Environmental Protection Agency (EPA) ambient air quality guidelines are meant to limit long-term exposures of toxins to safe levels. Unfortunately, there is little guidance for what constitutes a safe level from a one-time (or very infrequent) short exposure(s). In the case of mercury, a review of the derivation of the EPA ambient air quality standard shows that it implicitly assumes a tissue burden model. The time dependence of the tissue burden is commonly described in terms of a half-life, a modeling assumption that presumes that the decline in the tissue burden after a single exposure can be approximately described as an exponential decay. In this article, we use a simple exponential tissue burden model to derive a time-dependent no observable adverse effect level (NOAEL) for mercury concentrations in air. The model predicts that tissue body burden will asymptotically approach the EPA air quality level for long exposure times, and reach workplace standard levels for exposures of a few hours. The model was used along with data on mercury levels from experimental work done by the Maine Department of Environmental Protection to evaluate the risks from a broken compact fluorescent lamp in a residential setting. Mercury levels approached the NOAEL only when the debris was left in an almost sealed room. Normal common-sense cleaning measures: removal of debris to an outside area, and ventilation of the room for several minutes, reduced exposures to less than 1% of the NOAEL.