Specifications

An article written by Munley et al., in 1986 looked at midwives’ exposure to nitrous oxide. They

found mean exposures that were not significantly less than the Swedish 8hr TWA of 100 ppm

and one average exposure was 360 ppm. These values were based on an unscavenged nitrous

oxide delivery system and they noted that with scavenging in place the exposure could be

reduced by a factor of 2 to 5. They noted that even with scavenging systems in place, they were

unable to decrease the midwives’ 8 hour time weighted average (TWA) exposure to less than 25

ppm (NIOSH and WCB OF BC standard).

Heath et al., in 1994 conducted a study looking at nitrous oxide pollution in the delivery suite in

a hospital in New Zealand. They found unacceptable levels of nitrous oxide in labour and

delivery suites that were unscavenged and that in most cases by adding a scavenging system the

levels dropped to below the NIOSH 8 hr limit of 25 ppm. They noted that with some patients the

levels were unacceptably high even with scavenging. The delivery room investigated in that

study had an air inlet in the centre of the ceiling, an exhaust grill at floor level and a total of 12

room air changes per hour (Heath et al., 1994).

2.2.2 Exposure Limits

The 8 hour exposure limit for nitrous oxide has been set at 100 ppm in Sweden, Denmark,

Germany, Italy, Norway and the United Kingdom. The Health Services Advisory Committee,

(UK) stated reasons for setting the limit at this level were given including tests on rats at 1000

ppm for 8 hrs a day for the duration of gestation, and developmental toxicity was observed

(HSAC, 1995). The experiments were repeated at 500 ppm and no adverse effects on the fetus

were observed. The HSAC supports that nitrous oxide can inhibit the production of new cells

and therefore set its limit at 100 ppm, 1/5 the level of no effect. When examining how to

decrease exposure in labour and delivery the committee suggested that general exhaust (6-7

room air changes per hour) and good housekeeping practices would help keep exposure levels

below the permissible limit.