For management decisions related to effects of releases of toxic chemicals, it is useful to consider not only the expected or most likely impact of the release, but also the envelope of possible outcomes, the latter of which involves an assessment of the uncertainty in the modelling results. In this paper, we provide quantitative estimates of major sources of uncertainty in plume dispersion modelling and then provide a preliminary assessment of their effects. The sources of uncertainty investigated are wind speed and direction, plume rise, dispersion parameters, and stochastic effects not simulated by dispersion models.