Hazard Update [Health Devices Aug 1996;25(8):306-9]
ECRI continues to receive frequent inquiries from member hospitals about
scavenging cryosurgical units (CSUs) that use nitrous oxide (N2O) as the
cryogen (i.e., refrigerant gas). ECRI discovered the hazard presented by high CSU N2O
concentrations in 1979 while developing a draft standard under contract to the U.S. Food
and Drug Administration (FDA) that specifically required scavenging attachments on CSUs
that use N2O (Bruley 1980). We subsequently published two Hazard Reports
addressing the problem (Health Devices 8:293-4, October 1979, and 16:407-9,
However, confusion still exists about the need to scavenge this equipment,
appropriate scavenging methods, and available options that eliminate the need for
scavenging. This confusion is compounded, in part, by CSU manufacturers' representatives
who, according to member hospitals, tend to dismiss the issue when questioned about it. To
fully inform current medical, surgical, OR nursing, hospital clinical engineering, and
risk management staffs about this serious hazard, we are updating our earlier reports. Our
recommendations from these reports are repeated below; in short, exhausted N2O
must be scavenged.
The dangers of using N2O as a cryogen
The primary reasons that N2O is used as a cryogen are its
properties as an effective freezing agent and its availability in the hospital. Its
analgesic and pharmacologic effects, which can have adverse effects on personnel, play no
role in the operation or performance of the CSU. Using N2O as a cryogen is
acceptable only if the potential adverse effects are understood and measures are
taken to minimize personnel exposure. Repeated exposure to the extremely high
concentrations of N2O that result from CSU use can pose significant risks to
clinical and clerical personnel in the vicinity of the device. But, some physicians who
use N2O CSUs appear not to fully appreciate or understand the acute side
effects and long-term carcinogenic and teratogenic effects of chronic exposure.
Exposure to unscavenged CSUs has led to at least two lawsuits against a
CSU manufacturer. In those cases, a physician and his clerical assistant claimed injury
from daily exposure to the gas in the doctor's office. Both cases settled out of court for
substantial sums; however, in similar types of cases, hospitals might find themselves
named as defendants. For example, employees injured while working could be entitled to
Workers' Compensation benefits. It is also conceivable that a tort action could be brought
against a hospital or a physician's private practice that fails to minimize the risks
associated with N2O CSU use on the basis that the hospital or practice knew or
should have known of the hazards or intentionally failed to notify personnel of the risks.
Although the rate of N2O exhaust from CSUs and the resulting
ambient gas concentrations are not widely appreciated, they are easy to understand. The
current N2O exposure rate recommended by the National Institute for
Occupational Safety and Health (NIOSH) is 25 parts per million (ppm) averaged over the
period of administration (NIOSH 1977, Department of Health and Human Services [DHHS] 1994
Apr, DHHS 1994 May). In the OR, anesthesia machines typically exhaust only 2 to 4 L/min of
N2O during a surgical procedure. Whitcher et al. (1975) studied concentrations
of N2O exhausted from unscavenged anesthesia machines in operating and delivery
rooms having relatively high room-air exchange rates (10 to 21 changes/hr). At an exhaust
rate of 3 L/min, average room air concentrations of N2O ranged from 71 to 130
ppm. Short-term peak concentrations lasting several seconds ranged from approximately 500
to 1,200 ppm, depending on the height from the floor and the distance to the anesthesia
machine. At the end of anesthesia administration, the concentrations usually dropped to
below 25 ppm in less than 20 min.
Originally, concerns over exhausted N2O were raised based on
the chronic exposure of OR personnel to these relatively low levels of the gas. As a
result, scavenging of anesthesia machines is now the common and accepted practice. For
comparison, if an anesthesia machine exhausts 2 L/min, 120 L of N2O would be
exhausted during a typical 1 hr procedure. In contrast, N2O CSUs typically
exhaust 20 to 90 L/min during the freeze cycle; some models also exhaust the gas during
the thaw cycle. Therefore, a typical 3 min freeze application at 40 L/min will exhaust the
same amount of N2O as a 1 hr anesthesia procedure.
Furthermore, CSUs are frequently used in treatment rooms in a clinic or a
doctor's office where airflow exchange rates are very low (2 to 4 changes/hr, far lower
than in a typical OR). As such, N2O concentration in that room and in
adjacent rooms and nearby office areas can be excessive. In the
treatment room itself, levels will typically range from 4,000 to 7,000 ppm and
may take up to 4 hr to wash out to the 25 ppm level. Levels in nearby clerical
areas can also be high for long periods (see the table). In the lawsuits discussed above, it was calculated that the treatment
room concentrations ranged from 8,000 to 12,000 ppm.
