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Controlling Exposures to Nitrous Oxide During Anesthetic Administration
NIOSH ALERT: 1994
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.
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].
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.
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].
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].
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.
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:
-- informs workers about proper work practices, controls, equipment, and protective gear that should be used when working with N2O.
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:
-- Monitoring of air in the worker's personal breathing zone
-- Environmental (room air) monitoring
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:
Take the following steps to control N2O exposure from anesthetic scavenging systems:
-- validated to measure air flow within 5% of actual air flow,
-- permanently connected to the scavenging system vacuum line, and
-- positioned so that it is always visible to the operator.
Maintain the flowmeter by cleaning and recalibrating it according to the manufac-turer's recommendations.
Room Ventilation. Take the following steps to assure that the ventilation system effectively removes waste N2O:
Use the following work practices to control N2O exposures:
-- the scavenging mask is secured over the patient's nose or face.
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:
As specified by the NIOSH Respirator Decision Logic [NIOSH 1987b], the minimum level of protection for an air-supplied respirator is provided by a half-mask respirator operated in the demand or continuous-flow mode.# More protective air-supplied respirators are described in the NIOSH Respirator Decision Logic.
-- regular training of personnel,
-- periodic environmental monitoring,
-- respirator fit testing,
-- maintenance, inspection, cleaning, and storage, and
-- selection of proper NIOSH-approved respirators.
The respiratory protection program should be evaluated regularly by the employer.
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
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
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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: