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About two-thirds of the world's population is expected to live in cities by the year 2020 and, according to the United Nations, approximately 3.7 billion people will inhabit urban areas some ten years later. As cities grow, so do the number of buildings that characterise them:<注1> office towers, factories, shopping malls and high-rise apartment buildings.<注2> These structures depend on artificial ventilation systems<注3> to keep clean and cool air flowing to the people inside. We know these systems by the term "air-conditioning".
Although many of us may feel air-conditioners bring relief from hot, humid or polluted outside air, they pose many potential health hazards.<注4> Much research has looked at how the circulation of air inside a closed environment—such as an office building—can spread disease or expose occupants to harmful chemicals.
One of the more widely publicised dangers is that of Legionnaire's disease,<注5> which was first recognised in the 1970s. This was found to have affected people in buildings with air-conditioning systems in which warm air pumped out of the system's cooling towers was somehow sucked back into the air intake,<注6> in most cases due to poor design. This warm air was, needless to say, the perfect environment for the rapid growth of disease-carrying bacteria originating from outside the building,swheresit existed in harmless quantities. The warm, bacteria-laden air was combined with cooled, conditioned air and was then circulated around various parts of the building.<注7> Studies showed that even people outside such buildings were at risk if they walked past air exhaust ducts.<注8> Cases of Legionnaire's disease are becoming fewer with newer system designs and modifications to older systems, but many older buildings, particularly in developing countries, require constant monitoring. (来源：英语杂志 http://www.EnglishCN.com)
The ways in which air-conditioners work to "clean" the air can inadvertently<注9> cause health problems, too. One such way is with the use of an electrostatic precipitator, <注10> which removes dust and smoke particles from the air. What precipitators also do, however, is emit large quantities of positive air ions<注11> into the ventilation system. A growing number of studies show that overexposure to positive air ions can result in headaches, fatigue<注12> and feelings of irritation.
Large air-conditioning systems add water to the air they circulate by means of humidifiers.<注13> In older systems, the water used for this process is kept in special reservoirs,<注14> the bottoms of which provide breeding grounds<注15> for bacteria and fungi which can find their way into the ventilation system. The risk to human health from this situation has been highlighted by the fact that the immune systems of approximately half of workers in air-conditioned office buildings have developed antibodies to fight off the organisms found at the bottom of system reservoirs.<注16> Chemical disinfectants, called "biocides", that are added to reservoirs to make them germ-free, are dangerous in their own right in sufficient quantities, as they often contain compounds such as pentachlorophenol, which is strongly linked to abdominal cancers.<注17>
Finally, it should be pointed out that the artificial climatic environment created by air-conditioners can also adversely affect us. In a natural environment, whether indoor or outdoor, there are small variations in temperature and humidity. Indeed, the human body has long been accustomed to these normal changes. In an air-conditioned living or work environment, however, body temperatures remain well under 37℃, our normal temperature. This leads to a weakened immune system and thus greater susceptibility to diseases such as colds and flu.<注18>
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