Background, aim, and scope Propagation of parametric uncertainty in life cycle inventory (LCI) models is usually performed based on probabilistic Monte Carlo techniques. However, alternative approaches using interval or fuzzy numbers have been proposed based on the argument that these provide a better reflection of epistemological uncertainties inherent in some process data. Recent progress has been made to integrate fuzzy arithmetic into matrix-based LCI using decomposition into α-cut intervals. However, the proposed technique implicitly assumes that the lower bounds of the technology matrix elements give the highest inventory results, and vice versa, without providing rigorous proof. Materials and methods This paper provides formal proof of the validity of the assumptions made in that paper using a formula derived in 1950. It is shown that an increase in the numerical value of an element of the technology matrix A results in a decrease of the corresponding element of the inverted matrix A -1 , provided that the latter is non-negative. Results It thus follows that the assumption used in fuzzy uncertainty propagation using matrix-based LCI is valid when A -1 does not contain negative elements.Discussion In practice, this condition is satisfied by feasible life cycle systems whose component processes have positive scaling factors. However, when avoided processes are used in order to account for the presence of multifunctional processes, this condition will be violated. We then provide some guidelines to ensure that the necessary conditions for fuzzy propagation are met by an LCI model. Conclusions The arguments presented here thus provide rigorous proof that the algorithm developed for fuzzy matrix-based LCI is valid under specified conditions, namely when the inverse of the technology matrix is non-negative. Recommendations and perspectives This paper thus gives the conditions for which computationally efficient propagation of uncertainties in fuzzy LCI models is strictly valid.Keywords Fuzzy uncertainty propagation . Matrix-based LCA . Uncertainty 1 Background, aim, and scopeIn a seminal paper on uncertainty analysis in life cycle assessment (LCA), Heijungs (1996) discussed the issue of combining lower and upper values of LCA parameters to obtain an idea of the uncertainty range of LCA results. In fact, it was concluded that this was not a feasible approach for LCA systems of a sufficiently large size. The argument was that it cannot be predicted a priori if for a certain LCA parameter the lower or the upper value will produce the lowest LCA result. "This would imply that all combinations of upper and lower values must be tried in order to find the upper and lower values of the result. If there are 10,000 figures used as input data-a typical number for a mediocre LCI-the number of combinations is 2 10000 , a number which