Although many of us may feel air-conditioners bring relief from hot, humid or polluted outside air, they pose manypotential health hazards. Much research has looked at how the movement of air inside a closed environment---such as anoffice building---can spread disease or expose people in the building to harmful chemicals.One of the more widely publicized dangers is that of Legionnaire’s disease, which was first recognizedinthe 1970s. This was found to have affected people in buildings with air-conditioning systems in which warmair pumped out of the system’ cooling towers was somehow sucked back into the air intake (通风口),in mostcases due to poor design. The warm air, filled with bacteria,was combined with cooled, conditioned air andwas 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 pipes.Large air-conditioning systems add water to the air they circulate by means of humidifiers(湿度调节器).Inolder systems, the water used for this process is kept in special reservoirs, the bottoms of which providebreedinggrounds for bacteria which can find their way into the ventilation (通风)system. The risk to human health from this situation has been highlighted by the fact that the immunesystems (免疫系统)of approximately half of workers in air-conditioned office buildings have developed the ability to fight off the organisms found at the bottom of system reservoirs. But chemicals called “biocides”are added to reservoirs to make them germ-free, and they are dangerous in their own right in sufficient quantities, as they often contain compounds strongly linked tocancers.Finally, it should be pointed out that the artificial climatic environment created by air-conditioners canalso 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 workingenvironment, however, body temperatures remain well under37℃, our normal temperature. This leads to a weakened immune system and thus greater exposure to diseases such as colds and flu.3.The fact that about half of workers developed the ability to fight off the bacteria may__.