The CELSPAC – FIREexpo biomonitoring study investigates the long-term effects of chemical exposure on firefighters' wellness and fitness. It aims to provide science-based measures to minimize the health risks of the firefighting occupation. Here, we present the study design, cohort profile, and first results with respect to internal per- and polyfluoroalkyl substances (PFAS) and polycyclic aromatic hydrocarbons (PAH) levels in study participants. Participants (n = 166) were divided into three subcohorts: i) newly recruited firefighters, ii) professional firefighters with several years' experience, and iii) the control group. Participants underwent physical performance tests, provided information on their lifestyle and diet, and urine and blood samples 1–4 times within an 11-week period. 12 serum PFAS and 10 urinary hydroxylated PAH (OH-PAH) levels were determined using HPLC-MS/MS and compared between subcohorts and samplings. The association of internal exposure with reported lifestyles and occupational factors was investigated using Spearman's correlation, principal component analysis, and multivariate regression analysis. ΣPFAS levels in firefighters were significantly higher than in the control group and were mostly associated with the length of firefighting career, age, blood donation, and population size. 10.9 % and 7.6 % of measurements exceeded the HBM-I or HBM-II value for PFOS and PFOA, respectively. Urinary ΣPAH levels increased significantly after training with burning wooden pallets, but none of them exceeded the no observed genotoxic effect level. Firefighters’ occupational exposure, its sources, and pathways, need to be systematically monitored and investigated on a long-term and individual basis. The CELSPAC – FIREexpo study helps to clarify the degree of occupational exposure to the given compounds and the subsequent risks to firefighters.