The study methodology is based on a crossover design on firefighting training under different conditions, with characterization of exposure and assessment of cardiovascular, cancer and reproductive effect biomarkers. The study will be performed in cooperation with the Danish Emergency Management Agency.~The study will have one baseline session, while conscripts are in a classroom, and three sessions of firefighting-related exercises under different fire conditions, used currently in firefighting training programs in Denmark. The three firefighting training sessions will be controlled for equivalent work exercise using full protective gear and under different ambient conditions of firefighting training:~Firefighting equivalent work (no fire), with exercises performed in a clean environment, without fire (no ambient temperature, soot or smoke). This type of exercise precedes or complements the training under real fire conditions.~Firefighting under wood fire (wood fire), with exposure to ambient heat, smoke and soot. This is the most common training scenario used by Danish Emergency Management Agency training centres.~Firefighting under gas fire (gas fire), with exposure to ambient heat, and expectably less smoke and soot than with wood fire. These conditions are used in some Danish training centres, with logistical advantages (ease of turning or putting out the fire and managing the fire fuel) and unknown effect relating to exposure prevention (smoke and soot).~The order of the firefighting sessions will be as randomized as possible, and according to a selection of three sequence order options (no-wood-gas; wood-gas-no and gas-wood-no). Each campaign (corresponding to each rescue course) would desirably have one of these session sequences. It is not possible to blind the participants to the different scenarios, neither the field staff, but all the samples will be blinded for the subsequent laboratory analysis. The training sessions will have 1-3 weeks in between (accordingly with programmatic educational course schemes).
Epidemiological studies based on Danish registries have observed that Danish male firefighters have more cardiovascular disease, infertility diagnose and a trend to increased risk of cancer than other Danish employed males. Firefighting activities include a combination of stressors such as strenuous work under heat, smoke and soot known to be able to affect cardiovascular and reproductive health, with smoke and soot also being known to increase the risk of cancer.~The training facilities of real-fire extinguishing exercises in Denmark operate using wood or natural gas fire, which will have differential gradients of smoke, soot and possibly heat. The investigators will use different training conditions to create gradients of the different stressors and investigate health effects thereof. With this approach, the investigators expect to be able to evaluate the individual contribution of the different stressors in markers of cardiovascular, cancer and reproductive health risk. The project will include approx. 35 young conscript participants on a firefighting course, followed in four sessions, three firefighting training sessions under different fire conditions (no fire, wood fire and gas fire) and one control scenario.