Even when providing the same nominal rate of outdoor air (OA), different ventilation systems distribute air in different ways, affecting the occupants differently depending on the dwelling, on source disposition and strength, on occupants behavior, and on the cooling or heating system. This paper will report on simulations that compare ventilation systems by assessing their impact on exposure, for each system, It will examine the potential exposure variance for ordinary occupancy patterns (as opposed to extreme or ideal cases) and the influence of source emission patterns and house geometry. These simulations take into account the unsteady, occupancy-tied aspect of ventilation such as bathroom and kitchen exhaust fans. As most US homes have central HVAC systems, the results can be used to make recommendations and adjustments for distribution and mixing to residential ventilation standards such as ASHRAE Standard 62.2. This paper will then analyze the results heeded by increasing mixing of indoor air on the occupants exposure to contaminants, showing that increasing mixing reduces the effect of the other variables that affect exposure, and vice versa.