Dealing with Gases

The Department of Transportation’s Hazard Class Two contains all of the gases. There are many definitions of what exactly a gas is, but one of the easiest to understand is that used by Merriam Webster that states that a gas is “a fluid (such as air) that has neither independent shape nor volume but tends to expand indefinitely.” 


A gas will expand to fill the size of any container that it occupies and will continue to expand until acted upon by an outside force. Think of an expanding gas like a rising column of smoke. In a structure fire, smoke will expand and fill the room until visibility is reduced to zero. In an outside fire, the column of smoke will rise into the atmosphere until it is dissipated by the wind. Gases will act in much the same manner.

All gases can be divided into four basic categories: flammable, non-flammable, oxidizers and poisonous. As with all hazardous materials shipments, they are only required to be placarded to the highest hazard. Therefore a poisonous gas can also be flammable or vice-versa. Gases are always stored and shipped in some sort of pressurized container. It may be as small as a cylinder to power a pellet gun or as large as a railroad tank car or fixed facility tank containing hundreds of thousands gallons. Regardless of the size or contents of the container, the fact that the contents are under pressure must always be respected. To do anything less invites disaster.

In responding to hazardous materials emergencies, we must understand a few terms that pertain to gases. You do not need to be a Hazardous Materials Technician to understand these terms. In fact, we are just reviewing terms that you learned about when you took your Hazardous Materials Operations course. The first term relates to flammable gases and that is the Flammable Range. The Flammable Range is the area between the Lower Flammable Limit (LFL) and the Upper Flammable Limit (UFL). If the concentration of the flammable gas in air is less than the Lower Flammable Limit, the mixture is “fuel lean” and will not burn. There is too much oxygen and too little fuel for the mixture to burn. Conversely, if the concentration of the flammable gas in air is more that the Upper Flammable Limit, the mixture is “fuel rich” and also will not burn. There is too much fuel and too little oxygen for the mixture to burn. This situation is by no means to be considered safe. A sudden influx of fresh air from a ventilating system or open window could introduce enough oxygen to drop the mixture directly into the Flammable Range. This could have catastrophic consequences.

While the Flammable Range is associated with flammable gases, do not be lulled into a false sense of security if a material is placarded as a non-flammable gas. As hard as it may be to understand, some non-flammable gases CAN AND WILL burn! According to CFR49 173.115, in order for a gas to be placarded as flammable, one of the following two conditions must be met:

  1. The gas must be ignitable at 101.3 kPa (14.7 psi) when in a mixture of 13 percent or less by volume with air; or
  2. The gas must have a flammable range at 101.3 kPa (14.7 psi) with air of at least 12 percent regardless of the lower limit. A gas such as ammonia will burn, but it is not placarded as flammable because it has a relatively narrow Flammable Range of 16 to 25 percent.

The Vapor Density of the gas must also be known. The Vapor Density of regular air at sea level is one. Any gas with a Vapor Density greater than one will be heavier than air and will sink to low lying areas such as basements, sewers or storm drains. Any gas with a Vapor Density less than one will be lighter than air and will rise into the atmosphere. Depending on the product in question, this could create an extremely hazardous area of concern downwind. At best, the wind will dissipate the product until it is no longer a concern.

Most gases are heavier than air with only a few being lighter than air. If you can remember the simple acronym 4H MEDIC ANNA, you can remember the gases that are lighter than air. These lighter than air gases are Hydrogen, Helium, Hydrogen Cyanide, Hydrogen Fluoride, Methane, Ethylene, Diborane, Illuminating Gases, Carbon Monoxide, Acetylene, Neon, Nitrogen and Ammonia.

The Vapor Density and Flammable Range can easily be found on the Safety Data Sheet (SDS). The SDS can be found in fixed facilities or with hazardous materials shipments. In the era of smart phones, this information can easily be located by searching for the product name or United Nations Identification Number on any Internet search engine or a wide variety of hazardous materials related smart phone applications.

The primary hazard in dealing with any gas is a BLEVE (Boiling Liquid Expanding Vapor Explosion). Gases are pressurized to change them from a gaseous state to a liquid state for ease of storage and transportation. Any pressurized container that is exposed to excess heat will cause the liquid product to boil producing vapor. This vapor production will increase the pressure inside the container. If the pressure exceeds the rating of the pressure relief valve or design pressure of the container, a BLEVE will occur.

A BLEVE will cause the container, or the remains of the container, to rocket in any manner of directions. It is impossible to predict in which direction the container will travel, so don’t even try. Even the smallest container — think an aerosol can — will become a lethal missile traveling at high rates of speed. Make no mistake; being struck by one of these containers, or parts thereof, will kill you. Don’t become a statistic!

Poisonous gases present a great difficulty for first responders. They vaporize easily and very dangerous to life, even in small amounts.  They are also known to be so toxic to humans as to pose a hazard to health during transportation. Poison gases are also presumed to be toxic because of laboratory testing. When reviewing an SDS for a poisonous gas, you will see that the material was more than likely tested on laboratory animals such as mice or rats as it is illegal and unethical to test these materials on human beings. Even SCBA may not provide enough protection for the First Responder as these materials can easily be absorbed through the skin. Poisonous gases will always require the response of a Hazardous Materials Team with Level A Chemical Protective Equipment.

The last gases we will discuss are oxidizers. Oxidizers will not burn like a flammable gas but they will support combustion. Oxygen is a common oxidizer and can be found in many residential occupancies. A patient on home oxygen that suffers a structure fire in their residence will see the oxygen they depend on intensify the fire.

It is impossible to tell you how to respond to every gas emergency you may encounter during your career in the fire service. In order to keep your skills sharp, you must maintain proficiency with both Safety Data Sheets and the Department of Transportation’s Emergency Response Guidebook. Reading a placard and being able to interpret the SDS or the ERG could mean the difference between a successful incident and a lawsuit. It could literally be the difference between life and death.

Until next time, stay safe!

Mark Schmitt is a Captain/HazMat Specialist for the Greensboro Fire Department assigned to the Foam/ARFF Task Force and a veteran of 25 years in the fire service. The majority of his career has been spent in Special Operations. He holds a Master of Public Administration in Emergency Management and is a graduate of the National Fire Academy’s Executive Fire Officer Program. He has taught numerous hazardous materials courses for the Greensboro Fire Department, local community colleges and the North Carolina Office of the State Fire Marshal in addition to serving as a contract instructor with the National Fire Academy.

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