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Controlling Exposures to Nitrous Oxide During Anesthetic Administration

NIOSH ALERT: 1994
DHHS (NIOSH) Publication No. 94-100

WARNING!

Workers exposed to nitrous oxide (N2O) may suffer harmful effects.

The National Institute for Occupational Safety and Health (NIOSH) requests assistance in controlling exposures of workers to nitrous oxide (N2O) during the administration of anesthetic gas in medical, dental, and veterinary operatories. NIOSH concluded in 1977 that exposure to N2O causes decreases in mental performance, audiovisual ability, and manual dexterity [NIOSH 1977b]. A recent study of workers [Rowland et al. 1992] and several experimental animal studies [Corbett et al. 1973; Vieira 1979; Vieira et al. 1980, 1983] indicate that adverse reproductive effects may also result from chronic exposure to N2O.

This Alert presents control measures for preventing or greatly reducing exposure to N2O during the administration of anesthetic gas. These control measures should be part of a comprehensive written safety and health plan for workers. NIOSH requests that safety and health officials, editors of appropriate journals, manufacturers of anesthetic equipment, union representatives, employers, and managers bring the recommendations in this Alert to the attention of all workers who are at risk.

BACKGROUND

N2O is used as an anesthetic agent* in medical, dental, and veterinary operatories. This gas is also used as a foaming agent for whipped cream, an oxidant for organic compounds, a nitrating agent for alkali metals, and a component of certain rocket fuels [Beard 1982; Suruda and McGlothlin 1990].

In 1977, NIOSH published a technical report entitled Control of Occupational Exposure to N2O in the Dental Operatory. This report presented methods for limiting the concentration of waste N2O to 50 parts per million (ppm) during administrationCa limit based on the technical feasibility of existing controls [NIOSH 1977a]. Since publication of this technical report, data collected by NIOSH have shown occupational exposures as high as 300 ppm in hospital operating rooms [NIOSH 1985] and exposures higher than 1,000 ppm in dental operatories equipped with scavenging systems** (properly operating scavenging systems have been shown to reduce N2O concentrations by more than 70%) [McGlothlin et al. 1988, 1990].


*In dental operatories, agents that cause conscious sedation (such as N2O) are commonly referred to as "analgesic agents."
**Scavenging systems use local exhaust ventilation to collect waste gases from anesthetic breathing systems and remove them from the workplace.

To determine why occupational exposures to N2O are excessive even when scavenging systems are used, NIOSH has studied work practices*** and engineering controls@ for dental operatories. This work environment was chosen because N2O is frequently used as the sole anesthetic agent in dental operatories and exposures there tend to be more difficult to control than in general operating theaters. Control is more difficult in dental operatories because only the patient's nose is covered during anesthetic administration and scav-enging, but both the nose and mouth can be covered in general operating theaters.
***Work practices are procedures followed by employers and workers to control hazards.
@Engineering controls are hazard controls designed into equipment and workplaces.

HEALTH EFFECTS

Animal studies have shown adverse reproductive effects in female rats exposed to airborne concentrations of N2O [Corbett et al.1973; Vieira 1979; Vieira et al. 1980, 1983]. Data from these studies indicate that exposure to N2O during gestation can produce adverse health effects in the offspring.

Several studies of workers have shown that occupational exposure to N2O causes adverse effects such as reduced fertility [Rowland et al. 1992], spontaneous abortions, and neurologic, renal, and liver disease [Cohen et al. 1980]. A recent study [Rowland et al. 1992] reported that female dental assistants exposed to unscavenged N2O for 5 or more hours per week had a significant risk of reduced fertility compared with unexposed female dental assistants. The exposed assistants had a 59% decrease in probability of conception for any given menstrual cycle compared with the unexposed assistants. For dental assistants who used scavenging systems during N2O administration, the probability of conception was not significantly different from that of the unexposed assistants. Since environmental exposures were not measured during these epidemiologic studies, no dose-effect relationship could be established.

