Many of us may feel air-conditioners bring relief from hot, humid or polluted outside air, they gave rise to many 【B1】______ health hazards. Much research has looked at 【B2】______ the circulation of air inside a closed environment such as an office building can spread disease or 【B3】______ occupants to harmful chemicals. One of the more widely publicized dangers is 【B4】______ of Legionnaire’’s disease (军团病) , 【B5】______ was first recognized in the 1970s. This was found to 【B6】______ people in buildings with air-conditioning systems in which warm air pumped out of the system’’s cooling towers was 【B7】______ sucked back into the air intake(人口，进口) ,in most cases 【B8】______ poor design. This warm air was, needless to say, the perfect environment for the rapid growth of disease-carrying bacteria 【B9】______ from outside the building, where it existed in harmless quantities. The warm, bacteria-laden air 【B10】______ 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 【B11】______ air exhaust ducts (废气管). 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, 【B12】______ constant monitoring. The ways 【B13】______ air-conditioners work to "clean" the air can inadvertently(无心地) cause health problems, too. One such way is with the use of an electrostatic precipitator(静电滤尘器), which 【B14】______ dust and smoke particles from the air. What precipitators also do, 【B15】______ , is emit large quantities of positive air ions(正离子) into the ventilation system. A growing number of studies show that overexposure to positive air ions can 【B16】______ headaches, fatigue and feelings of irritation. Finally, it should be pointed out that the artificial climatic environment created by air-conditioners can also adversely 【B17】______ us. In a 【B18】______ environment, whether indoor or outdoor, there are small variations in temperature and humidity. Indeed, the human body has long 【B19】______ these normal changes. In an air-conditioned living or work environment, however, body temperature remain well under 37°C ,our normal temperatures. This 【B20】______ a weakened immune system and thus greater Susceptibility(易感性) to diseases such as colds and flu.