Seemingly conflicting results shows the difficulty of research and the need for more of it.

After a review of the scientific literature, the National League of Cities issued a press release stating, "A study released today titled 'Assessing State Firefighter Cancer Presumption Laws And Current Firefighter Cancer Research,' found there is inadequate scientific research to determine a link between working as a firefighter and an elevated risk of contracting cancer." 

Sep 18, 2015
By Sara Jahnke

http://www.firerescue1.com/fire-products/fire-rehab/articles/3218914-Why-firefighter-research-sends-mixed-signals/