Controlling Exposures to
Nitrous Oxide During Anesthetic
Administration
NIOSH ALERT: 1994 |
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].
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.
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
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:
·
Monitor airborne concentrations of N2O.
·
Implement appropriate engineering controls, work practices, and
maintenance procedures.
·
Institute a worker education program that
-- describes standard operating procedures for
all tasks that may expose workers to N2O, and
--
informs workers about proper work practices, controls, equipment, and
protective gear that should be used when working with N2O.
· Use the guidelines in the following
section to minimize worker exposures to N2O.
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:
· Monitor for N2O when the anesthetic equipment is installed and every 3 months
thereafter.
·
Include the following types of monitoring:
-- Leak testing of equipment
--
Monitoring of air in the worker's personal breathing zone
--
Environmental (room air) monitoring
· Prepare a written monitoring and
maintenance plan for each facility that uses N2O. This plan should be developed by
knowledgeable persons who consider the equipment manufacturers'
recommendations, frequency of use, and other circumstances that might affect
the equipment.
· Perform air monitoring by gas-bag
sampling [NIOSH 1984] or real-time sampling [McGlothlin
et al. 1989].
When real-time sampling is conducted
to obtain personal exposure data, attach the sampling train to the lapel of the
worker on the side closest to the patient; N2O concentrations in this location
are most representative of those in the worker's breathing zone. Diffusive
samplers (referred to as passive dosimeters) are commercially available and may
be useful as initial indicators of exposure [Bishop and Hossain
1984].
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:
· Use connection ports with
different-diameter hoses for N2O and O2 to reduce the possibility of
incorrectly connecting the gas delivery and scavenging hoses [Dorsch
and Dorsch 1984].
· Check all rubber hoses, connections,
tubing, and breathing bags daily and replace them when damaged or when
recommended by the manufacturer.
· Following visual inspection, perform
leak testing of the equipment and connections by using a soap solution to check
for bubbles at high-pressure connections. For a more thorough inspection of all connectors,
use a portable infrared spectrophotometer (such as a Miran
1A or 1B) calibrated for N2O detection [NIOSH 1984].
· Check both high-and low-pressure
connections (such as O-rings) regularly, as they may become worn; replace them
periodically, according to the manufacturer's recommendations.
· Evaluate the N2O and oxygen mixing
system for leaks when it is first installed and periodically thereafter,
according to the manufacturer's recommendations.
· Ensure that gas cylinders are safely
handled, used, and stored as specified by the National Research Council [NRC
1981] and as required by OSHA [29 CFR@@ 1910.101].
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:
· Supply scavenging masks in a variety of sizes so that the mask always fits
comfortably and securely over the patient's nose or face.
· Use an automatic interlock system to
assure that the N2O cannot be turned on unless the scavenging system is also
activated. N2O
should never be used without a properly operating scavenging system.
· Make sure that the scavenging system
exhaust rates (flow rates) are approxi-mately 45 liters per minute (L/min) to minimize leakage of N2O. Flow
rates of less than 40 L/min may result in significant leakage around the mask
[NIOSH 1977a]. Monitor the flow rate with a flowmeter
that is
-- 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.
· Use scavenging vacuum pumps that are
powerful enough to maintain a scavenging flow rate of at least 45 L/min at each
nasal mask regardless of the number of scavenging units in use at one time.
· Vent N2O from all scavenging vacuum
pumps to the outside of the building away from fresh air intakes, windows, or
walkways. Scavenging
system exhaust should not be vented into a recirculating
ventilation system.
Room Ventilation. Take the following steps to assure
that the ventilation system effectively removes waste N2O:
· If concentrations of N2O are above 25 ppm in work areas, increase the air flow into the room or
increase the percentage of outside air to allow for more air mixing and further dilution
of the anesthetic gas. Maintain a balanced air supply
and exhaust system so that N2O does not contaminate adjacent areas.
· If concentrations of N2O are still above
25 ppm, use supplementary local ventilation@@@ in
conjunction with a scavenging system to reduce N2O exposure in the operatory
[Mickelsen et al. 1993]. The effectiveness of this
ventilation depends on its location with respect to the patient and the airflow
rates. Do not work between the patient and the exhaust duct, where contaminated
air would be drawn through the worker's breathing zone.
· Dilute N2O and remove contaminated air
from the work area by placing fresh-air vents in the ceiling; direct the supply
of fresh air toward
the floor and the operating area. Place exhaust-air vents at or near the floor.
Work Practices
Use
the following work practices to control N2O exposures:
· Inspect the anesthestic
delivery systems and all connections before starting anesthetic
gas administration.
Make sure that breathing bags, hoses, and clamps are in place before turning on
the anesthetic machine.
· Connect the scavenging mask properly to
the gas delivery hose and the vacuum system.
· Do not turn on the machine delivering
N2O until
-- the
vacuum system scavenging unit is operating at the recommended flow rate of 45
L/min, and
-- the scavenging mask
is secured over the patient's nose or face.
· Fasten the mask according to the
manufacturer's instructions to prevent leaks around the mask during gas
delivery.
· Do not fill the breathing bag to
capacity with N2O; an overinflated bag can cause
excessive leakage from the scavenging mask. The breathing bag should collapse and expand
as the patient breaths. This bag activity shows that the proper amounts of N2O
and air are being delivered to the patient.
· Flush the system of N2O after surgery by
administering oxygen to the patient through the anesthetic
equipment for at least 5 minutes before disconnecting the gas delivery system.
· Encourage patients to minimize talking
and mouth-breathing during dental surgery. When mouth-breathing is apparent,
avoid the patient's breathing zone to the extent possible.
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:
· Wear air-supplied respirators.
Air-purifying respirators (that is, respirators that remove N2O from the air
rather than supply air from a clean source) should not be used because
respirator filters do not efficiently remove N2O.
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.
· When respirators are used, the employer
must establish a comprehensive respiratory protection program as outlined in
the NIOSH Guide to Industrial Respiratory Protection [NIOSH 1987a] and as required by the
OSHA respiratory protection standard [29 CFR 1910.134]. Important elements of
this standard are
-- an
evaluation of the worker's ability to perform the work while wearing a
respirator,
-- 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
NIOSH
Telephone: (513) 841B4321
We greatly appreciate your
assistance in protecting the health of
Richard A. Lemen, Ph.D.
Acting Director, National Institute for Occupational Safety and
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RR [1982]. Nitrous oxide, N2O. In: Clayton GD, Clayton
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EC, Hossain MA [1984]. Field
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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: