Firefighters are exposed to chemical mixtures that can pose risks to their health. Understanding the relationship between exposure and effect is crucial to understanding and mitigating the risks. In this thesis, the levels of DNA damage in whole blood samples of the CELSPAC – FIREexpo study participants will be analyzed. For this, comet assay that detects single and double DNA strand breaks, and oxidized purines will be used. The DNA damage levels measured in the whole blood might be affected by the different ratios of leucocyte subpopulations present in the blood. The leucocyte subpopulations might differ in susceptibility to DNA damage resulting from chemical exposure. The main objective of the thesis will therefore be to explore the link between the blood differential count and occupational exposure and training activities of firefighters, and the relationship between the blood differential count and the observed DNA damage levels. This work will contribute to the assessment of the impact of occupational exposure of firefighters on the levels of their DNA damage.