Gas Properties

Knowing the gas density, the flammable range, and toxicity of the gas will help in understanding the hazards associated with the gas.

Gas density is defined as the relative weight of a gas compared to air, which has an arbitrary value of one. If a gas density is less than one it will generally rise in air. If the gas density is greater than one it will generally sink in air.

Some gases such as carbon dioxide can cause you to suffocate by displacing oxygen in the air. This is referred to as a simple asphyxiant. A simple asphyxiant by itself is not toxic but can be hazardous because of the oxygen deficient atmosphere that it may create. Oxygen deficient atmospheres contain less than 19.5% oxygen. Simple asphyxiants are especially dangerous in confined and enclosed spaces because the gas gets trapped and has nowhere to go.

Chemical asphyxiants reduce the body's ability to absorb, transport, or utilize inhaled oxygen. They are often toxic at very low concentrations (a few ppm). Chemical asphyxiants include carbon monoxide and hydrogen sulfide.

When flammable gases and vapors are mixed with air in the proper proportions in the presence of a source of ignition, an explosion can occur. The proper proportion is called the flammable range. The flammable range includes all concentrations of flammable vapor or gas in air, in which a flash will occur or a flame will travel if the mixture is ignited. The lowest concentration of a gas or a vapor in air capable of producing a flash of fire in presence of an ignition source is referred to as the lower explosive limit (LEL). The highest concentration of a gas or a vapor in air capable of producing a flash of fire in presence of an ignition source is referred to as the upper explosive limit (UEL). These are usually expressed in terms of percentage by volume of vapor in air.

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