In this world of 7 billion people, the global rural-urban balance of populations has tipped irreversibly in favour of cities. But what, exactly, is a "city" in 2011?

Hania Zlotnik, the director of the Population Division of the United Nations Department of Economic and Social Affairs cautions against assuming too easy a definition because governments and urban areas themselves define "city" in numerous ways and their boundaries can shift, sometimes for political, demographic or economic reasons. Metropolitan areas spreading over large territory are absorbing or overtaking compact cities, sometimes merging with other metros along heavily populated corridors. Urban populations may also be counted in different ways from country to country or city to city.

The Population Division's World urbanisation Prospects: The 2009 Revision calls these huge population centres "urban agglomerations." Tokyo emerges as the world's largest urban area under this definition, with 36.7 million people, which is more than a quarter of the national population. Tokyo is followed by Delhi, with 22 million; Sao Paulo, 20 million; Mumbai, 20 million; Mexico City, 19.5 million; New York-Newark, 19.4 million; Shanghai, 16.6 million; Kolkata, 15.5 million, Dhaka, 14.7 million, and Karachi, 13 million. Each of these cities reflects a different pattern or path of planning and governance and a different composition of affluence and poverty.

Without planning, cities can grow absent-mindedly, spread over every available empty space and overrun the ability of public services, where they exist, to meet demands or cope with the growth of slums. Property developers, corporations, migrant workers, government bureaucracies, and public institutions seeking room to expand all play roles in the growth, reshaping or, lately in a number of countries, contraction of cities. While many cities face overwhelming challenges, others have the potential to bring the benefits of urban life to their residents.

Advocacy organisations, civic associations and bolder, better informed residents demand to be heard. In China, where in the past Government decisions on urban development projects were not easily challenged, a spirit of participation is arising, most recently over where to put waste incinerators in the Beijing area, a United Nations official working on environmental issues said.

How planners and politicians deal with urbanisation demonstrates some differing policies and programmes for coping with rapid urban growth, or how to rectify mistakes that allowed it to happen without good planning or preparation. But though cities may have different histories and challenges, the aims of city officials almost everywhere are similar. They say they want to create better and more secure environments, with acceptable levels of public services and infrastructure and meet the explosion of motor and pedestrian traffic.

Perspectives on urbanisation

In recent years, there has been a debate about whether more urban living is to be deplored for the mushrooming of slums where sanitation is nonexistent, epidemic diseases can thrive, exploitation is rampant and physical dangers lurk where law is absent and order may be left to criminal gangs, or to be welcomed for the opportunities city life offers-jobs, access to health services, family planning, schools and more economic openings for women. Boosting the opportunities while minimizing the dangers and difficulties are the main challenges of development in the today's urban transitions.

Urban trends are not uniform, however. In India, for example, traditional city centre populations are shrinking as peripheral areas grow, statistics show. Mumbai is often cited as a prime example. New figures from the 2011 census show that in Maharashtra, the centuries-old city of Thane, formerly a middle class satellite suburb 43 kilometres northeast of Mumbai, has swelled with a soaring slum population. Thane now has 9.84 per cent of the state's people - 11 million in numerical terms. That is a jump of almost 36 per cent growth in a decade. Mumbai city proper, with 3.14 million people, recorded a negative growth rate of 5.75 per cent during the same period.

Amitabh Kundu, a professor of economics at the Centre for the Study of Regional Development and dean of the School of Social Sciences at Jawaharlal Nehru University in New Delhi says that some of India's major cities are experiencing what he calls "degenerative peripheralisation" - where the people are driven out by the high cost of living and the scarcity of jobs that pay a decent wage to live in ad hoc settlements on the periphery of metropolitan areas. In those peripheral settlements, people have lost the advantages of both urban and rural life. Kundu says recent efforts to clean up and beautify cities in India, applauded by many, are changing the cities' characters, not necessarily for the better.

Kundu sees international economic considerations driving the change. "Fast developing countries, especially in Asia, are trying to get access to the global capital market, and the only way they can do that is through their big cities," Kundu says. As foreign investment and capital rise, so do prices, and city life becomes more expensive. Many of the improvements to India's cities are mainly benefiting the middle class, he adds.

