Complex sentence: External factors that by themselves are not threatening or stressful are deemed such for someone experiencing PTSD, often reported stressful is hearing a song on radio or seeing a type of object that may remind the person with PTSD of prior threatening events.
Split into simpler sentences: External factors that by themselves are not threatening or stressful are deemed such for someone experiencing PTSD. Triggers can be stressful, such as when a person reports stress when hearing a song on radio or seeing a type of object that may remind the person of prior threatening events.