Build-up refers to the accumulation of product residue on hair strands, which can occur when a product isn't easily washed away, resulting in multiple layers that weigh down the hair over time. While humectants may have comedogenic properties, they don't need to be removed with sulfate-containing shampoos. Therefore, despite the potential for clogging skin pores, humectants won't contribute to build-up on the hair strands. It is not an issue to use a comedogenic humectant on the strands but should be avoided on the scalp.