"Big cities are losing the poor because they can't afford to live there," Kundu said. "Earlier, people would pick up something like 1000 rupees and come to Delhi and look for a job for a month. Now with 1000 rupees you can't stay for a week. So the percentage of the poor in Delhi has gone down from about 55 per cent in three decades to 7 per cent."

The result? "We are sanitising our cities," Kundu says. "Sanitisation means making the environment clean, ... clearing the slums, pushing out the low-income colonies." And in the process, cities' miss out on any opportunity to transform the urban poor into drivers of growth and development and instead perceive illiterate, unskilled workers only as liabilities to health, hygiene and law and order, he argues.

Altering the social balance of cities in India is an important subject for study by demographers and economists because 410 million of the country's 1.2 billion people already live below the poverty line. That represents one-third of all the world's poor, according to the World Bank, which also notes that Indian income gaps are widening.

"In Mumbai-the central district-the growth has come down dramatically," he said. "Same thing with Chennai, with Hyderabad, with Kolkata - all the major cities, all the urban central districts. Earlier, somebody would come from the rural areas and start polishing shoes or pushing a cart. Those jobs are diminishing as rural-to-urban migration is going down, Kundu said. He and other demographers say that India needs to develop small and medium-sized cities, which may be more accessible to the poor and have the potential to provide jobs.

Faujdar Ram, director of the Indian Institute for Population Sciences, a degree-granting, university-level institution, said that even though people on marginal or even middle class incomes have been pushed out of Mumbai city, they still want to work there. He said there are commuters coming into the city from numerous outlying areas, including Pune, 163 kilometres to the southeast, where population growth has also been rapid. Pune is now connected to Mumbai by a six-lane motorway that cuts travel time for those with cars or money for intercity buses.

"Why are people coming from Pune?" Ram asks. "Pune needs jobs there." Meanwhile, public commuter transportation handling ever larger numbers of passengers needs improvement, he said. Mumbai's commuter trains are notorious for their overcrowding, slow service and sexual harassment of female passengers.

New opportunities for women

For many women, there is a positive side to the evolution of central Mumbai, says Sajana Jayraj, writing for Media Matters, a nongovernmental organisation which works in development communications and studies women in urban settings. Expanding service industries and the technology sector bring many more women into the city to work and advance their education and skills. What she calls "a growing tribe of young women, working and studying at the same time" make a daily journey of two hours or more from the inner and outer suburbs of Mumbai.

They are a different kind of urban migrant, well-educated and leading middle class lives, frequently balancing careers and families. "Women peeling vegetables in the train back home is a common sight," Jayraj wrote.

The pattern of peripheral low-income growth is evident in Thane, where about 30 per cent of the population now live in slums. In Bhim Nagar, one of those settlements, there are 10 to 15 people living in each small (often one-room) house, residents say. Many if not most of the men are unemployed or can find only casual labour at that distance from Mumbai. Women do better because they are able to work as domestic servants, but those jobs also do not carry any longterm security or benefits. Survival from day to day is the goal of most families. No one is certain if or when they could be evicted from their homes crowded together along muddy lanes.

"Slums are complex," said Ram at the Indian Institute for Population Sciences in Mumbai. "Most people are renters, and the owners are local leaders, elected politicians." In Indian cities, politicians use slum settlements and shantytowns as "vote banks" in their electoral districts. It is in their interest to keep the poor there in sizeable numbers.

But slumlords are coming into competition with property developers who see unincorporated territory with uncertain ownership as prime targets for private development, Ram said. When property developers have powerful backers, a slum can be bulldozed out of the way with little warning, and its families scattered. Only rarely are private developers required to provide a portion of low-income housing, Ram said.

Although rural-to-urban migration is declining, the state of Maharashtra will continue to attract unskilled migrants looking for work because language barriers are not insurmountable in the polyglot Mumbai area, Ram said. But language may be a barrier for Hindi-speaking northerners, for example, who seek to fill jobs in cities in other regions, such as Kerala, Tamil Nadu, and Karnataka, where labour shortages are occurring.

