Hazardous Chemical Information System (HCIS)

Nitrous oxide

E/S first adopted in 1990

Documentation notice: NationalOccupational Health and Safety Commission documentation available for thesevalues. For this standard considerations, such as, economic, social ortechnological implications, or sampling and analytical limitations, havealso been taken into account.

No standard should be applied without reference to Guidance on the interpretation of Workplace exposure standards for airborne contaminants.

1. IDENTITY

2. CHEMICAL AND PHYSICAL PROPERTIES

Nitrous oxide is a colourless, non-flammable gas at room temperature witha slightly sweet odour. Its physical and chemical properties include:

3. MAJOR INDUSTRIAL USES

Nitrous oxide is used mainly as an anaesthetic for both major and minorsurgery, either alone or in combination with other anaesthetics such ashalothane.

4. HEALTH EFFECTS

The health effects of nitrous oxide have been reviewed ^(1,2,3) .

The American Conference of Governmental Industrial Hygienists (ACGIH) has reviewed ⁽¹⁾ the anaesthetic, haematological, carcinogenicand foetotoxic effects of the gas. In this review, the ACGIH has also takeninto consideration the findings of several retrospective epidemiologicalstudies carried out in the late 1970's, which indicated an increased risk ofspontaneous abortions in operating room nurses and anaesthetists. Theyconsider there is a similarity of effects between epidemiological and animalstudies and recommends a threshold limit value (TLV) of 50ppm for nitrousoxide. This level was set to "prevent embryofetal toxicity in humans(resulting in an increased risk of spontaneous abortion) and significantdecrements in human psychomotor and cognitive functions or other adversehealth effects in exposed personnel" ⁽¹⁾

However, the methodology and the interpretation of findings of theepidemiological studies considered by the ACGIH, have beenquestioned by some other reviewers ^(4,5,6) .Spence ⁽⁶⁾ has conducted a prospective studyin the U.K. from 1977-86 and reported from the unpublished data that therewas a relationship between child birth weight and women anaesthetists orsurgeons working in operating theatres, but "at the moment there is noobvious relationship between hours worked or speciality and miscarriage".This observation can be attributed to the improved study methodology andthe changing exposure pattern associated with the introduction of scavengingsystems which has led to lower exposure to anaesthetic gases in operatingtheatres.

The possible interaction effect of nitrous oxide withother anaesthetic gases requires careful consideration. The US NationalInstitute for Occupational Safety and Health (NIOSH) has recommended ⁽²⁾ an exposure standard of 25ppm for nitrousoxide, based on the findings of the study of Bruce & Bach ⁽⁷⁾ such that decrements in performance,cognition, audiovisual ability and dexterity have been observed in humanvolunteers at an nitrous oxide exposure level of 50ppm. The decrementsincreased when a mixture of 50ppm nitrous oxide and 1ppm halothane was used.

5. OCCUPATIONAL EXPOSURE

Crea & Grygorcewicz ⁽⁸⁾ surveyed 40hospitals, 25 dental surgeries and 26 veterinary surgeries, where the airconcentrations of halothane and nitrous oxide, alone or in combination, weremeasured in the breathing zone of the anaesthetists and surgeons usinginfrared monitoring technique. With properly maintained anaesthetic gasdispensing equipment and scavenging systems in operation, the concentrationsof nitrous oxide gas in the hospital operating theatres could be kept be1ow25ppm. In dental surgeries where relative analgesia was used with ascavenging mask, the average concentrations of nitrous oxide during theoperation were slightly higher, ranging from 30-40ppm, but lowerconcentrations were also recorded.

Similar but slightly higherresults were also reported by Rajhans et al ⁽⁹⁾ and Gardner ⁽¹⁰⁾ , using personaldosimetry.

6. RECOMMENDATION FOR EXPOSURE STANDARD

To revent neurobehavioural effects to most workers, the ExposureStandards Working Group recommends a time-weighted average exposure standardof 25ppm for nitrous oxide. This level should also provide a safety marginfor preventing other health effects such as embryofoetal toxicity. TheWorking Group also recognises that occupational exposure to nitrous oxide canbe kept below this level if active scavenging equipment is used andmaintained properly.

1. American Conference of Governmental Industrial Hygienists, Documentation of The Threshold Limit Values and Biological Exposure Indices, 5th ed., Cincinnati, Ohio, 1986

2. National Institute for Occupational Safety and Health (NIOSH). Criteria for a Recommended Standard...Occupational Exposure to Waste Anesthetic Gases and Vapors, DHEW(NIOSH) Publication No.77-140, Washington DC, March, 1977

3. National Board of Occupational Safety and Health, Consensus Report for Nitrous Oxide, Scientific Basis for Swedish Occupational Standards. III, p.36-42, Solna, 1982

4. de Silva PE, "The reproductive hazards of exposure to anaesthetic gases", Australian Institute of Occupational Hygiene Annual Conference, Melbourne, 1982

5. Tannebaum TN & Goldberg RJ, "Exposure to anaesthetic gases and reproductive outcome", J Occup Med, 27, 659-668, 1985

6. Spence AA, "Environmental pollution by inhalation anaesthetics", Br J Anaesth, 59, 96-103, 1987

7. Bruce DL & Bach MJ, "Effects of trace anaesthetic gases on behavioural performance of volunteers", Br J Anaesth, 48, 871-875, 1976U

8. Crea J & Grygorcewicz, A Survey of Waste Anaesthetic Gases inHospital Operating Theatres, Dental and Veterinary Surgeries, SouthAustralian Health Commission, Adelaide, June 1988

9. Rajhans GS et al, "Hygiene aspects of occupational exposure to waste anaestheticgases in Ontario hospitals", Ann Occup Hyg, 33, 27-44, 1989

10. Gardner RJ, "Inhalation anaesthetics - exposure and control: astatistical comparison of personal exposures in operating theatres with andwithout gas scavenging", Ann Occup Hyg, 33, 159-173, 1989

Documentation notice:Entries carrying a notice for National Occupational Health and SafetyCommission documentation indicate that these substances have been reviewed indetail by the Exposure Standards Expert Working Group and that documentationsupporting the adopted national values is available in the NationalCommission's Documentation of the Exposure Standards[NOHSC:10003(1995)].