WORKERS EXPOSED

More than 424,000 workers (i.e., dentists, dental assistants, and dental hygienists) practice dentistry in the United States [BLS 1990]. In 1983, the American Dental Association (ADA) reported that 35% of all dentists used N2O to control pain and anxiety in their patients [ADA 1983]. The ADA 1991 Survey of Dental Practice indicated that 58% of dentists reported having N2O anesthetic equipment, and 64% of those practitioners also reported having a scavenging system [ADA 1992]. The percentage of pediatric dentists using N2O increased from 65% in 1980 to 88% in 1988 [Davis 1988].

OCCUPATIONAL EXPOSURE LIMITS

The Occupational Safety and Health Administration (OSHA) does not currently have a standard for N2O.

The NIOSH recommended exposure limit (REL) for N2O is 25 ppm as a time-weighted average (TWA) during the period of anesthetic administration [NIOSH 1977b]. This REL is intended to prevent decreases in mental performance, audiovisual ability, and manual dexterity during exposures to N2O. An REL to prevent adverse reproductive effects cannot be established until more data are available.

The American Conference of Governmental Industrial Hygienists (ACGIH) threshold limit value (TLV ) for N2O is 50 ppm as an 8-hour TWA [ACGIH 1993]. The 1991 Documentation of the Threshold Limit Values and Biological Exposure Indices states that "control to this level should prevent embryofetal toxicity in humans and significant decrements in human psychomotor and cognitive functions or other adverse health effects in exposed personnel" [ACGIH 1991].

CONCLUSIONS

A large population of health care workers is potentially exposed to N2O, and NIOSH has documented cases in which exposures substantially exceed existing RELs. NIOSH has concluded that exposure to N2O causes decreases in mental performance, audiovisual ability, and manual dexterity. Data from animal studies demonstrate that exposure to N2O may cause adverse reproductive effects. Studies of workers exposed to N2O have reported adverse health effects such as reduced fertility, spontaneous abortion, and neurological, renal, and liver disease. The recommendations in this Alert should therefore be followed to minimize worker exposures.

RECOMMENDATIONS

Engineering controls, work practices, and respirators (when necessary) should be used to minimize the exposure of workers to N2O. Employers should ensure that their workers are adequately protected from N2O exposure by taking the following steps:

  • GUIDELINES FOR MINIMIZING WORKER EXPOSURES

    Exposure Monitoring

    Exposure monitoring should be the first step in developing work practices and worker education programs, since measurements of N2O are needed to determine the type and extent of controls that are necessary. Follow the guidelines below to minimize worker exposures:

  • Engineering Controls and Maintenance Procedures

    The following engineering controls and maintenance procedures have been shown to be feasible and effective in reducing exposure to N2O during anesthetic administration.

    Anesthetic delivery. Excessive exposure to N2O may occur as a result of leaks from the anesthetic delivery system during administration [McGlothlin et al. 1989]. The rubber and plastic components of the anesthetic equipment are potential sources of N2O leakage because they may be degraded by the N2O and the oxygen as well as by repeated sterilization.

    Figure 1 illustrates sources of possible leaks from anesthetic delivery systems in dental operatories. These sources include leaks from the high-pressure connectionsCthat is, from the gas delivery tanks, the wall connectors, the hoses connected to the anesthetic machine, and the anesthetic machine (especially the on-demand valve). Low-pressure leaks occur from the connections between the anesthetic flowmeter and the scavenging mask. This leakage is due to loose-fitting connections, loosely assembled or deformed slip joints and threaded connections, and defective or worn seals, gaskets, breathing bags, and hoses.

    Take the following steps to control N2O exposure from anesthetic delivery systems:


  • @@Code of Federal Regulations. See CFR in references.

    Scavenging systems. Control of N2O at the scavenging mask is the next priority after control of N2O leakage from the anesthetic equipment. Leakage from the scavenging mask can be one of the most significant sources of N2O exposure because the breathing zone of a dentist or dental assistant is within inches of the mask [McGlothlin et al. 1989]. NIOSH research has reported breathing-zone concentrations of N2O above 1,000 ppm [McGlothlin et al. 1988].

    Take the following steps to control N2O exposure from anesthetic scavenging systems:

  • Room Ventilation. Take the following steps to assure that the ventilation system effectively removes waste N2O:


  • @@@Local ventilation is the use of a hood or duct to capture N2O at the source.