The allure of jobs

Farther to the northeast, about 60 kilometres from Mumbai but still considered part of its greater metropolitan area, the city of Bhiwandi is a case study in the meeting of Indian industrialisation and urbanisation. For many years, Bhiwandi was a small town known for its handloom weavers. Then with electrification and the introduction of power looms, it became the "Manchester of India," with the largest number of textile mills in the country, eclipsing the traditional work of farmers, fishing people, merchants and spice traders.

The looms of Bhiwandi employ a major part of the city's workforce, but the textile factories, running around the clock, always need more labour and so a large number of migrants from other Indian states have become a fixture of the population. In Bhiwandi, young men continue to arrive from the poor states of northern India, Uttar Pradesh in particular, to work in factories that look like a picture of 19th century England.

Bhiwandi might be a good example of an economically sustainable and self-contained small city if life were updated to make the environment more congenial and healthier. The jobs are dirty, hot and dangerous. Huge, squalid, stifling sheds packed with looms often have no running water or toilets. But migrants, virtually all men and boys, stay here for years or decades, in essence becoming residents, because life here is better than at home and their earnings keep distant families and villages afloat.

In sweat-drenched tank tops and cheap trousers, wearing only flip-flops or sandals on their feet, they sit at clattering looms running at ear-splitting noise levels, with scant protective devices to protect them from the huge machinery's moving parts. Workers say they suffer industrial accidents and health problems: electric shocks, injuries from flying shuttles, skin infections and tuberculosis. Their windowless living quarters appear like concrete cartons stacked on several floors, where up to 10 workers may sleep in shifts in one room. Scores of them share a communal toilet and water tap.

Workers who have spent years in the mills were eager to tell their stories for this report. Nagendra Tiwari, age 42, from Gorakhpur in Uttar Pradesh, came to Bhiwandi in 1988 when his father, a poor farmer, could not afford to find husbands for his five daughters and Nagendra was forced to migrate in search of money for his sisters' dowries. He had to leave behind a wife and four children. As a high school graduate with management skills, he moved from mill to mill, but the work was never easy. "We worked in 12-hour shifts, and were paid every 15 days. There were no days off." Tiwari earned less than the equivalent of $20 a month, based on piece work, and paid 250 rupees (about $5.60) a month in rent for a room he shared with three other men.

When he finally found a mill owner who allowed him to go to weekly lectures on HIV prevention run by the local branch of the Family Planning Association of India, he threw himself into the safe-sex campaign with enthusiasm and vigor. "For six months, I waited eagerly for Fridays," he said. "I lost a cousin to AIDS in my village and I wanted to go back and talk to the villagers, who know nothing about AIDS." Because of the large number of men living without families in Bhiwandi, the sex industry flourishes.

Impressed with his dedication and communications skills, the local Family Planning Association, FPA, made him a peer group educator, and then a staff member, but Tiwari says he still lives with the loom workers, most from his home state. About 20,000 single migrant workers (out of 400,000 in the area) are covered by the local Family Planning Association's HIV/AIDS awareness, prevention and testing projects, which also teach workers about other sexually transmitted diseases and general reproductive health issues. Workers say they have benefited from this urban experience, and they carry the information back home on annual breaks to educate others.

Despite the hardships and dangers of their daily work, the men insist there is no alternative, no future, in their home villages and towns. Only one, Shyam Narayan Prajapati, a 45-year-old university graduate who is now also on the local FPA staff, said that though he worked in the textile mills for more than 20 years, he still has hopes of going home to Uttar Pradesh. He wants to enter politics to help fight corruption and poor economic performance in his state.

The workers know that the city and the industry need them, and that is their insurance policy. Santlal Bind, who returns north to see his family as often as he can and admits to being too exhausted to do much more in Bhiwandi than work, eat and sleep, nevertheless does not worry about his future or about losing a mill job because of the skills he has learned as a weaver. "If I go home," he said, "I can always find a job in any loom when I return."