ACGIH threshold limit value-time-weighted average (TLV-TWA) refers to the time-weighted average airborne concentration of a substance, for a normal 8-hour workday and a 40-hour workweek, to which nearly all workers may be repeatedly exposed, day after day, without adverse effect.

Adjustable Pressure-Limiting (APL) Valve, also known as a "pop-off" valve, is a user-adjustable valve that releases gases to the atmosphere or a scavenging system and is intended to provide control of the pressure in the breathing system. The volume of gas above that needed to achieve the required patient pressure is vented.

Air is the elastic, invisible mixture of gases (chiefly nitrogen and oxygen) that may be used with medical equipment; also called medical air.

Anesthesia machine is equipment intended for dispensing and delivering anesthetic gases and vapors into a breathing system.

Anesthesia system is any of a variety of assemblies designed to administer an anesthetic.

Anesthetic agent is a drug that is used to reduce or abolish the sensation of pain, e.g., halothane, enflurane, isoflurane, desflurane, sevoflurane, and methoxyflurane.

Anesthetic agent vapor is the gaseous phase of an anesthetic agent that is normally a liquid at room temperature and atmospheric pressure.

Anesthetic gas is any gaseous substance, e.g., nitrous oxide, used in producing a state of anesthesia.

Anesthetic vaporizer is a device designed to facilitate the change of an anesthetic from a liquid to a vapor.

Anesthetizing location is any area in a facility where an anesthetic agent or drug is administered in the course of examination or treatment. This includes operating rooms, delivery rooms, emergency rooms, induction rooms, and other areas.

Area sample is a sample collected at a fixed point in the workplace. The data from the area sample may or may not correlate with an individual’s personal sample results due to the often high degree of variability in exposures.

Breathing system is a gas pathway in direct connection with the patient's lungs, through which gas flow occurs at respiratory pressures, and into which a gas mixture of controlled composition may be dispensed. The function of the breathing system is to convey oxygen and anesthetic gases to the patient's lungs and remove waste and anesthetic gases from the patient's lungs. Scavenging equipment is not considered part of the breathing system. The system is also referred to as breathing or patient circuit, respiratory circuit or system.

Breathing system, semiclosed is a system that allows some of the expired gases to leave the circuit; the remainder mixes with the fresh gases and is reinhaled. A CO2 absorber is used in this system.

Breathing tubes are large-bore, nonrigid tubes composed of rubber or plastic and used in most breathing systems to convey gases to and from the patient's airway. They are usually corrugated to prevent obstruction due to kinking and increase flexibility.

Breathing zone is defined as the area immediately adjacent to the employee’s nose and mouth; a hemisphere forward of the worker’s shoulders with a radius of approximately 6 to 9 inches.

Calibrated vaporizer is an instrument designed to facilitate the change of a liquid anesthetic into its vapor and to add a controlled amount of this vapor to the fresh gas flow.

Carbon dioxide (CO2) is a colorless, odorless gas, and is a normal end product of human metabolism. It is formed in the tissues and eliminated by the lungs.

Carbon dioxide absorber is a device used to remove CO2 chemically from exhaled patient gas. Primarily used in the closed or semiclosed circle breathing system, which requires carbon dioxide absorption to make reinhalation of previously exhaled gas possible.

Carcinogenicity is the ability of a substance to cause cancer.

Check valves are also known as unidirectional valves, one-way valves, and inspiratory and expiratory valves (refer to definition of unidirectional valve).

Common (fresh) gas outlet is the port through which the mixture of gases and vapors dispensed from the anesthesia machine is delivered to the breathing system. Also referred to as the machine outlet.

Compressed gas is defined as any material or mixture having in the container an absolute pressure exceeding 40 psig at 70°F or having an absolute pressure exceeding 104 psig at 130°F.

Connectors are fittings intended to join together two or more components.

Cylinder supply source is a cylindrical-shaped tank that is color-coded and pin-indexed or Compressed Gas Association (CGA) valve-specific and used to contain a specified medical gas. It supplies compressed gas to the anesthesia machine if a pipeline supply source is not available or if the pipeline fails. Cylinders range in size from B (smallest) to H (largest).

Cylinder pressure gauge monitors the pressure of gas within a cylinder.

Diameter Index Safety System (DISS) provides threaded noninterchangeable (gas-specific) connections for medical gas lines at pressures of 200 psig or less to minimize the risk of misconnection.

