Hazards of Anesthetic Gases
American Association of Zoo Veterinarians Conference 1998
Dean H. Riedesel, DVM, PhD, DACVA
Department of Veterinary Clinical Sciences, College of Veterinary Medicine, Iowa State University, Ames, IA, USA


That portion of the fresh anesthetic gas delivered through the breathing circuit which is not metabolized by the patient and exits through the pop-off valve is called waste anesthetic gas. This waste anesthetic gas has been associated with numerous health problems in individuals that chronically inhale polluted air: hepatic disease, renal disease, immunosuppression, bone marrow depression, abortion, infertility, birth defects, cancer, pruritus, and a wide range of central nervous system (CNS) disturbances. It is advisable to minimize or eliminate exposure to anesthetic pollution for two reasons: 1) health hazard and 2) National Institute of Occupational Safety and Health (NIOSH) requirements. This paper will outline the hazards and requirements for waste anesthetic gas.


The first report of health hazards associated with anesthesia personnel was in 1967.20 Vaisman’s survey of 15% of the Russian anesthesiologists practicing at that time revealed a high incidence of spontaneous abortion among the females and headache, fatigue, irritability, nausea, and pruritus in the entire group.

Cohen et al.3 found an increased incidence of miscarriage among operating room nurses. Further reports recorded increased frequency of spontaneous abortion, congenital defects, and cancer in female anesthesiologists and nurse anesthetists.4,5,10 However, a conflicting report published in 1979 found no significant incidence of reproductive abnormalities in female operating room workers.8

The National Institute of Occupational Safety and Health (NIOSH) initiated a study in the mid-1970s.6 The study identified an increased prevalence of congenital anomalies in offspring of male and female operating room personnel, increased spontaneous abortions in female workers but not in the wives of male workers. Women developed more hepatic and renal disease, while men developed only hepatic disease. Women operating room workers also had a higher incidence of leukemia and lymphoma.

In 1977, NIOSH made recommendations concerning exposure levels and monitoring methods.6 The recommended standards were arbitrarily set at 2 ppm for halogenated agents when used alone, 0.5 ppm when halogenated agents are used with nitrous oxide, and 25 ppm for nitrous oxide. Exact toxic levels were never determined. A subsequent study in the United Kingdom from 1977–1984 surveyed 11,520 female medical school graduates working in hospitals. Anesthesiologists were not found to have an increased risk of infertility, spontaneous abortion, congenital abnormalities, or cancer.13

Recently, the American Society of Anesthesiology sponsored a review of previously published studies.1 An increased incidence of spontaneous abortion in women working in the operating room was suggested, but the accuracy of the data was questioned. Further prospective studies were suggested to determine if trace anesthetic levels in the air breathed by operating room personnel leads to adverse health outcomes. Currently, with scavenging and other safety measures in place, it does not appear that there are proven hazards to personnel working in the operating room.14

Veterinarians are usually considered to be at less risk than human operating room personnel, because veterinarians spend less time per day in this potentially polluted environment.11,18 But veterinarians tend to perform quite a few mask and box inductions, plus monitor recovering animals confined in small cages and stalls.16,21 Thus, veterinarians may offset limited time of exposure by excessive levels of exposure.

Surveys of veterinary private practice and institutional surgery rooms revealed that breathing levels as high as 20–30 ppm were occasionally found, but levels were usually around 4–5 ppm.7,15,17,21,22 This is about double the NIOSH maximum recommendation but less than the 10 ppm found in most human operating rooms. Scavenging these waste gases made dramatic reductions possible. One surgery suite at the University of Georgia decreased from 36.6 to 0.85 ppm with active scavenging.7

Recovery rooms are also important for veterinarians, because our patients are frequently placed into small cages or stalls to wake up, and their exhaled gases are not efficiently removed. A study was done at Cornell University, measuring halogenated anesthetic levels at the cage door of recovering dogs.16 It took about 2 hours for halothane levels to decrease below the NIOSH maximum recommendation. A relatively large animal with large tidal volumes (5 L) in a small recovery stall, and a veterinarian or technician sitting at the head could lead to significant exposure levels.

