Acute toxicity results from exposure to a toxin either froma single event or from multiple events within a short time span,usually less than 24 hours. On the other hand, chronic toxicitystems from repeated exposures at lower levels over a longerperiod. Chronic exposure to carbon monoxide (CO) reportedlymanifests in a confusing array of unrelated symptoms and isdifficult to diagnose. The source of CO in a chronic exposureincident is typically a fuel-burning appliance in a residence ora workplace setting. An engineering analysis of the potentialsource and the surroundings can provide valuable insight onthe likelihood of the appliance as a viable source. Such analysescan yield -- within a reasonable degree of engineeringcertainty -- conservative estimates of the level the individualwas exposed to, and the duration and frequency of each exposure.In this paper, we present a transient material balance todetermine volume-averaged, time-dependent concentrationswithin a residence. We highlight the techniques used to estimateeach term of the material balance equation as they applyto a real-world scenario. For example, we employed computationalfluid dynamics as well as visual examination of a gasfiredappliance to evaluate the magnitude of a source term inthe material balance.