Health issues facing so-called "megacities" like Tokyo, Mumbai or New York are poised to become a huge challenge for global policy as the cities grow, experts at the World Health Summit said.
Megacities, roughly defined as cities with a population of more than 10 million, are springing up around the world as people increasingly migrate from the fields to massive, sprawling urban centres.
There will be 27 of these megacities by 2020, up from 19 in 2007, said Victor Rodwin, director of the World Cities Project at New York University, with most located in Asia, South America or Africa.
Moreover, the cities themselves are growing at a ferocious rate. "For every minute that I speak, a new person is going to be moving into Lagos, Kinshasa or Dhaka," said Ricky Burdett from the London School of Economics.
One in every 25 people on the planet will be living in a megacity by 2025, predicted Francisco Armada Perez, an official from the World Health Organisation.
Health issues found elsewhere are exacerbated in megacities. Diseases such as AIDS, SARS or H5N1 bird flu can spread like wildfire, especially through slums, where one-third of urban dwellers live.
Overcrowding and poor sanitation foster tuberculosis, another major challenge facing health officials in megacities.
Burdett pointed out also that 300 people die every day in car accidents in India. "That's equivalent to a jumbo jet crashing every single day and no one ever talks about it," he said.
In addition, three-quarters of global CO2 emissions come from urban areas, meaning that small alterations in the way people in cities consume energy could have a massive effect on climate change.
"We now have a new issue, a new field of study, which is the field of urban health," said Rodwin, speaking on a high-level panel on megacities.
Nevertheless, the health problems faced in megacities are not confined to the developing world, the experts said.
"If you walk through the poor areas of Paris or London, you can find the same health problems as you might find in Mumbai or Lagos," according to Alfred Spira, from France's Institute of Public Health Research.
Life expectancy drops by around one year for every stop travelled eastwards on London's west-east Jubilee Line, Burdett pointed out.
In April, the World Health Organisation launched a major campaign to "open up public spaces to health", by urging some 1,000 cities to close off portions of streets to traffic and encourage exercise in public parks.
In 2009, the UN Under Secretary General for Humanitarian Affairs John Holmes, predicted that megacities were ripe for a "megadisaster" due to climate change and rising sea levels as several are located in coastal areas.
Timothy Evans, from the BRAC school of public health in Bangladesh, called for more involvement with people on the ground in slums but pointed to some examples that showed improvements could be made.
"Who is going to solve these problems? It might not be the class that is born with a golden spoon in their mouth," said Evans.
"If our politicians can't speak the language of the favela, then they may not be interested or able to solve their problems," he added, referring to the Brazilian word for slum.
However, he noted a scheme in Dhaka whereby gynaecology centres had been introduced within the slums that had slashed the mortality of women in half.
"Success is possible," he said.
The 2nd World Health Summit in Berlin brings together around 1,000 of the leading global experts of health issues and runs until October 13.
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