Nitrous oxide (N2O) was found responsible for genetic and reproductive toxicities, whereas it is widely used in paediatric care units where most healthcare providers are women of childbearing age. This motivated investigating occupational overexposure and overexposure factors in several paediatric hospital units. A cross-sectional study was carried out in seven healthcare units. On each of 34 healthcare providers, air samples were extracted (portable pumps and Tedlar® bags) and N2O quantified (gas chromatography, pulsed discharge ionization detection, and infrared spectrometry). The data allowed calculating mean instantaneous exposures. The mean instantaneous exposure was: i) four times higher in closed vs. open treatment rooms; ii) two times higher in case of use vs. non-use of N2O; iii) significantly higher in junior vs. senior healthcare providers (by 12%); and, iv) higher during presumably short vs. presumably long procedures (by 20%). Overexposures to N2O were mainly seen in the emergency unit and in day hospitals for thoracic/abdominal diseases and nephrology. Overexposures were frequent during short-duration procedures; among 88 N2O measurements, 77 (87.5%) exceeded the 200 ppm threshold over 15 minutes. The overexposures call for dedicated treatment rooms (with adequate equipment and ventilation), more efficient anaesthetic practices, appropriate training, and regular checks.