To the Editor.— The report by Thal et al (242:2418, 1979) on the use of an oxygen-nitrous oxide mixture to provide analgesia for patients during emergency transport on mobile intensive care units is interesting but raises some concerns. One consideration is the exposure of mobile unit personnel to the exhaled nitrous oxide (N 2 O). It is likely that high levels of N 2 O may accumulate within the confined space of a mobile unit. Long-term exposure of transport personnel may affect their performance 1 as well as longterm safety. If N 2 O use continues, proper scavenging should be imple mented. In addition, although the side effects and contraindications to using N 2 O were noted, it should be reemphasized that this is not a benign drug. In critically ill patients, it may obtund reflexes. Furthermore, N 2 O may affect the cardiovascular system by reducing cardiac output and increasing