Nitrous oxide exposure on the labour ward.

Occupational exposure to nitrous oxide may be increased in confined working environments where ventilation and scavenging are either ineffective or unavailable. This is particularly the case on the labour ward. To assess the exposure of midwives working on the labour ward, midwives were asked to wear personal nitrous oxide samplers during their shift periods. The nitrous oxide exposure during a total of 242 midwife shift periods was monitored in two hospitals. Seven (3%) of these midwife shifts demonstrated mean exposure levels of over 500 parts per million (ppm), the highest reaching 1638 ppm. Fifty-six (23%) had NZO levels over 100 ppm and 129 (53%) had levels above 25 ppm. During the 111 midwife shift periods where midwives did not work in a room where Entonox was in use, their mean exposure was 22 ppm (median 12 ppm), however in four of these midwife shift periods the mean exposure level exceeded 100 ppm. Chronic exposure to high levels of nitrous oxide which exceed the recently proposed UK standard of 100 ppm and those in force in the USA (25 ppm) and Sweden (100 ppm) are frequently encountered by midwives.