The ability of fibrous nanomaterials used in thermal insulation (such as asbestos or fiberglass) to cause disease is not related to their chemical composition, but instead to their geometry and size. The macrophages that normally defend lung tissue are unable to eliminate foreign bodies that are too large. An experiment on rats showed that regular unprotected inhalation of multiple types of inert fibrous nanomaterials causes pulmonary lesions. This poses a challenge for the use of inert nanofiber felts in construction, which had been deemed to be less harmful than the asbestos it replaced. In reality, they could present the same health risks for those handling it.