Ethylene, methanol, ethanol, and ammonia are nowadays known indirect biomarkers for various diseases and malfunctions of the human body. The objective of the present research was to investigate the origin of the trace volatile compounds observed in breath exhaled via the mouth, which is the usual way to carry out breath analysis. Therefore, we have initiated experiments to measure the concentration of the ammonia, ethylene, ethanol and methanol gases in the breath exhaled via the mouth and nose, in order to identify the major trace gas compounds for breath analysis. All the measurements were made before and after brushing with toothpaste and baking soda. Mouth-exhaled breath vs. nose-exhaled breath was investigated using a CO 2 laser photoacoustic system (LPAS), a well-known method in the field of trace gas detection, used in our study for quantitative multi-component determination. The levels of all four trace gases are much lower for nose breathing compared with mouth breathing. Compared to initial concentrations, the levels of all four trace gases are much lower after brushing with toothpaste and after brushing baking soda. To analyze and identify the major trace gas compounds from breath, the volunteers should be instructed to use toothpaste before each breath test sampling, to keep the oral bacteria under control. Even though it is conceivable that some compounds may have their origins in the nasal passages and sinuses, to avoid mouth contamination of the breath samples also nose breath analysis should be carried out. In order to identify the major sources of ethylene, ammonia, ethanol, and methanol compound the measurements should be carried out of both nasal-breathing route and mouth-breathing route. These findings may lead to a procedure whereby subjects should give breath samples so as endogenous biomarkers are less possible contaminated by other sources.