Mixed cellulose ester (MCE) filters, used routinely to collect dust samples from air for fiber analysis, are the only filter type that can be prepared for both phased contrast microscopy and transmission electron microscopy analyses. However, whenever fiber counts require collecting dust masses <100 μg on a single filter under variable relative humidity (RH) conditions, historically noted effects of humidity on MCE filter mass can hinder accurate estimates of dust mass, measured as loaded minus unloaded filter mass (M). In this study, a baseline set of hundreds of paired measures of change in RH versus M over different time intervals were obtained over a 5-day period for replicate series of 40 unloaded 37-mm MCE filters under varying RH conditions at a nearly constant temperature. Similar baseline data were obtained for 25-mm MCE filters. Linear regressions fit to these data allow improved estimates of dust mass loaded onto MCE filters from measures of M and RH made before and after loading occurs. Using established theory, these relationships were generalized to address temperature variation as well, and examples of numerical applications are provided.