[Measurement of nitrous oxide in exhaled air: feasibility, physiological constraints and implications of screening; a critical analysis].

Recreational use of nitrous oxide is rapidly increasing in France and across Europe, accompanied by a rise in neurological, cardiovascular and psychiatric complications. In this context, portable devices, particularly those based on infrared spectroscopy (IR), are being proposed to detect N2O several hours after inhalation. However, physiological data show that N2O has very low blood solubility, diffuses rapidly, and is almost completely eliminated within approximately 30 minutes, with a half-time of about 5 minutes. The prolonged detection reported in profile and is based on controlled conditions that are difficult to extrapolate to real-world settings. Exhaled concentrations are strongly influenced by ventilation, cardiac output and breathing technique, resulting in significant interindividual variability. From an analytical perspective, headspace gas chromatography-mass spectrometry (HS/GC-MS) remains the laboratory reference method but is not suitable for rapid field screening. Infrared spectroscopy offers a promising portable alternative, yet it still faces methodological limitations, variability issues and a lack of robust validation. Based on current knowledge, a breath test could serve as a qualitative screening tool for recent exposure, but it does not yet allow reliable quantification or robust medico-legal interpretation without further scientific evidence and an appropriate regulatory framework.