Abstract: A method for monitoring breathing air of a person breathing via a face mask (8). In the method pressure of inhaled air and partial pressure of oxygen before oxygen enters in the face mask (8) are measured. A partial pressure of oxygen in lungs of the person is estimated on the basis of the measured pressure of inhaled air and the measured partial pressure of oxygen. Also at least one physical characteristics of lungs is taken into account. The estimated partial pressure of oxygen in lungs is compared with a threshold. An indication of impending hypoxia is provided to the person, if the comparison indicates that the estimated partial pressure of oxygen in lungs is at a level lower than the threshold. The present disclosure also relates to an apparatus (6) for implementing the method.