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Overwhelming evidence shows that human activities are affecting the global climate, with serious implications for public health. Catastrophic weather events, variable climates that affect food and water supplies, new patterns of infectious disease outbreaks, and emerging diseases linked to ecosystem changes, are all associated with global warming and pose health risks.

Climate and weather already exert strong influences on health: through deaths in heat waves, and in natural disasters such as floods, as well as influencing patterns of life-threatening vector-borne diseases such as malaria.

Continuing climate change will affect, in profoundly adverse ways, some of the most fundamental determinants of health: food, air and water, according to WHO Director-General Dr Margaret Chan. Areas with weak health infrastructure - mostly in developing countries - will be the least able to cope without assistance to prepare and respond.

This fact file describes current and projected effects of climate change on health.

Over the last 50 years, human activities - particularly the burning of fossil fuels - have released sufficient quantities of carbon dioxide and other greenhouse gases to affect the global climate. The atmospheric concentration of carbon dioxide has increased by more than 30% since pre-industrial times, trapping more heat in the lower atmosphere. The resulting changes in the global climate bring a range of risks to health, from deaths in extreme high temperatures to changing patterns of infectious diseases.

From the tropics to the arctic, climate and weather have powerful direct and indirect impacts on human life. Weather extremes - such as heavy rains, floods, and disasters like Hurricane Katrina that devastated New Orleans, USA in August 2005 - endanger health as well as destroy property and livelihoods. Approximately 600 000 deaths occurred worldwide as a result of weather-related natural disasters in the 1990s, some 95% of which took place in developing countries.

Intense short-term fluctuations in temperature can also seriously affect health - causing heat stress (hyperthermia) or extreme cold (hypothermia) - and lead to increased death rates from heart and respiratory diseases. Recent studies suggest that the record high temperatures in western Europe in the summer of 2003 were associated with a spike of an estimated 70 000 more deaths than the equivalent periods in previous years.

Increasing global temperatures affect levels and seasonal patterns of both man-made and natural air-borne particles, such as plant pollen, which can trigger asthma. About 300 million people suffer from asthma, and 255 000 people died of the disease in 2005. Asthma deaths are expected to increase by almost 20% in the next 10 years if urgent actions to curb climate change and prepare for its consequences are not taken.

Rising sea levels - another outcome of global warming - increase the risk of coastal flooding, and could cause population displacement. More than half of the world's population now lives within 60 kilometres of shorelines. Some of the most vulnerable regions are the Nile delta in Egypt, the Ganges-Brahmaputra delta in Bangladesh, and small island nations such as the Maldives in the Indian Ocean, and the Marshall Islands and Tuvalu in the Pacific Ocean. Floods can directly cause injury and death, and increase risks of infection from water and vector-borne diseases. Population displacement could increase tensions and potentially the risks of conflict.

More variable rainfall patterns are likely to compromise the supply of fresh water. Globally, water scarcity already affects four out of every 10 people. A lack of water and poor water quality can compromise hygiene and health. This increases the risk of diarrhoea, which kills approximately 1.8 million people every year, as well as trachoma (an eye infection that can lead to blindness) and other illnesses.

Water scarcity encourages people to transport water long distances and store supplies in their homes. This can increase the risk of household water contamination, causing illnesses.

Climatic conditions affect diseases transmitted through water, and via vectors such as mosquitoes. Climate-sensitive diseases are among the largest global killers. Diarrhoea, malaria and protein-energy malnutrition alone caused more than 3 million deaths globally in 2002, with over one third of these deaths occurring in Africa.

Malnutrition causes millions of deaths each year, from both a lack of sufficient nutrients to sustain life and a resulting vulnerability to infectious diseases such as malaria, diarrhoea, and respiratory illnesses. Increasing temperatures on the planet and more variable rainfalls are expected to reduce crop yields in many tropical developing regions, where food security is already a problem. Mali is a good example. Unless adaptive measures are taken, climate change is projected to approximately double by the 2050s the percentage of its population at risk of hunger and associated health effects.

Steps to reduce greenhouse gas emissions or lessen the health impacts of climate change could have positive health effects. For example, promoting the safe use of public transportation and active movement - such as biking or walking as alternatives to using private vehicles - could reduce carbon dioxide emissions and improve public health. They can not only cut traffic injuries, but also air pollution and associated respiratory and cardiovascular diseases. Increased levels of physical activity can lower overall mortality rates.