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  3. (1)About two-thirds of the world’s population is expected to live in cities by the year 2020 and, according to the United Nation

(1)About two-thirds of the world’s population is expected to live in cities by the year 2020 and, according to the United Nation

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问题     (1)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 characterize them: office towers, factories, shopping malls and high-rise apartment buildings. These structures depend on artificial ventilation systems to keep clean and cool air flowing to the people inside. We know these systems by the term"air-conditioning".
    (2)Although many of us may feel air-conditioners bring relief from hot, humid or polluted outside air, they pose many potential health hazards. 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.
    (3)One of the more widely publicised dangers is that of Legionnaire’s disease, 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, 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, where it 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. Studies showed that even people outside such buildings were at risk if they walked past air exhaust ducts. 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.
    (4)The ways in which air-conditioners work to "clean "the air can inadvertently cause health problems, too. One such way is with the use of an electrostatic precipitator, which removes dust and smoke particles from the air. What precipitators also do, however, is to emit large quantities of positive air ions into the ventilation system. A growing number of studies show that overexposure to positive air ions can result in headaches, fatigue and feelings of irritation.
    (5)Large air-conditioning systems add water to the air they circulate by means of humidifiers. In older systems, the water used for this process is kept in special reservoirs, the bottoms of which provide breeding grounds 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. 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.
    (6)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.
The main purpose of the passage is to______.
选项 A、introduce the dangers of air-conditioning
B、explain the defects of old air-conditioning
C、illustrate how air-conditioning affects people’s health
D、advocate abandoning old air-conditioning
答案A
解析 主旨题。作者从第二段开始就指出空调在给我们带来凉爽的同时也会给我们的健康带来危害。第三段介绍了一种空调病——军团病的致病原理。接下来介绍了空调系统中存在的一些致病物质，给使用空调的人的健康带来了重大威胁。全篇反复出现hazards、dangers、problems等单词，可见，本文主旨是介绍空调的危害，故[A]为答案。[B]和[C]都是文章的部分内容，不全面，故排除；作者并未倡导淘汰旧的空调系统，故排除[D]。
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