Send letter to journal:
Re: Occupational exposure of midwives to nitrous oxide on delivery suites

Dear Editor

In our opinion, the article by Henderson et al.[1] Occupational exposure of midwives to nitrous oxide on delivery suites is in need of some remarks.

In the paper a serious problem seems to be the presence of nitrous oxide in samples collected at the beginning of the shift. Many years ago, during the first studies about the evaluation of N₂O in urine , we have frequently observed "uncommon" concentration of N₂O in urine of exposed and unexposed subjects. The phenomenon was kept under control and disappeared when urine samples were treated with a little quantity of H₂SO₄ ( 0.2 ml). For this reason in the paper" N₂O in urine as biological Index of Exposure in Operating Room personnel"[2] we suggested: "(…) Approximately 10 ml of urine were collected from all the subjects at the end of the exposure period in 120 gastight glass vials with airtight plugs. Caps were rapidly replaced in the vials to prevent any significant loss of N₂O. The vials contained 0.2 ml sulfuric acid in order to avoid the in vitro production of N₂O ( probably due to microflora activity.(…)"[3]

Another point we consider very important is that the subjects must void the bladder rapidly in areas known to be free of nitrous oxide, otherwise a significant contamination of samples can occur. In conclusion, we think that among the simple precautions that should be taken to avoid significant errors (avoiding collection of urine samples in places contaminated with N₂O, carrying out collection rapidly and using airtight collection vials so as to avoid any major loss of dissolved anaesthetic) one point should be emphasized in view of its importance: storage of urine before analysis can produce an endogenous formation of N₂O originating from the oxidation processes of the nitrogen compounds present in biological liquids. Experiments performed to study this phenomenon have shown that the process is inhibited if the urine is kept acid. If, as a precaution, a few drops of strong acid are added to each collection vial before urine samples are collected, neoformation of nitrous oxide will be safely avoided and the urine samples may then be stored as long as required prior to the analysis.

References

(1) K A Henderson, I P Matthews, A Adisesh, and A D Hutchings. Occupational exposure of midwives to nitrous oxide on delivery suites. Occup Environ Med 2003; 60: 958-961.

(2) M Imbriani, S Ghittori, G Pezzagno , E Capodaglio. Nitrous Oxide in Urine as Biological Index of Exposure in Operating Room Personnel. Appl Ind Hyg 1988; 3:223-227.

(3) World Health Organization (WHO): Health Hazards from nitrates in drinking water; Environmental Health 1. 1985 Report on a WHO meeting . Copenhagen 33-34.