
Priti Parikh, a chartered engineer researching slum issues at the University of Cambridge, explains: "In India, for example, the rainy season is a particular problem because the mud roads get flooded, and slums tend to be in areas of poor drainage [because welldrained land is already occupied by higher value developments]. Then the sewage and the storm water get mixed up, leading to disease, and the standing water leads to malaria."
For these reasons, slumdwellers suffer what Anna Tibaijuka, executive director at UN-Habitat, the human settlements programme, calls the "urban Penalty". She explains: "They have worse health [because of poor sanitation] and they are affected by the worst effects of industrial pollution. If there is a flood or a disaster, it's the poor who always suffer."
So far, there seems little prospect of solving these problems in most cities. Even the UN's own "millennium development goals" feature nothing more ambitious than a vow to "improve the lives of at least 100m slumdwellers by 2020". Given the numbers involved, this is simply tinkering at the edges.
The scale of the problem is daunting. More than one-quarter of the developing world's urban population -- more than 560m people -- lack access to clean water and sanitation, and about 1.6m people a year die as a direct result ' The World Health Organisation estimates that as much as one-quarter of global disease is caused by environmental problems that, if tackled, could save up to 13m lives a year. More than 40 per cent of malaria cases and an estimated 94 per cent of sickness and death from diarrhoeal diseases -- two of the biggest killers of children -- are preventable.
Few developing country governments could hope to afford the vast sums needed to clean up their slums. The UN estimates that just meeting its millennium development goal on urban poverty would cost $67bn (euro52bn, £35bn). But by 2020, another 400m people will be living in slums, and these people will need $20bn a year in assistance to gain access to basic services and amenities. Annual spending on slums, from both public and private funds, amounts to between 5 and 10 per cent of the sums needed.
What lessons does history teach about tackling the scourges urbanisation brings in its wake? London's Dickensian misery was alleviated only through a massive programme of public works in the second half of the 19th century. The sewage system built in the wake of the Great Stink of 1858 [from effluent in the Thames] took advantage of existing waterways and serves the city to this day.
Himanshu Parikh, Priti's father and director of Buro Happold Engineers, has pioneered a similar solution in the form of "slum networking". This involves making use of the natural contours of an area rather than relying on pumps to bring water to the surface. Water courses can be diverted and used to flush lavatories, while sewage can be treated using natural methods such as planting reed beds. Mud tracks can be paved so as to act as storm drains when water needs to overflow from the courses it has carved out, ensuring they do not turn into open sewers.
Ms Parikh adds that the most successful developments she has seen involve local people, businesses and government working together on slum improvement projects. These have tended to be on a small scale, involving between 800 and 1,200 houses at a time. Projects on a larger scale can become bogged down in bureaucracy and the need to co-ordinate a greater number of interested parties.
The UN-Habitat report on the world's cities identified strong central government as another essential ingredient in effective slum improvement. "Consistency in political commitment is crucial in mobilising long-term support for slum upgrading," the report notes. Clear, publicly stated goals and explicit political pronouncements on slums are part of this. Morocco, for instance, has set itself the goal of becoming a slumfree country by 2010.
Ms Tibaijuka says central government needs to direct migration better in order to avoid congestion in the most populous slums. This need not involve controversial forced clearances; instead, strategies can be developed to help people migrating from the countryside find shelter in the cities best able to accommodate them.
Governments also need to be powerful enough to enforce environmental regulations. Multinational companies investing in the developing world are increasingly aware of the reputational risk they can run from generating dangerous pollution. Ms Tibaijuka points to the disaster at Union Carbide in Bhopal in 1984, in which thousands of people died when a dangerous gas leaked from a pesticide plant, as one of the most serious examples of how companies can fail, and the consequences they face as a result.
Heavy industry is not the worst form of pollution slumdwellers must contend with; sometimes, the source is closer to home. Indoor air pollution caused by open fires for cooking and heating in the home is the fourth biggest killer of the world's poor, according to the World Health Organisation. Bronchitis, pneumonia and other respiratory illnesses contracted from breathing in smoke particles kill an estimated 1.6m people each year. The majority of victims are women and children, as they tend to spend most time in the home.
There are some simple solutions to this form of pollution, such as betterdesigned cooking pots that retain more heat. In China, there are plans to produce a coal-derived liquid fuel called dimethyl ether, which burns more cleanly and can be used for domestic cooking and heating, and in transport.
Outdoor air pollution is also worsening in many cities as road traffic increases, and urban regions are responsible for most greenhouse gas emissions. China is home to 16 of the world's 20 most air-polluted cities, says the UN, and over the past 10 years the concentration of pollutants in China's air has increased by 50 per cent.
Land reform can be an important weapon in the battle against urban poverty. Slumdwellers suffer from an inherent insecurity because they rarely own title to their land. This leaves them vulnerable to government interference and the whims of developers and reduces their incentive to improve their areas.
None of these issues are easily addressed but, as the UN's report makes clear, the problems incubating in the world's slums can no longer be ignored. Moving from Dickensian squalor to the fashionable restaurants and penthouses of today's Clerkenwell took well over 100 years. Given the speed at which slums are spreading today, urban dwellers must hope the 21st century will yield a more rapid solution than the 19th.