Applications and Case Studies320 Fig. 3.8-1 Schematic diagram of the main factors affecting indoor particle concentration · ·Macromodels are based on the material (mass, number) balance equation and the assumption of perfect air mixing. The indoor space is divided into one or several compartments, in which uniformity of a considered parameter (e.g. concentration or particle emission rate) is assumed over the time of interest. The macromodels are characterized in the form of ordinary differential equations, and provide information about the temporal variation of particle concentration levels indoors. No spatial variation data are available, as only the average values are provided for each compartment or zone. Depending on the type, range, and the number of parameters involved (e.g. sinks, sources), the macromodels can differ significantly in their complexity and capabilities.Micromodels are based on the full form of Navier-Stokes equations containing the spatial and time dependency of all indoor environment variables, such as air velocity vectors, temperature, and species concentration. Micromodels may have one, two, or three dimensions depending on the flow problem and application. Advanced numerical methods, such as finite volume and finite element methods, are needed to solve the equations. The micromodels are more complex and require more computational power compared to macromodeling; however, the predictive capability of the model is enhanced.Both modeling approaches are important, and the choice of approach to be followed depends on the application, the information required, issues to be addressed, understanding of the problem, input data, and the computational power availability. A key to choosing the best model for an application is understanding how it addresses various parameters relevant to that application.