Understanding how nanoparticles interact with pulmonary surfactant monolayer (PSM) is quite important for safe biomedical applications of nanoparticles and evaluating both health and environment impacts.In this work,we propose a possible origin of the pulmonary nanotoxicity of graphene-based nanoparticles that comes from a rigidifying effect of graphene nanosheets on PSM,which is observed from our molecular dynamics simulations.This,in reality,indicates that once adsorbed on the PSM surface,the inhaled graphene is hard to be removed from the PSM partially because the expiration or PSM compression is locally restrained,possibly leading to graphene accumulation onthe PSM.