The purpose of this study was to examine the frequency of firefighters’ heat-related illness (HRI) in the line of duty with relation to resting/working hours and the behaviour of taking off PPE during rest periods based on nationwide survey (N=674). The results showed that 74.8% of firefighters experienced HRI symptoms and 5% of firefighters suffered from symptoms of HRI 20 times or more in a year. This study also showed a statistically significant correlation between working hours and removing PPE with the experience of HRI symptoms (p<0.05). In addition, removing PPE except the helmet was correlated with resting hours (p<0.001; Gloves: p=0.051), which indicates that removing PPE is related to resting time. This study revealed that firefighters routinely experienced mild HRI symptoms and the fatalities were just a tiny fraction. It is also emphasized that attention should be given on the significant role of taking off PPE during rest and on the standardized firefighters’ rehabilitation followed by its application.