Methods of Reducing Pollution and Exposure9,12,18

1.  Collection pop-off valves
Old pop-off valves have numerous holes that open directly into the room. Collection pop-off valves direct all the outflow to a single port (19 mm), and this type of pop-off should be installed on all machines. Nonrebreathing circuits have a lot of waste anesthetic gas, and bag-tail-end valves can be used for gas collection if the circuit does not have a collecting port. Most old ventilators cannot be retrofitted with collection devices and need to be replaced with models that have collection ports.

2.  Interface
An interface needs to be positioned between the disposal system and the pop-off valve. The interface contains positive (+10 cm H2O) and negative (-0.5 cm H2O) pressure relief valves so the animal will not be harmed if the disposal system malfunctions. A reservoir is an essential part of the interface also. This reservoir bag accepts large volumes of gas that exceed the disposal system capacity to immediately remove.

3.  Disposal system
Disposal systems vary and include active vacuum, ceiling or wall vents, direct passive connections to the outside, and charcoal filters. Venting to the floor is unacceptable. Passive systems have no pumps or fans and rely on the pressure difference from the pop-off to the outside air for the movement of gas. Large-bore tubing (clothes drier venting) reduces resistance, and short distances to the outside wall are preferred. Connection to a non-recirculating room vent is sometimes possible. But most building air handling systems are recirculating. It may be necessary to install a separate exhaust fan that connects to the attic or outside wall. Charcoal filters are effective if low flows are utilized and careful records are kept on utilization. Because of their expense and short effective life (12–15 hours), they should only be used when transporting animals or as a temporary solution. Active evacuation systems develop a negative pressure and exhaust to the outside atmosphere. Central surgical vacuums or dedicated systems for anesthetic gases can be used.

4.  Leak detection
Other than the gas coming out of the pop-off, there are several other sources of leakage from anesthetic machines. Always pressure test the machine and circuit for leaks before usage. Gas leaks around the soda lime canister are very common. The machine circuit should be maintainable at 30 cm H2O with an oxygen flow of less than 250 ml/minute, or the pressure drop should be less than 5 cm H2O in 30 seconds. Endotracheal tube cuffs are another common source of pollution. Check the cuffs ahead of time and allow time for slow leaks to be apparent.

5.  Inductions with inhalants
Avoid mask or chamber inductions if possible. When necessary, perform these procedures in large, well-ventilated rooms.

6.  Recovery
Let animals breathe 100% oxygen on the anesthetic machine for the first 5–10 minutes of recovery. Frequently empty the reservoir bag into the scavenging system and refill with the flush button.

7.  Filling vaporizers
Use a bottle adapter to fill vaporizers and fill the vaporizers when very few people are around (end of the day).

Monitoring Anesthetic Gases

Monitoring exposure levels can be done with badges that are worn for a day and then sent out for analysis. They are expensive to use routinely but may be valuable for pregnant employees or reoccurring situations that may be considered hazardous.18


There is definite potential for health hazards with exposure to anesthetic gases, and chronic exposure to anesthetic pollution should not be ignored. Install a scavenging system, maintain the anesthetic machine, and educate your employees on the safe administration of anesthetic gases.

Legally, employees have a “right to know” what hazardous chemicals they may use or encounter in the workplace and know how to protect themselves from the adverse effects of these chemicals. The anesthetic gases are considered hazardous chemicals and fall under the OSHA Hazard Communication Standard (HCS).14 This federal law, which was enacted in 1988, requires employers to keep a list of hazardous chemicals in the workplace, maintain a file of Material Data Safety Sheets (MSDS), and train their employees in the safe and proper use of these hazardous chemicals.2,19,23

Literature Cited

1.  American Society of Anesthesiology. 1974. Ad hoc committee report on effects of trace anesthetic agents on health of operating room personnel. Occupational disease among operating room personnel: a national study. Anesthesiology 41:321–340.

2.  American Veterinary Medical Association. 1990. Guide to chemical hazard communication. J Am Vet Med Assoc 196:224.

3.  Cohen, E.N., J.W. Belville, and B.W. Brown. 1971. Anesthesia, pregnancy, and miscarriage: a study of operating room nurses and anesthetists. Anesthesiology 35:345–347.

