Future flows of emissions and waste from society to the environment can be estimated either as a percentage of the future stock or as a delayed input. The first approach is based on a static model where concentration is the driving force and is generally preferable for ease of calculation. The second approach is based on a dynamic model, where ageing is the driving force and knowledge of the life span is needed. We present the conditions under which the calculations based on a static model will produce acceptable approximations for a dynamic system.