NEW YORK — When observing the chaotic growth of the modern city, the more erudite of urban planners will reminisce wistfully on how different it is from its ancient Greek counterpart, the polis, which Italian architectural historian Leonardo Benevolo once described as “dynamic but stable, in balance with nature and growing manageably even after reaching large dimensions.”

The rapid and uncontrolled sprawl of today’s cities breeds alarm not only among urban planners and architects but also among public health experts, for the apparent randomness of the urban dynamic is robbing the population of its basic health and well-being through environmental pollution, shrinking green areas, inadequate housing, overburdened public services, mushrooming makeshift settlements on the outskirts, mounting anomie and the sheer numbers of neighbors who do not know each other.

Beijing, a city of over 17 million inhabitants, exemplifies this social alienation. Until the early 1980s, the Chinese capital was constructed as a multitude of siheyuans, or one-story complexes built around a common courtyard inhabited by three or four families who shared a single kitchen and water spigot. These courtyards were connected by narrow hutongs that formed a north-south, east-west grid.

This open structure greatly facilitated contact between neighbors, encouraged the sharing of resources, fostered relations between contiguous families, and enabled the elderly to care for children and share with them their passion for songbirds. These structures were described as “collections of small rural villages.” Until the mid-1980s, only a few skyscrapers disrupted this landscape. Today, with few exceptions, sterile towers have supplanted the villages. This change has dramatically altered the fabric of human relations.

Physical isolation has led to more crime, destroyed local solidarity and contributed to the fragmentation of family groups. As the distance between home and work soared, workers had to devote what was once valuable family time to exhausting commutes in overcrowded buses or subways.

According to Chen Xitong, a former mayor of Beijing, “the capital is growing increasingly ugly and it is steadily losing its Chinese character. Most of the modern high-rises, with their boring concrete facades, look like dominoes set down in the landscape without a plan.”

Rapid urbanization is related in part to population growth and rural migration. In 2008 the world reached an important milestone: For the first time in history more than half of its human population — 3.3 billion people — were living in urban areas. By 2030 their number is expected to swell to almost 5 billion. The economic future of many new urbanites will depend largely on decisions made now.

The unchecked growth of the cities is also due to migration from other countries. Common denominators are rural poverty, the search for better social and employment opportunities, or flight from political persecution and violence.

Take Colombia, where the urbanization process — unlike in most other Latin American countries — was stimulated, and to some extent defined, by episodes of violence occurring principally in rural areas. Since the 1930s, violence has been an inescapable fact of Colombian civilian life.

As families were uprooted and displaced by successive waves of violence, they fled en masse to the country’s main cities, where the majority now reside in poverty-stricken marginal areas. As a result of the violence either witnessed or experienced firsthand, many of Colombia’s young generation have internalized the culture of aggression into which they were born.

Large migrations will intensify as changing climatic conditions lead to abandoned, flooded or arid environments. Serious health problems will follow as a result of the stresses linked to migration itself and from the civil strife it could cause. Every year, climate change is a contributing factor in the deaths of approximately 300,000 people, according to the Global Humanitarian Forum.

A climate refugee is one who is forced to relocate, either to a new country or to a new location within their country, due to the consequences of global warming. The number of environmental refugees will reach 150 million over the next 50 years, according to professor Norman Myers of Oxford University.

In Africa, desertification and its agricultural consequences are displacing increasingly more people. Approximately 10 million people in Africa have been forced to migrate over the past two decades as a result of environmental degradation.

In addition, most people in Africa move into mostly marginal urban areas because of poverty, environmental degradation, political persecution and religious strife. Food insecurity and lack of basic services in rural areas encourage migration to cities where too often people become marginalized.

The bidonvilles of French-speaking West Africa, the ishish of Arab countries, kampungs of Indonesia and villas miseria of Argentina may contain 30 to 60 percent of a Third World city’s population, says Worldwatch Institute.

Many governments try to discourage migration from rural areas to the cities, but they are generally unsuccessful. Since large cities enjoy preferential treatment because of infrastructure and industrial development, they serve as magnets for the “have-nots.”

Regardless of the big city’s allure, many observers now feel that conditions for the ever-growing numbers of urban poor are most likely worse than for their rural counterparts. The true dimensions of this phenomenon remain elusive, according to World Health Organization expert Dr. I. Tabibzadeh, because the poor are either omitted from official statistics or are not considered separately.

Migrations between countries is usually stimulated by factors similar to those responsible for internal migration. The Latin American country that has produced the greatest number of migrants is Mexico. Among Mexicans living abroad, 99 percent can be found in the United States, where income opportunities are greater.

In the Southern Cone, Argentina is the main destination of migrants from Paraguay, Uruguay and Bolivia. In Central America and the Caribbean, the U.S. is the most frequent destination. There are also significant migratory flows from the Dominican Republic to Venezuela and Puerto Rico, and from Haiti to the Dominican Republic.

Several European countries have attracted Africans, and many sub-Saharan people have migrated to north Africa. In addition, the traditional pattern of migration within and from Africa is changing as a male-dominated process becomes feminized.

The characteristics of migration in Asia have been shaped by political and economic changes in recent decades. It is estimated that more than 6 million migrants are working in East and Southeast Asia. Until the recent economic crisis, oil-rich Arab countries attracted large numbers of Asian workers.

The chaotic growth of today’s cities can no longer be ignored. The great challenge is how to improve the quality of urban life by ensuring harmonious growth. Cities can — and should — learn from the experiences of other cities with similar characteristics. Urban planners, public health and environmental experts, politicians and the communities themselves must work together. Perhaps it is still possible to reach that urban ideal mentioned by Hippocrates some 2,600 years ago: a balance between the human organism and its environment.

Cesar Chelala, M.D., is a public health consultant for several international organizations.

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