1.3 Medical Gases, Hazards & Permitted Uses

The Gases an Installer Pipes

NFPA 99 defines medical gas and medical support gas broadly. The provisions apply to piped systems for oxygen, nitrous oxide, medical air, carbon dioxide, helium, nitrogen, instrument air, and their mixtures. Where a rule names one gas, it applies only to that gas.

Medical support gases are used mainly to power equipment in patient care procedures. They may be piped to areas with a support purpose and, where appropriate, to laboratories. A support gas system may not carry oxidizing gases other than air, and it may not carry gases meant for patient or staff breathing.

Why Oxidizers Change the Rules

Oxygen and nitrous oxide do not burn, but they make other materials burn far more readily and intensely. NFPA 99 requires designers and installers to consider the fire and explosion hazards of positive-pressure gas systems and vacuum systems in design, installation, testing, operation, and maintenance. For central supply materials, the code requires the following:

• Oxygen-compatible construction wherever oxygen, nitrous oxide, their mixtures, or any mixture containing more than 23.5% oxygen will be handled, at the temperatures and pressures involved.
• No polymeric material in interconnecting hose that carries oxygen at gauge pressures above 435 psi (3000 kPa). Cylinder pressures are far higher than pipeline pressure.
• Low-temperature materials wherever components can see cryogenic temperatures.

Installers see the same hazard in daily work:

1. Oil, grease, and cutting lubricants inside a pipe are fuel. A sudden pressure rise (adiabatic compression) or a fast-moving particle can ignite them in an oxygen stream.
2. Oxide scale and chips left from brazing or cutting become particulates that travel to patient equipment.
3. A cross-connection, where one gas is piped to another gas's outlet, can deliver the wrong gas to a patient. That is why NFPA 99 requires gas-specific outlets, labeling, cross-connection tests, and gas concentration tests.

Quality Standards for the Gases

Medical air gets extra attention. It may come from cylinders, bulk containers, or medical air compressors, or it may be reconstituted from oxygen USP and oil-free, dry nitrogen NF. It must meet the medical air USP requirements, contain no detectable liquid hydrocarbons, and contain less than 25 ppm gaseous hydrocarbons. It may hold no more than 1 mg/m3 of permanent particulates sized 1 micron or larger.

Permitted Uses: Piping Patient Gases Only Where They Belong

Central supply systems for oxygen, medical air, nitrous oxide, carbon dioxide, and the other patient gases may be piped only to medical gas outlets. Those outlets must be in areas where licensed medical professionals direct the use of the gases for purposes consistent with:

1. Direct respiration by patients
2. Clinical application to a patient, such as carbon dioxide insufflation or purging heart-lung machine blood pathways
3. Medical device applications directly related to respiration
4. Power for medical devices used directly on patients
5. Calibration of the devices in items 1-4
6. Simulation centers for educating, training, and assessing health care professionals

Several related limits come up on exams:

• Medical air sources connect only to the medical air distribution system. They serve only human respiration and the calibration of respiratory devices. Medical air can never feed a shop air line.
• Support gas central supplies may not be piped to, or used for, anything except medical support applications.
• Medical-surgical vacuum and WAGD may not be used for nonmedical applications, such as steam condensate return.