1. The amount by which the water vapor in the air must be increased to achieve saturation without changing the environmental temperature and pressure. The saturation deficit may be expressed in terms of a vapor pressure deficit, an absolute humidity deficit, or a relative humidity deficit. 2. The physiological saturation deficit: “The difference between the amount of vapor actually present in the air (i.e., the absolute humidity) and amount that saturated air at body temperature contains (viz. , about 45 gm per cubic m). ”