Abstract: Disclosed is a method for monitoring cell density during cell expansion resulting from a cell culture process in a bioreactor comprising the steps of: a) cultivating cells in a bioreactor culture chamber according to a cell culture process having cell culture parameters; b) during said process, introducing cell culture fluid inputs and generating waste materials; c) determining the amount of volatile organic compounds (VOCs) and their chemical species in the waste materials; and d) estimating the density or population of cells in the bioreactor based on said determination.