Add spaces: Whilesomebelieveasbestos-relateddiseaseisa"thresholdphenomenon,"whichrequiresacertainlevelofexposurefordiseasetooccur,othersbelievethereisnosafelevelofasbestos.
While some believe asbestos-related disease is a "threshold phenomenon," which requires a certain level of exposure for disease to occur, others believe there is no safe level of asbestos.