Scavenging N2O from CSUs
On most CSUs purchased during the last decade, scavenging N2O
is simple. Most have a scavenging outlet to which the user attaches a long plastic hose
(typically one-half inch in diameter). In most ORs, scavenged gas can be easily and safely
vented into a nonrecirculating exhaust air vent (i.e., a vent through which air is
exhausted to the outdoors). Some ORs also have a dedicated system for safely venting
scavenged anesthetic gases; depending on its flow, such a system may also be suitable for
scavenged N2O from a CSU. In contrast, safely venting scavenged gas from
treatment rooms can be difficult. These rooms have recirculated ventilation; therefore,
the scavenged gas cannot be vented into the return air system. Options for venting N2O
from rooms with recirculated ventilation include placing the scavenging hose out a window
or through a small hole to the outdoors that has been drilled in the wall of the room
where the CSU is used.
Some hospitals report that CSU manufacturers recommend placing the vent
hose down the trap in any convenient sink. Never do this. Such a practice probably
violates most local plumbing codes. In addition, you cannot know where the gas will
eventually exhaust. N2O is heavier than air and may vent from a dry trap in a
sink or drain within the building on a floor below. Because of the high gas flows, such
venting could temporarily pressurize the plumbing system and allow gas to vent through wet
traps as well.
Some CSU manufacturers also recommend that the units be vented into the
hospital's centrally piped medical/surgical suction system. Never do this. The high
CSU gas flows can temporarily compromise an entire suction system if vented into it. A CSU
so vented in one OR could adversely affect the piped suction being used in a neighboring
room and place a surgical patient at risk.
Some older N2O CSUs can be factory-modified or retrofitted with
field kits to permit gas scavenging. However, some older units cannot be easily scavenged;
replacing them with newer models that can be scavenged or switching to carbon dioxide (CO2)
as the cryogen (as discussed below) are the only acceptable options.
Using CO2 as an alternative cryogen
Using CO2 as a cryogen is often an acceptable—and even
preferred—alternative to N2O, mostly because chronic exposure to this gas
exhausted from CSUs presents no health hazards. No carcinogenic or teratogenic risks are
associated with its use. Concerns about the use of CO2 relate to its asphyxial
properties during acute, high-concentration exposures. However, it is extremely unlikely
that the NIOSH limit for exposure to CO2 (5,000 ppm averaged over an 8 hr
period [NIOSH 1994]) could be exceeded by a CSU, even under the most demanding clinical
circumstances. While room air concentrations of CO2 may exceed 5,000 ppm during
a cryosurgical procedure, our studies with N2O suggest that the concentration
in even a poorly ventilated treatment room would drop to below 5,000 ppm within 15 min or
less. If averaged over an 8 hr day, the concentration from one procedure would be about
150 ppm. A clinician would have to perform more than 30 procedures in 8 hr (an unlikely
occurrence) in a poorly ventilated room to exceed the limit for average CO2
An additional concern of some physicians experienced with N2O
CSUs is whether CO2 is cold enough to adequately freeze tissues: the minimum
temperature for CO2 is –79° C, compared with
–89° C for N2O. In fact, CO2 can
provide the same freezing depth for the majority of clinical applications as N2O,
although the freezing time required is somewhat longer. We have found that once physicians
understand this, they are less reluctant to switch to CO2.
- Determine whether you own an N2O CSU (some CSUs can be
operated with either N2O or CO2).
- Always scavenge N2O CSUs and vent them to the
outside, away from any air-intake ducts. Do not vent them into a sink, drain
trap, air recirculation duct, or the piped medical/surgical suction
- Contact your unit's manufacturer to request information on scavenging
the N2O exhaust. If your equipment currently has an N2O
scavenging port, you will probably be able to order the proper size and type
of exhaust hose from the manufacturer. Or, you can order hose from a local
supplier. An exhaust hose is no more inconvenient or obtrusive than an
electrical power cord.
- If the CSU is used in an OR with nonrecirculated exhaust-air
ventilation, place one end of the exhaust hose approximately 0.5 m (1 to 2 ft) into the
exhaust air vent. If vents are inconveniently located (e.g., near the ceiling), consider
permanently installing a short length of exhaust hose through the vent grille. Equip the
exposed hose end with a connector appropriate for attachment to the CSU exhaust hose. If
the OR has a dedicated system for venting scavenged anesthetic gases, determine the flow
capacity for that system. If the capacity is over 100 L/min (3.5 ft3/mm), the N2O from the CSU can
be vented through that dedicated system if this is more convenient than
venting through the return air system.
- For treatment rooms in clinics and physicians' offices (and for ORs where
the N2O cannot be vented as discussed above), the N2O exhaust
hose can be vented to the outside through a window or a small hole drilled
in the windowsill or wall of the room where the equipment is used. Such
holes typically have a pipe of larger diameter than the vent hose and are
capped on the inside when not in use.