Epidemiology is the study of health and illness in human populations. It is the study of trends and events in similar populations, for example, one exposed to a chemical and one not exposed.

Excess gases are those gases and anesthetic vapors that are delivered to the breathing circuit in excess of the patient’s requirements and the breathing circuit’s capacity. These gases are released from the breathing circuit via the APL or pop-off valve or the ventilator pressure relief valve and are ultimately removed from the breathing circuit by the waste gas scavenging system.

Exhalation check valve, also known as expiratory unidirectional valve, refers to that valve placed in the vicinity of the CO2 absorber that ensures that exhaled gases flow away from the patient and into the absorber.

Flow control valve, also known as the needle valve, controls the rate of flow of a gas through its associated flow meter by manual adjustment of a variable orifice.

Flowmeter is a device that measures and indicates the flow rate of a gas passing through it.

Gas is defined as a formless fluid that expands readily to fill any containing vessel, and which can be changed to the liquid or solid state only by the combined effect of increased pressure and decreased temperature.

Gas-tight seal is a connection that does not allow bubbling when immersed in water and subjected to a differential pressure.

General anesthesia is a state of unconsciousness in which there is an absence of pain sensation.

Hanger yoke is a device used to attach a reserve gas cylinder to the anesthesia machine. The functions of the hanger yoke are to orient and support the cylinder, provide a gas-tight seal, and ensure a unidirectional flow of gas into the machine. It is pin-indexed according to a gas-specific safety system in order to prevent the connection of a cylinder of one gas to a yoke intended for another.

HVAC system, also known as the heating, ventilating, and air conditioning system, supplies outdoor replacement (make-up) air and environmental control to a space or building. It conditions the air by supplying the required degree of air cleanliness, temperature and/or humidity.

Inhalation check valve, also called inspiratory unidirectional valve, refers to the valve placed in the vicinity of the CO2 absorber that ensures that the gases flow toward the patient.

Medical gas is any gaseous substance that meets medical purity standards and has application in a medical environment. Examples are oxygen, nitrous oxide, helium, air, nitrogen, and carbon dioxide.

Medical gas mixture is a mixture of two or more medical gases to be used for a specific medical application.

NIOSH RELs (recommended exposure limits) are occupational exposure limits recommended by NIOSH as being protective of worker health and safety over a working lifetime. These limits are generally expressed as 8- or 10-hour TWAs for a 40-hour workweek. The REL may also be expressed as a short-term (TWA) exposure limit or a ceiling limit.

Nitrous oxide (N2O) is used as an anesthetic agent in medical, dental, and veterinary operatories. It is a weak anesthetic with rapid onset and rapid emergence. In hospitals, it may be used with oxygen as a carrier gas for other, more potent anesthetics. In dental offices, it is administered with oxygen, primarily as an analgesic (an agent that diminishes or eliminates pain in the conscious patient) and as a sedative to reduce anxiety.

Nonrecirculating ventilation system takes in fresh outside air and processes it by filtering and adjusting the humidity and temperature. The processed air is circulated through the various rooms in a facility, and then all of it is exhausted to the atmosphere. Whatever volume of fresh air is introduced into a room is ultimately exhausted outdoors.

Occupational exposure to waste anesthetic gases includes exposure to any inhalation anesthetic agents that escape into locations associated with, and adjacent to, anesthetic procedures. Such locations include, but are not limited to, operating rooms, delivery rooms, recovery rooms, and dental operatories.

Oxygen (O2) is an element which, at atmospheric temperatures and pressures, exists as a colorless, odorless, tasteless gas. Its outstanding properties are its ability to sustain life and to support combustion. Although oxygen is nonflammable, materials which burn in air will burn much more vigorously and create higher temperatures in oxygen or oxygen-enriched atmospheres.

Oxygen flush valve is a separate valve designed to rapidly supply a large volume of oxygen to the breathing system.

PACU (post anesthesia care unit) is also known as the recovery room.

Patient end is the end of the component part nearest the patient.

PEEP valve is a device installed in the exhalation limb of the patient circuit that allows positive end-expiratory pressure to be delivered to the patient's airway and adjusted as needed.

Personal sample is a sample collected from an individual’s breathing zone.

Pin Index Safety System is a safeguard to eliminate cylinder interchanging and the possibility of accidentally placing the incorrect gas on a yoke designed to accommodate another gas. Two pins on the yoke are so arranged that they project into the cylinder valve. Each gas or combination of gases has a specific pin arrangement.

