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On a dusty construction site in western China, Mr Tan is just another anonymous migrant labourer. But, the unassuming former farmer is also the face of a complex web of crises threatening global health.

On one level, 24-year-old Mr Tan is a symbol of China's economic success, and similar growth patterns over recent decades in other developing economies. Half a million rural Chinese migrant workers pour into Chongqing alone every year, making it the world's biggest city -- and building site -- fuelling China's growth engine, even if somewhat less vigorously since the global financial downturn. Long term forecasts suggest another 350 million rural villagers will converge on industrializing cities in China alone by 2025; some pulled by new opportunities, others pushed by poverty and food shortages caused, in part, by the climatic disruptions of global warming.

For Mr Tan, the lure of Chongqing is, in his words, "the big money". Living in a shed, amid the fine, choking concrete dust and surrounded by hectares of concrete, high rise skeletons, Mr Tan earns 1,000 yuan a month, more than ten times what he eked out of the land and enough to allow him to survive in the city and send money home to the village. His is a familiar tale of aspiration.

But, on Mr Tan's building site, a number of the world's most intractable and emerging health threats are converging. Just as the advances of the Industrial Revolution came at a cost to the millions who left the land for Europe's factories and slums, the very jobs which have pulled hundreds of millions out of abject poverty in the late twentieth and early twenty-first centuries also come with new risks.

For the world's marginalized communities, economic globalization often means less job security in itinerant industrial and labouring jobs and new toxic threats and accident risks at work. In urban shanty towns, inadequate sanitation and services combined with crowded living conditions greatly increases health risks and vulnerability to communicable diseases.

Then, there are new emerging threats, unique to our times. In Chongqing, and many cities like it, armies of male migrant labourers are cut adrift from village social structures for months and years at a time. With the erosion of social cohesion, high risk lifestyles of cheap sex and drugs can follow; undermining economic gains and threatening the transmission of HIV/AIDS and serious related infections such as drug-resistant tuberculosis.

At the same time rising population mobility resulting from mass travel facilitated by modern transport networks, migration and forced population displacement are providing unprecedented opportunities for the transmission of many communicable diseases -- nationally and globally -- greatly jeopardizing the health of people on the move and elevating the risk of pandemics.

And, that's before we consider the health impacts of environmental degradation caused by rapid development. In the world's industrializing cities, air and water pollution routinely exceed safe levels, exposing tens of millions of people to respiratory risks and diverting meagre family incomes to purchasing safe drinking water. In many poor rural areas, large scale deforestation and resource stripping has led to devastating soil erosion, water shortages and contamination.

Single Biggest Threat

Less immediately apparent, but potentially the single biggest health threat this century, is climate change. This is no longer an abstract notion. We know climatic variations -- particularly the droughts and floods which are disrupting agriculture and extreme weather events which in turn damage infrastructure -- are hitting vulnerable developing nations the hardest, and will wreak havoc on the world's poorest people in the coming decades.

China's official news agency, Xinhau, reported sustained extreme heat in Chongqing in September 2009, as well as water shortages affecting hundreds of thousands of people in the city. Without the resources to "adapt", such as air-conditioning, low-paid workers toiling outdoors and living under baking, tin roofs are exposed to health threats posed by intense heat and pollution. In surrounding rural areas, livestock and crops are suffering, pushing more farmers off the land.

As early as 2000, the World Health Organization attributed 2.4 per cent of worldwide diarrhoea and 6 per cent of malaria cases to climate change. The first large scale, quantifiable impacts on human health are likely to be changes to the geographic range and seasonality of some infectious diseases, including vector-borne infections such as malaria and dengue fever and food-borne infections such as salmonellosis, which peak in warmer months. We have also begun to identify as "climate change casualties" the victims of extreme weather events, such as the 27,000 deaths associated with abnormally high temperatures in the European summer of 2003.

However, the future public health consequences loom even larger. Much attention is focused on rising sea levels and areas which face inundation -- a situation which could drive mass people movements. But, what of the wider scale disruption to food production with changing rainfall patterns, including longer droughts and more intense floods, and the consequent economic losses and food shortages forcing mass migration and exacerbating civil strife?

