Abstract: A method is described to perform on-line detection of stress levels or changes in humor of individuals. The approach is particularly advantageous because analysis can be performed in real time or, at least, comprising a delay shorter than 30 s, and because it is non-invasive and non-degrading; and actually the individual under study do not need to be aware of being studied. More specifically, the present invention is based on the analysis of lactic acid and other volatiles concentrations in the vapors released by the individual (through the skin and the breath). The inventors have proved that said concentrations respond nearly instantaneously when the individual is subjected to a sudden stressful stimulus.