- Use CO2 for CSUs if a) scavenged N2O cannot be
safely or conveniently vented or b) the N2O cannot be
scavenged because of the design of the CSU.
- If you currently have CSUs that can be operated with either N2O
or CO2, strongly consider using CO2, even if scavenging is possible,
because it is intrinsically safer. After switching to CO2, the clinician using
the equipment will have to become accustomed to the changes in technique (primarily
increasing the freezing time) required when using this cryogen. If your unit requires a
different gas cylinder yoke for CO2 tanks, purchase the correct one rather than
removing the safety indexing pins on the existing N2O
yoke. Removing the pins will defeat a standard safety system and create yet
- Remove from service those N2O units that cannot be scavenged
or converted to use with CO2. For future acquisitions of cryosurgical
equipment, purchase units that are operated on CO2. This eliminates the need for scavenging.
- If it is absolutely necessary to use an unscavenged N2O CSU
while awaiting proper scavenging modifications or before switching to a CO2
CSU, use it in an extremely well-ventilated area, such as an OR. Personnel should leave
the room as soon as possible after the procedure is completed and should be exposed to
such procedures as little as possible. In no case should pregnant staff members be
present during use of an unscavenged N2O CSU.
Pregnant patients are not at risk because they are not subject to chronic
- Disseminate this information to physicians on your staff who may use
CSUs in their private practices, particularly gynecologists, dermatologists,
otolaryngologists, and general surgeons.
American National Standards Institute (ANSI).
American national standard for anesthetic equipment—Scavenging systems
for excess anesthetic gases.
ANSI, 1982; ANSI publication no. Z79.11.
American Society of Anesthesiologists (ASA). Waste anesthetic gases in
operating room air: A suggested program to reduce personnel exposure. ASA Ad Hoc
Bruce DL, Bach MJ. Effects of trace concentrations of anesthetic gases
on behavioral performance of operating room personnel. Washington (DC): Dept. of
Health, Education, and Welfare (HSS); publication HEW (NIOSH) 76-169.
Bruley ME. A study of safety and performance requirements of
cryosurgical devices. Springfield (VA): FDA/BMDDP, National Technical Information
Service, 1980 Sep; publication no. PB 81 124943.
Cohen EN, Brown BW, Wu M, et al. Anesthetic health hazards in the
dental operatory [Abstract]. Anesthesiology 1979 Sep;51(suppl):S-254.
Department of Health and Human Services (DHHS):
NIOSH warns: Nitrous oxide continues to threaten health care workers [Update]. Cincinnati, OH: Publications
Dissemination, DSDTT, 1994 May 25, 2 p.; DHHS (NIOSH) publication no.
Request for assistance in controlling exposures to nitrous oxide during
anesthetic administration [Alert].
Cincinnati, OH: Publications Dissemination, DSDTT, 1994 Apr, 11 p; DHHS
(NIOSH) publication no. 94-100.
National Institute for Occupational Safety and Health (NIOSH):
NIOSH pocket guide to chemical hazards. Department of Health and Human Services (NIOSH), 1994
Occupational exposure to waste anesthetic gases and vapors: Criteria
for a recommended standard. Washington (DC):
Dept. of Health, Education, and Welfare (HSS), 1977; DHEW (NIOSH)
publication no. 77-140.
OSHA enforces nitrous oxide recommendation that has not been brought out
as a standard. Hosp Week 1979 Aug 24;15:2.
Whitcher C, Piziali R, Sher R, et al. Development and evaluation of
methods for the elimination of waste anesthetic gases and vapors in hospitals.
Washington (DC): Dept. of Health, Education, and Welfare (HSS), 1975; HEW (NIOSH)
publication no. 75-137.
Wray RP. Cryoprobe leakage of nitrous oxide into operating room air. Anesthesiology
of N2O in Physicians' Offices*
Room Concentration,** ppm
Area Concentration,** ppm
After 30 Min
After 60 Min
After 30 Min
After 60 Min
* Bruley ME. A study of
safety and performance requirements of cryosurgical devices. Springfield, VA:
FDA/BMDDP, National Technical Information Service, 1980 Sep; publication no. PB 81 124943.
** Measured in center of room, 1.25 m (4 ft) above floor. Measurements
*** Data point could not be obtained.
- Cryosurgical Units [18-051]
- Cryosurgical Units, General-Purpose [11-067]
- Cryosurgical Units, Ophthalmic [11-068]
Cause of Device-Related Incident
Device factor: Design/labeling error
External factor: Medical gas and vacuum supplies
Support system failures: Error in hospital
policy; Lack or failure of incoming and pre-use inspections; Poor prepurchase
Mechanism of Injury or Death
Exposure to hazardous gas