Athletes, such as marathon runners and cyclists, often resort to training at higher altitude areas, in order to optimize their endurance. The atmosphere in such locations has lower oxygen content, which forces the human body to compensate for the reduced oxygen uptake by increasing hemoglobin concentration in blood. Thusly, oxygen transfer is improved, and consequently the performance in sports too. This kind of training is not a viable alternative for recreational athletes, who cannot afford such costs. Commercially available products simulating high altitude training conditions currently rely on the augmentation of dead space to achieve the desired effect. In addition, absorbers are employed in order to bind excess carbon dioxide, thus substantially increasing airway resistance. This kind of high altitude training devices do not offer an accurate adjustment of oxygen and carbon dioxide, which in turn does not allow the user to have regular and effective training sessions. This can be changed however, with the novel altitude-training device presently introduced. HypoxieFit is based on technologies that stem from the field of heart-lung machines, and blood enrichment with oxygen. Oxygen and carbon dioxide content in the aspired air can be precisely adjusted by means of thin hollow fibers. Hence, carbon dioxide is effortlessly and accurately eliminated by diffusion, which renders the application of adsorbers obsolete. The absence of any filters and adsorbers in HypoxieFit minimizes airway resistance, making the use of it incomparably more pleasant. Furthermore, the attainment of precise values facilitates interval-training sessions. The HypoxieFit has the potential to become an enrichment for every athlete; each training session being individually adapted. The positive effect of repeated short-term oxygen deficiency on the human body has been presumed for a long time, and extensive research has been conducted on this topic. Its therapeutic effects are also being increasingly investigated. Thus, it is also expected to have a positive impact especially on chronic pulmonary diseases (e.g. COPD) and on cardiac system disorders (e.g. arterial hypertension).