In the era of mechanical ventilation, requirements for outdoor air derive primarily from the need for odor control. The odors arise from human beings and their activities (e.g., smoking). The experimental foundation for ventilation requirements arose from psychophysical experiments on body odor and were performed more than 40 years ago. Those data, incomplete in their time, must now be supplemented with results obtained by modern methods of psychophysics and analytical chemistry. These techniques, properly applied and blended with engineering creativity, could yield means of ventilation that would save enormous amounts of energy now used to heat, cool, humidify, or dehumidify fresh (outdoor) air.