  • Work Practices

    Use the following work practices to control N2O exposures:

  • Respiratory Protection

    Workers should wear respiratory protection when N2O concentrations are not consistently below 25 ppm; however, practical considerations may prevent them from wearing such protection. Therefore, it is essential that employers use the engineering controls and work practices described in this Alert to reduce N2O concentrations below 25 ppm.

    When N2O concentrations are not consistently below 25 ppm, workers should take the following steps to protect themselves:


  • #Note: The assigned protection factor (APF) for this class of respirator is 10. The APF indicates the level of protection provided by a class of respirator [NIOSH 1987b]. An APF of 10 means that the respirator should reduce the air concentration of N2O for the wearer by a factor of 10 (or to 10% of the concentration without respiratory protection).

  • ACKNOWLEDGMENTS

    Barbara L. Dames and James D. McGlothlin, Ph.D., were the principal contributors to this Alert. Comments, questions, or requests for additional information should be directed to the following:

    Dr. Laurence J. Doemeny
    Acting Director, Division of Physical Sciences and Engineering
    NIOSH
    4676 Columbia Parkway
    Cincinnati, OH 45226

    Telephone: (513) 841B4321

    We greatly appreciate your assistance in protecting the health of U.S. workers.

    Richard A. Lemen, Ph.D.
    Acting Director, National Institute for Occupational Safety and Health
    Centers for Disease Control and Prevention

    REFERENCES

    ACGIH [1991]. Documentation of the threshold limit values and biological exposure indices. 6th ed. Cincinnati, OH: American Conference of Governmental Industrial Hygienists, pp. 1134B1138; documentation for nitrous oxide revised as of 1992.

    ACGIH [1993]. 1993B1994 threshold limit values for chemical substances and physical agents and biological exposure indices. Cincinnati, OH: American Conference of Governmental Hygienists.

    ADA [1983]. Dentists' desk reference: materials, instruments and equipment. 2nd ed. Chicago, IL: American Dental Association, pp. 413B414.

    ADA [1992]. The 1991 survey of dental practice: general characteristics of dentists. Chicago, IL: American Dental Association, p. 11.

    Beard RR [1982]. Nitrous oxide, N2O. In: Clayton GD, Clayton FE, eds. Patty's industrial hygiene and toxicology. 3rd rev. ed. Vol. 2C. New York, NY: John Wiley and Sons, pp. 4096B4097.

    Bishop EC, Hossain MA [1984]. Field comparison between two nitrous oxide (N2O) passive monitors and conventional sampling methods. Am Ind Hyg Assoc J 45(12):812B816.

    BLS [1990]. Outlook 2000. Washington, DC: U.S. Department of Labor, Bureau of Labor Statistics, Bulletin 2352, pp. 51, 54.

    CFR. Code of Federal regulations. Washington, DC: U.S. Government Printing Office, Office of the Federal Register.

    Cohen EN, Gift HC, Brown BW, Greenfield W, Wu ML, Jones TW, et al. [1980]. Occupational disease in dentistry and chronic exposure to trace anesthetic gases. J Am Dent Assoc 101(1):21B31.

    Corbett TH, Cornell RG, Endres JL, Millard RI [1973]. Effects of low concentrations of nitrous oxide on rat pregnancy. Anesthesiology 39:299B301.

    Davis MJ [1988]. Conscious sedation practices in pediatric dentistry: a survey of members of the American Board of Pediatric Dentistry College of Diplomates. Pediatr Dent 10(4):328B329.

    Dorsch JA, Dorsch SE [1984]. Understanding anesthesia equipment: construction, care and complications. Baltimore, MD: Williams & Wilkins, pp. 24B26.

    McGlothlin JD, Jensen PA, Cooper TC, Fischbach TJ, Fairfield CL [1990]. In-depth survey report: control of anesthetic gases in dental operatories at University of California at San Francisco Oral Surgical Dental Clinic, San Francisco, CA. Cincinnati, OH: U.S. Department of Health and Human Services, Centers for Disease Control, National Institute for Occupational Safety and Health, Report No. ECTB 166B12b.