The "World Development Report 2010: Development and Climate Change", released in advance of the UN Climate Change Conference in Copenhagen in December 2009, cites evidence that global warming of 2?C above pre-industrial temperatures could, for example, result in permanent reductions in annual per capita consumption of 4 to 5 per cent in Africa, already the world's most vulnerable continent.

Climate variability is a feature of Africa's history, but the frequency and severity of both floods and droughts have increased sharply in recent years, and climate projections indicate that this trend will intensify. This is likely to have devastating consequences for rain-fed agriculture, which employs about 70 per cent of Africa's population, the report says.

An earlier World Bank report notes; "[the world's] poor communities will be especially vulnerable. They tend to have limited adaptive capacities and are much more dependent on climate-sensitive resources such as local water and food supplies." The Intergovernmental Panel on Climate Change has estimated that between 75 and 250 million people will be exposed to increased water stress due to climate change by 2020, most of them in poor countries already facing multiple health challenges.

At the same time, the world is facing a food crisis driven by multiple causes, including climate variability and market distortions. During early 2008, international prices of all major food commodities reached their highest levels in almost 50 years; pushing world hunger through the one billion mark. The global financial downturn since threatens to plunge 55 to 90 million more people into poverty this year alone, according to the United Nations.

From a global health perspective, the growing diversity and magnitude of health-related crises around the world is converging into a potentially disastrous perfect storm.
Doing better than this

Clearly, we should be doing better than this. Never before in history has the world had such a wealth of knowledge, skills and resources invested in keeping its communities healthy. Development assistance for health grew substantially from $5.6 billion in 1990 to $21.8 billion in 2007,[1] accompanied by new global initiatives for mobilizing and channelling funds.

Yet, the gap between the extraordinary promise of modern medicine and the reality of the world's disease burden is arguably growing. One third of the world's population is infected with tuberculosis and 350-500 million people suffer from malaria; and in a world with 33 million people living with HIV, for every one person who gains access to life-saving drugs, another two new infections occur. These "big three" diseases alone kill six million people a year, before we even consider compounding factors such as children left unvaccinated, mothers dying in childbirth or workers exposed to industrial pollutants.

Part of the problem is simply that the crises are outrunning the solutions. With global health fires burning on so many fronts, many good programmes end up merely putting out spot-fires.

Just as challenging, though, is the increasingly complex, intersecting nature of heath threats. If climate change reduces agricultural output, for example, economies of survival follow; more men are likely migrate to cities than women, breaking down social structures and leaving women and children in dire poverty in villages. HIV risks rise because of changing living conditions and poverty-fed commercial sex industries. If entire communities are forced to migrate away from inundated, drought-affected or flooded regions, a host of health risks follow associated with overcrowding, and inadequate shelter, services and nutrition.

International agencies do recognize these intersecting challenges, yet much of the global health effort is currently focused on "care and cure" for specific diseases, not the overwhelming global web of risk-factors which underpin health outcomes. The result is competition for resources among advocates of particular health crises, when progress really depends on understanding how these crises intersect. An essential first step is better "vulnerability-mapping", to determine, at a local level, which of the long list of globally-significant health threats are impacting local populations. And, as no health threat exists in isolation, bridges must be built across institutions and between key players to maximize resources. The answer is not to haggle over resource allocation or to seek to move funds from one problem to another, but to look for synergies which allow funds to combat multiple crises at the same time.

A human rights-based approach -- framed in terms of the right to the highest attainable standard of health, and the right to the basic determinants of health such as clean water and food, shelter, education, health services and equality -- is the most promising. Through this lens, the circumstances of those most at risk can be holistically addressed.

But, instead of "thinking globally and acting locally", there is a strong case for turning the adage around. To understand the complexity of the global health challenge, and to formulate effective responses, we need to understand what's happening on the ground and to tap into the creativity and energy of civil society to help formulate responses. Consider the global micro credit movement, now with over 600 million clients. When Professor Muhammad Yunus used money from his own pocket to finance the first group of destitute women basket weavers in Bangladesh in the 1980s, he was addressing gender inequity, poverty alleviation, and in turn, health.

By "thinking locally and acting globally", the lives of the hundreds of millions like Mr Tan can inform global policy responses.