Pipeline supply source is a permanently installed piped distribution system that delivers medical gases such as oxygen, nitrous oxide, and air to the operating room.

Pneumatic means pertaining to or operated by air or other gas under pressure.

Power outlet is an accessory outlet located on an anesthesia machine that supplies a driving gas for auxiliary equipment such as a ventilator. Driving gas is normally oxygen, but medical air may be used.

Pressure relief valve is a mechanical device that eliminates system overpressure by allowing the controlled or emergency escape of liquid or gas from a pressurized system. The relief valve may or may not be adjustable.

PSIG stands for pounds per square inch gauge, which is the difference between the measured pressure and surrounding atmospheric pressure. Most gauges are constructed to read 0 at atmospheric pressure.

Recirculating ventilation system returns part of the exhaust air to the air supply duct. The system takes in an amount of fresh outside air that varies as a function of the outside temperature. Air exhausted from a room is filtered for particulate matter and bacteria, not anesthetic gases, and then recirculated through several rooms by means of a common mixing (plenum) chamber. In this process, some fresh air is added and an equal amount of recirculating air is exhausted.

Recovery room is the patient care location where recovering patients are awakened and stabilized and/or awakened after surgical anesthesia. Anesthetic gases are exhaled by recovering patients (who received inhalation anesthetics) as they breathe.

Reservoir bag is also known as the respiratory bag or breathing bag. It allows accumulation of gas during exhalation so that a reservoir is available for the next inspiration. It provides a means whereby anesthesia personnel may assist or control ventilation. It can serve, through visual and tactile observation, as a monitor of a patient’s spontaneous respirations and acts to protect the patient from excessive pressure in the breathing system.

Respiration is the process by which a rapid exchange of oxygen and carbon dioxide takes place between the atmosphere and the blood coming to the pulmonary capillaries. Oxygen is taken up, utilized in metabolic processes, and a proportional amount of carbon dioxide is released.

Scavenging is defined as the collection of excess gases from the breathing circuit and removal of these gases to an appropriate place of discharge outside the working environment.

Scavenging system is defined as a device (assembly of specific components) that collects and removes the excess anesthetic gases that are released from the breathing circuit. Scavenging systems are also called evacuation systems, waste anesthetic gas disposal systems, and excess anesthetic gas-scavenging systems.

Source-control technology is an engineering control designed to collect and remove excess anesthetic gases at the point of origin (i.e., from the breathing circuit or in close proximity to the patient’s mouth and nose). It can be either a scavenging system or local (auxiliary) exhaust ventilation system.

Source sample is a sample collected at the origin of contamination (source of emission).

Tracheal tube also called the endotracheal tube, intratracheal tube, and catheter is inserted into the trachea and is used to conduct gases and vapors to and from the lungs.

TWA is a time-weighted average concentration. It is a way of expressing exposure such that the amount of time spent exposed to each different concentration level is weighted by the amount of time the worker was exposed to that level.

Unidirectional valve is a valve that allows gas flow in one direction only. Two unidirectional valves are used in each circle system to ensure that the gases flow toward the patient in one limb of the circle breathing system and away in the other. They are usually part of the absorber assembly.

Vapor is the gaseous phase of a substance which at ordinary temperature and pressure exists as a liquid.

Ventilation is (1) the physical process of moving gases into and out of the lungs. (2) It is also defined for the purposes of industrial hygiene engineering as a method for providing control of an environment by strategic use of airflow. The flow of air may be used to provide either heating or cooling of a work space, to remove a contaminant near its source of release into the environment, to dilute the concentration of a contaminant to acceptable levels, or to replace air exhausted from a space.

Waste anesthetic gases are those gases that are inadvertently released into the workplace and/or can no longer be used. They include all fugitive anesthetic gases and vapors that are released into anesthetizing and recovery locations, from equipment used in administering anesthetics under normal operating conditions, as well as those gases that leak from the anesthetic gas scavenging system, or are exhaled by the patient into the workplace environment. Waste gases are also those excess gases in the breathing circuit that are ultimately scavenged. Spills of liquid anesthetic agents also contribute to ambient levels of waste gases. Waste anesthetic gases may include N2O and vapors of potent inhaled volatile anesthetic agents such as halothane, enflurane, isoflurane, desflurane and sevoflurane.