Cause of Device-Related Incident
Device factors; External factors; Support system failures

Clinical Specialty or Hospital Department
Clinical/Biomedical Engineering; Dermatology; Obstetrics and Gynecology; OR / Surgery

Device Factors
Design / labeling error

Document Type
Hazard Reports

External Factors
Medical gas and vacuum supplies

Mechanism of Injury or Death
Exposure to hazardous gas

Support System Failures
Error in hospital policy; Lack or failure of incoming and pre-use inspections; Use of inappropriate devices

Tampering and/or Sabotage
*Not stated

User Errors
*Not stated

Cryosurgical Units [18-051]; Cryosurgical Units, General-Purpose [11-067]; Cryosurgical Units, Ophthalmic [11-068]

Nitrous Oxide (N2O) Cryosurgical Units Must Be Scavenged (Update)

Hazard Update [Health Devices Aug 1996;25(8):306-9]

Problem Summary

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[12]:293-4, October 1979, and 16[12]:407-9, December 1987).

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 exposure.

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.


  1. Determine whether you own an N2O CSU (some CSUs can be operated with either N2O or CO2).
  2. 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 system.
  3. 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.
  4. 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.
  5. 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.
  6. 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.
  7. 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 another risk.
  8. 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.
  9. 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 exposure.
  10. 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. New York: 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 Committee.

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. 94-118.

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 Jun.

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 1979 Sep;51(suppl):S-335.

Concentrations of N2O in Physicians' Offices*


Treatment Room Concentration,** ppm

Clerical Area Concentration,** ppm

Type of Practice


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 ±50 ppm.

*** 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 evaluation

Mechanism of Injury or Death

Exposure to hazardous gas

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