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In page Nitrous oxide:

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Nitrous oxide is a significant occupational hazard for surgeons, dentists and nurses. Because the gas is minimally metabolised in humans (with a rate of 0.004%), it retains its potency when exhaled into the room by the patient, and can intoxicate the clinic staff if the room is poorly ventilated, with potential chronic exposure. A continuous-flow fresh-air ventilation system or N2O scavenger system may be needed to prevent waste-gas buildup.[citation needed] The National Institute for Occupational Safety and Health recommends that workers' exposure to nitrous oxide should be controlled during the administration of anaesthetic gas in medical, dental and veterinary operators.[1] It set a recommended exposure limit (REL) of 25 ppm (46 mg/m3) to escaped anaesthetic.[2]