When a puff of contaminant is released in the atmosphere, concentration statistics at fixed points in space do not include the necessary information for a complete risk or consequence assessment. Also, if the released material is chemically reactive, estimating the concentration fluctuations at fixed points may lead to large errors and to an incorrect representation of the kinetic of the reaction. For puffs of contaminants, more complete information is given by the concentration statistics in a frame of reference moving with the center of mass. We present an interaction by exchange with the conditional mean (IECM) micromixing model to calculate the internal statistics of plumes and puffs. The model consists of a system of four Lagrangian stochastic equations for position, velocity, kinetic energy and spread in the relative frame of reference, and one micromixing equation for concentration. Results are compared to field and laboratory observations.