WASTE GASES

The central points concerning waste gases are that anesthetic vapors do enter the operating room atmosphere, their concentrations may be well controlled with proper scavenging, and although some data suggest that chronic inhalation of trace concentrations of anesthetics may be harmful, these studies are inconclusive.

Concentrations of waste gases are usually reported on a volume-per-volume basis in parts per million (ppm). For example, 100% halothane, such as in the saturated vapor above the liquid in a bottle or vaporizer, has a concentration of 1 million ppm. Similarly, 1% halothane represents 10,000 ppm. Waste nitrous oxide and halogenated anesthetics in the absence of scavenging may approach concentrations of 3000 and 50 ppm, respectively.^[1] The National Institute for Occupational Safety and Health (NIOSH) has recommended that the upper limits in the atmosphere of operating rooms be 25 ppm for nitrous oxide and 2 ppm for halogenated anesthetics (or 0.5 ppm for halogenated anesthetics used in combination with nitrous oxide).^[2] To put these figures in perspective, assuming that 1 mL of a volatile liquid anesthetic produces 200 mL of vapor, that volume of liquid spilled in a closed room measuring 20 by 20 by 9 feet results in a concentration of vapor of nearly 2 ppm. It is important to recognize that the NIOSH recommendations refer to a time-weighted average over the duration of exposure to the gas. In practice, the NIOSH recommendations are nearly impossible to achieve, although proper scavenging methods decrease concentrations by about 90%.^[1] The maximal concentration of halothane

3152

The universal use of scavenging systems in the operating room can lead to a false sense of security among operating room personnel. Although theoretically the scavenging of anesthetic gases should reduce contamination to levels below the NIOSH recommendations, it is clear that these levels are not consistently met in the routine practice of anesthesia. Mask inductions, circuit disconnects, and the use of laryngeal mask airways and cuffless endotracheal tubes all contribute to operating room contamination. Hence, exposure to anesthetic gases is more common in pediatric anesthesia, where mask inductions and uncuffed endotracheal tubes are the standard of care. Introduction of the laryngeal mask to anesthesia practice has increased operating room exposure to anesthetics in adult cases as well. One study of inhaled induction and maintenance with sevoflurane and nitrous oxide in adults resulted in violations of NIOSH standards 50% of the time.^[4]

The proper use of scavenging systems is essential. Anesthesia machines are not equipped to recognize unconnected scavenging systems, so failure of the system is not readily apparent. Kanmura and colleagues studied 402 anesthetic cases and found abnormally high ambient concentrations of N₂ O to be the result of mask ventilation (42%), unconnected scavenging system (19.2%), leak around pediatric endotracheal tubes (12.5%), and equipment leakage (11.5%). All scavenging system disconnects were due to human error rather than equipment failure.^[5] In this study a failed scavenging system produced higher contamination levels than did an inhaled induction.

Health Risks

Until the late 1960s, there was little concern about the potential for adverse effects on operating room personnel from inhaling trace concentrations of anesthetic gases and vapors. The decade that followed was marked by a flourish of interest. The inaugural report suggesting that there was a problem appeared in 1967 in the Russian literature.^[6] A multitude of retrospective reports followed, each suggesting that trace concentrations of anesthetics presented considerable risk to operating room personnel. Of these, three large studies conducted during the mid-1970s in the United States and the United Kingdom all concluded that the prevalence of abortion was substantially higher in female anesthesiologists than female physicians working outside the operating room. The incidence of congenital anomalies in children of male and female anesthesiologists was higher than in the control groups of physicians. Although the prevalence of malignancy was similar in both anesthesiologists and controls, liver disease was reported more often by male anesthesiologists.^[7] The authors of each of the reports recognized that problems were associated with reporting retrospective data; nevertheless, their observations provoked significant concern among anesthesiologists. Some readers may have discounted the possibility that other environmental factors in the operating room, such as radiation, stress, and contact with organic chemicals, might also be culpable.

The reports stimulated the appearance of numerous review articles, many of which critiqued the original data and reinterpreted them. Methodologic concerns, including the retrospective cohort design based on questionnaires completed by each respondent, were prominent in these critiques. Biases may have arisen for many reasons, including inadequate response rates; lack of control for confounding variables, such as age, nutrition, obstetric history, drinking, smoking, exposure to methylmethacrylate, and radiation; lack of quantification of the exposure to anesthetic vapors; losses to follow-up; inappropriate comparison groups; and lack of independent verification of the reported data.^{[8] [9]} Even the titles of some questionnaires, for example, "Effects of Waste Anesthetics on Health" and "Anesthetic Practice and Pregnancy," may have introduced bias by stimulating respondents who work with anesthetics to over-report their past experiences.^{[10] [11]} In any study of this type, self-reported historical information and disease outcomes are certain to be biased if the respondents have a preconceived idea of its goals.

In the late 1990s a Task Force on Trace Anesthetic Gases was convened by the American Society of Anesthesiologists (ASA) Committee on Occupational Health of Operating Room Personnel. The task force analyzed data from all of the available epidemiologic studies at the time and concluded that there was no proof of an association between occupational exposure to waste anesthetic gases and adverse health effects. In addition to the analysis of retrospective studies, their report, published in 1999, cited interim data from the only prospective survey in progress. This study monitored 11,500 female physicians in the United Kingdom and documented occupation, work practices, lifestyle, and medical and obstetric history, as well as hours of exposure and the use of scavenging equipment. This preliminary report showed the incidence of infertility, spontaneous abortion, and children with congenital abnormalities in female anesthesiologists to be the same as that in other physicians.^[12]

Even though no firm evidence suggests that trace concentrations of anesthetic gases present a health hazard, there is no definite proof to the contrary. Perhaps for this reason the Occupational Health and Safety Administration (OSHA) requires compliance with its Hazard Communication Standard on toxic and hazardous substances (29 CFR Part 1910.1200). This standard requires that operating room and recovery room personnel take courses on the potential for harm from these anesthetics. In any case, it makes sense to minimize our exposure to waste gases by exercising care when filling vaporizers with volatile anesthetics and preventing gases from venting into the operating room.^[13]