4.  Corbett, T.H., R.G. Cornell, K. Lieding, and J.L. Endres. 1973. Incidence of cancer among Michigan nurse anesthetists. Anesthesiology 38:260–263.

5.  Corbett, T.H., R.G. Cornell, J.L. Endres, and K. Lieding. 1974. Birth defects among children of nurse-anesthetists. Anesthesiology 41:341–344.

6.  U.S. Department of Health Education and Welfare, Public Health Service. 1977. Centers for Disease Control, National Institute for Occupation Safety and Health Criteria for a Recommended Standard Occupational Exposure to Waste Anesthetic Gases and Vapors. HEW Publication No (N105H), U.S. Government Printing Office, Washington, D.C.

7.  Dreesen, D.W., G.L. Jones, J. Brown, and C.A. Rawlings. 1981. Monitoring for trace anesthetic gases in a veterinary teaching hospital. J Am Vet Med Assoc 179:797–799.

8.  Ericson, A. and B. Kallen. 1979. Survey of infants born in 1973 or 1975 to Swedish women working in operating rooms during their pregnancies. Anesth Analg 58:302–305.

9.  Hartsfield, S.M. 1996. Anesthetic machines and breathing systems. In: Thurmon J.C., Tranquilli W.J., and Benson G.J., eds. Lumb & Jones’ Veterinary Anesthesia, 3rd ed. Baltimore: Lea & Febiger.

10.  Knill-Jones, R.P., L.V. Rodrigues, and D.D. Moir. 1972. Anaesthetic practice and pregnancy. Controlled survey of women anaesthetists in the United Kingdom. Lancet 1:1326–1328.

11.  Manley, S.V. and W.N. McDonell. 1980. Anesthetic pollution and disease. J Am Vet Med Assoc 176:515–518.

12.  Manley, S.V. and W.N. McDonell.1980. Recommendations for reduction of anesthetic gas pollution. J Am Vet Med Assoc 176:519–524.

13.  Maran, N.J., R.P. Knill-Jones, A.A Spence. 1996. Infertility among female hospital doctors in the UK. Br J Anaesth 76:581P.

14.  McGregor, D.G. 1997. Trace anesthetic gases in the operating room. ASA Newsletter 61:12–13.

15.  Milligan, J.E., J.L. Sablan, and C.E. Short. 1980. A survey of waste anesthetic gas concentration in U.S. Air Force veterinary surgeries. J Am Vet Med Assoc 177:1021–1022.

16.  Milligan, J.E., J.L. Sablan, and C.E. Short. 1982. Waste anesthetic gas concentrations in a veterinary recovery room. J Am Vet Med Assoc 181:1540–1541.

17.  Ruby, D.L., R.M. Buchan, and B.J. Gunter. 1980. Waste anesthetic gas and vapor exposures in veterinary hospitals and clinics. Am Ind Hyg Assoc J 41:229–231.

18.  Smith, J.A. 1993. Anesthetic pollution and waste anesthetic gas scavenging. Sem Vet Med and Surg 8:90–103.

19.  Tennyson, A.V. 1993. Reminder to veterinarians: observe OSHA requirement. J Am Vet Med Assoc 202:512.

20.  Vaisman, A.I. 1967. Working conditions in surgery and their effect on health of anesthesiologists. Eksp Khir Anesteziol 3:44–49.

21.  Ward, G.S. and R.R. Byland. 1982. Concentration of halothane in veterinary operating and treatment rooms. J Am Vet Med Assoc 180:174–177.

22.  Wingfield, W.E., D.L. Ruby, R.M. Buchan and B.J. Gunther. 1981. Waste anesthetic gas exposures to veterinarians and animal technicians. J Am Vet Med Assoc 178:399–402.

23.  Zuziak P. 1992. Ignoring business regulations can be hazardous to your practice. J Am Vet Med Assoc 201:379–380.


Speaker Information
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Dean H. Riedesel, DVM, PhD, DACVA
Department of Veterinary Clinical Sciences
College of Veterinary Medicine
Iowa State University
Ames, IA, USA

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