From About Health
Rates of PTSD in firefighters may be heightened more so than in other professions. See, many people will experience a potentially traumatic event at some point in their life. But just because you have experienced a traumatic event does not mean that you will definitely go on to develop PTSD. However, people who have experienced multiple traumatic events have been found to be at greater risk for developing PTSD. One group of people that may experience many traumatic events as part of their job — and thus be at a heightened risk for PTSD — is firefighters.
It is clear from these studies that there is a big range in PTSD rates among firefighters. This is likely due to a number of reasons, including how PTSD was assessed (through a questionnaire or interview), whether other emergency responders were also surveyed along with the firefighters, whether the firefighters were volunteer or not, and where the firefighters worked.