For many years Americans have enjoyed the news broadcasts and contemporary information that has been presented by Paul Harvey on the radio. In particular, one segment of his show that reveals very interesting facts is "The Rest of the Story". Harvey usually tells listeners much more behind the scene information than most people have heard on a certain topic. Usually, when the story is complete, the listener has a better understanding of why things are the way they are-the rest of the story, if you will.
This same concept also applies to hazardous materials response in many ways. One topic where the rest of the story information can actually increase responder safety is in the area of vapor density. Most responders have been taught a few things about vapor density but there is much more to the topic than what is usually included in most hazmat training sessions. As a matter of fact, vapor density is usually portrayed as a general concept but in the interest of safety a more comprehensive view is necessary. Therefore, precise computations concerning the concept of vapor density will better enable response personnel to respond safely to hazardous materials emergencies and to conduct more effective risk assessments.
Vapor density is defined as the relative weight of a gas or vapor compared to air, which has an arbitrary value of one. If a gas has a vapor density of less than one it will generally rise in air. If the vapor density is greater than one the gas will generally sink in air.
This concept is important for responders because it will indicate where the gas or vapors can generally be expected to be located at hazmat releases. Accordingly, responders can better select their staging and equipment set-up areas as well as the hazard isolation zones for the incident. Additionally, knowledge of where the gas may be found can better define the monitoring instrument strategy at each incident.
The only problem is the concept of vapor density, which has also been termed "specific gravity of vapor", offers only a vague value of where the gas or vapor may travel. In other words, response guidebooks such as the Department of Transportation Emergency Response Guidebook (ERG) which state that "vapors may be heavier than air and found in low areas" do not indicate how heavy the vapors will be. To be more precise, then, vapor densities can be calculated.
Density of Air
Air is a complex mixture of several gases with nitrogen and oxygen being the most prevalent. The composition of air at sea level by weight is;
- Nitrogen - 75.53%
- Oxygen - 23.16%
- Others - 1.31% (Argon, carbon dioxide, neon, helium, methane, krypton, nitrous oxide, hydrogen, xenon, ozone)
Air, by volume, is composed of the following gases;
- Nitrogen - 78.00%
- Oxygen - 20.95%
- Argon - .93%
- CO2 - .033%
- Others - < .003%
Additionally, air has a molecular weight of 29 atomic mass units (amu's) at sea level. In essence, this is the weight of a sample of air and which can be used for comparison purposes with other gases and vapors.
Density of Gases
Given a material's identity its molecular weight can be calculated by its chemical formula and in conjunction with a periodic table of elements. All atoms have mass and weight is the attraction of mass by gravity. For our purposes we refer to the mass of a compound as it's weight. Finding the chemical formula for a compound and adding the weight of all of its atoms can calculate molecular weight. For instance the molecular weight of anhydrous ammonia is 17 because the formula is NH3 where one atom of nitrogen (N) is 14 amu's and three atoms of hydrogen (H) is 3 amu's.
Computing Vapor Density
To compute a compound's vapor density simply divide the molecular weight of the compound by the molecular weight of air. This will provide a numerical value that can be compared to air's value of one.