The dose reduction factor is used in the evaluation of containment vessel spray systems that are designed for the removal of atmospheric contaminants in pressurized water reactors. This note examines the variability of this factor for the specific removal of elemental iodine. Two aspects of variability are considered. First, a method is given for estimating the variance of the dose reduction factor as a function of two random variables: (a) spray drop diameter, and (b) spray drop velocity. Second, an estimate of the variance is based on observations from three repeated Nuclear Safety Pilot Plant runs.