 mechanical systems are used in buildings to provide conditioned air , dissipate thermal loads , dilute contaminants , and maintain pressure differences . 
 the characteristics of these systems and their operations h implications for the exposures of workers to environmental tobacco smoke ( ets ) and for the control of these exposures . 
 this review describes the general features of building ventilation systems and the efficacy of ventilation for controlling contaminant concentrations . 
 ventilation can reduce the concentration of ets through dilution , but central heating , ventilating , and air conditioning ( hvac ) can also move air throughout a building that has been contaminated by ets . 
 an understanding of hvac systems is needed to develop models for exposures of workers to ets.imagesfigure 1figure 2figure 3 
Images