Human activity pattern-exposure models require accurate submodels for the exposures in microenvironments that people occupy, including those containing environmental tobacco smoke (ETS). The paper describes the Sequential Cigarette Exposure Model (SCEM), a general-purpose mathematical model developed for calculating the pollutant concentration time series in a well-mixed microenvironment of known volume when any cigarette smoking activity pattern occurs. The SCEM is based on solutions to the mass balance equation that represent smoking emissions for each cigarette as a rectangular input function over time, and the paper evaluates the performance of the SCEM in several experiments. The paper presents theoretical equations for the minimum, maximum, and mean of the pollutant concentration time series for any sequential smoking activity pattern, including a uniform smoking activity pattern input time series (that is, the 'habitual smoker'), and these equations are more general than those reported elsewhere in the literature. The model is validated for carbon monoxide (CO) and respirable particles in a controlled experiment in a well-mixed chamber with a cigarette smoking machine. (Copyright (c) 1992 Princeton Scientific Publishing Co., Inc.)