About two-thirds of the world’spopulation is expected to live incities by the year 2020 and,according to the United Nations,approximately 3.7 billion people will inhabiturban areas some ten years later.As citiesgrow,so do the number of buildings thatcharacterise them:~1office towers,factories,shopping malls and high-rise apartmentbuildings.~2These structures depend onartificial ventilation systems~3 to keep cleanand cool air flowing to the people inside.Weknow these systems by the term“air-conditioning”.Although many of us may feel air-conditioners bring relief from hot,humid orpolluted outside air,they pose many potentialhealth hazards.~4Much research has looked athow the circulation of air inside a closedenvironment—such as an office building—can spread disease or expose occupants toharmful chemicals.One of the more widely publicised About two-thirds of the world’spopulation is expected to live incities by the year 2020 and, according to the United Nations, approximately 3.7 billion people will inhabiturban areas some ten years later.As citiesgrow, so do the number of buildings that characterize them: ~ 1office towers, factories, shopping malls and high-rise apartmentbuildings. ~ 2These structures depend onartificial ventilation systems ~ 3 to keep cleanand cool air flowing to the people inside .Weknow these systems by the term “air-conditioning ”. of us may feel air-conditioners bring relief from hot, humid orpolluted outside air, they pose many potentialhealth hazards. ~ 4Much research has looked athow the circulation of air inside a closedenvironment-such as an office office-can spread disease or expose occupants toharmful Of the more widely publicized