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K.A. Henderson, I.P. Matthews, A. Adisesh, A.D. Hutchings.
Occup Environ Med 20023; 60; 958-961
Study conducted prior to December 2002 using 40 midwives within 2 hospitals covering a total of 46 midwife shifts.
Passive diffusion tubes were used to determine workplace exposure to nitrous oxide during the first 4hrs of their shift. In addition, pre and post shift urine samples were collected to ascertain biological update.
During 35 out of the 46 midwife shifts exposure levels were greater than the 100ppm TWA occupational exposure limit. The measurement range was 2.4~1300ppm, mean 313ppm, s.d. 358ppm.
In 43 of the 46 instances nitrous oxide was detected in the midwife urine post shift. The measurement range was 0~1102µg/l, mean 114 µg/l, s.d. 191µg/l.
Adjusting for baseline urine levels, there was a statistically significant correlation between urinary concentration and environmental concentration of nitrous oxide (p<0.001).
It was noted that 14 of the 20 delivery rooms where the study was conducted had less than the recommended 5~6 air changes per hour.
Approximately half of the midwives had baseline nitrous oxide present in their pre-shift urine sample, which was viewed as a surprising finding. The authors went on to postulate that the clearance time for nitrous oxide may be longer that currently thought.