Operatory ventilation and scavenger evacuation rate influence on ambient nitrous oxide levels

Investigations have shown that greater than Federally-recommended levels of ambient nitrous oxide often exist in the dental workplace. The purposes of this investigation were to examine the influences of (1) operatory ventilation rate and (2) scavenger evacuation rate on ambient nitrous oxide concentrations within a closed dental operatory. This laboratory investigation utilized a scavenging nasal mask assembly (Porter/Brown) to deliver 40% nitrous oxide in oxygen, at a total flow rate of 5 L/min, to a mannequin positioned supinely in a dental chair. Eight experimental groups were assigned based on combinations of operatory air exchange rates (0, 5, 10, or 15 room air changes per hour) and scavenger evacuation rates (18.8 L/min and 21.4 L/min). Ambient nitrous oxide levels were monitored at four locations (0, 15.3, 30.7, and 61.4 cm) from the nasal mask by means of an infrared spectrophotometer (Miran 1B). The operatory air exchange rate was measured with a flow hood (Short Ridge) and manipulated to one of the desired ventilation rates. Time-weighted average (TWA) concentrations of ambient nitrous oxide (ppm) were continuously recorded at 15-second intervals with a microprocessor (DL332F Datalogger) connected directly to a microcomputer. The results demonstrated that significantly reduced (p < 0.05) ambient nitrous oxide levels were achieved with increasing room air exchange rates and the high scavenger evacuation rate. It was concluded that operatory ventilation and scavenger evacuation rates were significant control measures available for reduction of ambient nitrous oxide to Federally recommended levels.