Simple asphyxiants are inert gases or vapors that displace oxygen from air when present in high concentrations. In low concentrations, they have no physiologic effects. Normally oxygen can be measured in air at 18% by volume or at a pO2 of 135 torr. Many simple asphyxiants have little or no warning properties. For some of the agents, the major hazard is their flammable or explosive properties (hydrogen, ethane, ethylene, acetylene, methane, propane, and propylene). [ACGIH] Simple asphyxiants include: H: Hydrogen, Helium; E: Ethane, Ethylene; N: Nitrogen, Neon; C: Carbon dioxide; A: Acetylene, Argon; M: Methane; and P: Propane, Propylene. [Harber, p. 523; Rom, p 556-60; LaDou, p. 558] When the O2 concentration at sea level is below 16%, subjects have decreased mental alertness, visual acuity, and muscular coordination. Loss of consciousness occurs at concentrations below 10%, and death occurs at concentrations below 6%. [AHLS, p. 38] Aliphatic hydrocarbon gases include methane (CH4), ethane (C2H6), propane (C3H6), butane (C4H10), isobutane (C4H10), and liquefied petroleum gas (LPG). LPG is a mixture mainly of propane, n-butane, and isobutane. These alkanes can cause CNS depression, cardiac sensitization, and simply asphyxiation. "Hypoxia, as well as the stress that may occur with high concentrations of these gases, can elevate epinephrine. Thus, the asphyxiant, CNS, and cardiac arrhythmia effects may act synergistically." At concentrations of 14% (140,000 ppm) and higher, alkanes reduce the concentration of oxygen from the normal of 21% to below the critical level of 18% needed for the heart and brain to function. [ACGIH] In this database, simple asphyxiants include the "inert" as well as the "relatively inert" gases like the aliphatic alkanes and the chlorofluorocarbons.