    McGlothlin JD, Jensen PA, Todd WF, Fischbach TJ [1988]. Study protocol: control of anesthetic gases in dental operatories. Cincinnati, OH: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control, National Institute for Occupational Safety and Health, Division of Physical Sciences and Engineering.

    McGlothlin JD, Jensen PA, Todd WF, Fischbach TJ, Fairfield CL [1989]. In-depth survey report: control of anesthetic gases in dental operatories at Children's Hospital Medical Center Dental Facility. Cincinnati, OH: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control, National Institute for Occupational Safety and Health, Report No. ECTB 166B11b.

    Mickelsen RL, Jacobs DE, Jensen PA, Middendorf PJ, O'Brien DM, Fischbach TJ, et al. [1993]. Auxiliary ventilation for the control of nitrous oxide in a dental clinic. Appl Occup Environ Hyg 8(6):564B570.

    NIOSH [1977a]. Control of occupational exposure to N2O in the dental operatory. Cincinnati, OH: U.S. Department of Health, Education, and Welfare, Public Health Service, Center for Disease Control, National Institute for Occupational Safety and Health, DHEW (NIOSH) Publication No. 77B171.

    NIOSH [1977b]. Criteria for a recommended standard: occupational exposure to waste anesthetic gases and vapors. Cincinnati, OH: U.S. Department of Health, Education, and Welfare, Public Health Service, Center for Disease Control, National Institute for Occupational Safety and Health, DHEW (NIOSH) Publication No. 77B140.

    NIOSH [1984]. Nitrous oxide: Method 6600. In: Eller PM, ed. NIOSH manual of analytical methods. 3rd ed. Cincinnati, OH: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control, National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 84B100.

    NIOSH [1985]. Health hazard evaluation report: Hennepin County Medical Center, Minneapolis, MN. Cincinnati, OH: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control, National Institute for Occupational Safety and Health, HETA 84B046B1584.

    NIOSH [1987a]. Guide to industrial respiratory protection. Morgantown, WV: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control, National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 87B116.

    NIOSH [1987b]. NIOSH respirator decision logic. Cincinnati, OH: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control, National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 87B108.

    NRC (National Research Council) [1981]. Prudent practices for handling hazardous chemicals in laboratories. Washington, DC: National Academy Press, pp. 75B78.

    Rowland AS, Baird DD, Weinberg CR, Shore DL, Shy CM, Wilcox AJ [1992]. Reduced fertility among women employed as dental assistants exposed to high levels of nitrous oxide. New Eng J Med 327(14):993B997.

    Suruda AJ, McGlothlin JD [1990]. Fatal abuse of nitrous oxide in the workplace. J Occup Med 32(8):682B684.

    Vieira E [1979]. Effect of the chronic administration of nitrous oxide 0.5% to gravid rats. Br J Anaesth 51:283B287.

    Vieira E, Cleaton-Jones JP, Austin JC, Moyes DG, Shaw R [1980]. Effects of low concentrations of nitrous oxide on rat fetuses. Anesth and Analgesia 59(3):175B177.

    Vieira E, Cleaton-Jones P, Moyes D [1983]. Effects of low intermittent concentrations of nitrous oxide on the developing rat fetus. Br J Anaesth 55:67B69.


    (WORKER FACTSHEET)

    Controlling Exposures to Nitrous Oxide
    During Anesthetic Administration

    WARNING!

    Workers exposed to nitrous oxide (N2O) may suffer harmful effects.

    Take the following steps to protect yourself from waste N2O in the workplace:

    1. Be aware that N2O may cause the following health effects:

    2. Make sure that the following monitoring procedures are performed when the anesthetic equipment is installed and every 3 months thereafter:

    3. Prevent leakage from the anesthetic delivery system through proper maintenance and inspection of equipment. Eliminate or replace the following:

    4. Control waste N2O with a well-designed scavenging system that includes the following:

    5. Make sure that the room ventilation is effectively removing waste N2O. If concentrations of N2O are above 25 ppm, take the following steps:


    This document was last updated on 8/5/96.


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