Cross contamination and entrainment in office environments are discussed. Airborne contaminants may be generated either within or outside office buildings. If generated outside, they may be brought inside as a result of cross contamination or entrainment. Cross contamination is discussed. Cross contamination occurs when a contaminant is present in an area of a building other than where it is generated and finds its way there without leaving the building. Conditions conducive to cross contamination include: dual or multi/use buildings; separate heating, ventilation, and air conditioning (HVAC) systems serving adjacent areas; change in space use relative to original design; or using a heat energy recovery system. It is noted that cross contamination is most often associated with dual or multipurpose buildings where production or laboratory activities are under the same roof with offices or classrooms. A common HVAC system can recirculate contaminants to the office areas causing complaints even though the resulting concentrations are far below the applicable occupational exposure criteria. Conditions conducive to entrainment include fresh air intakes that are located close to or down wind of exhaust stacks, exhaust stacks too low to discharge outside the turbulent wake caused by airflow over and around the building, and buildings under negative pressure. Corrective actions for air contamination resulting from cross contamination or entrainment include rebalancing HVAC systems, isolating contaminant generating areas, increasing the height of exhaust stacks, insulating and sealing cracks, providing adequate makeup air, and retaining a well trained maintenance staff.