Releases in still air form fog-like clouds. The combination vapor-and-fume cloud in this photo was caused by the heat of neutralization of a hydrofluosilic acid spill. The leak is being stopped by the author using a wood plug driven into the broken pipe nipple on the bottom nozzle flange of the 10,000-gallon tank. Vapors from flammable liquids such as gasoline form similar fog-like clouds, but they are not visible. In addition, heavier-than-air vapors flow downhill and collect in low spots.

(Photo courtesy of Chesterfield, Missouri Fire Protection District.)

The terminology emergency responders use to describe a haz-mat incident must be accurate. It is the basis for all communication on the scene. Responders must learn the correct terms to successfully communicate with industry representatives and haz-mat specialists. Words such as vapor, fume, and smoke have specific meanings and convey precise information. Responders who aren’t accurate in their descriptions could cause delay in mitigation anti increase risk to themselves and the community. Accuracy is especially important in an official report, which may be reviewed by the media or the courts.

The Chemical Manufacturers Association committee that produced the training video on the DOT’S Emergency Response Guide found that firefighters and industry representatives need to learn to more accurately describe emergency conditions. One critical area of concern is airborne releases.

The following definitions are a result of the CMA committee’s efforts. Included are general statements for situations of unknown origin and very specific descriptions for identified materials as well as sample statements and descriptions of release shapes.


Chemical cloud—An airborne mass of similar material from a chemical release; also referred to as an airborne release; not always visible. A chemical cloud can result from a release of dust, gas, smoke, vapor, mist, or fumes. The term chemical cloud can refer to a release before its origin is identified.

Cone-shaped cloud—a chemical cloud moving away from its source in a breeze or wind and resembling a cone; also referred to as a feather or plume-shaped cloud. Chemical clouds from any source (dust, gas, fumes) are carried along by the wind. If the velocity is high enough, this cloud will form a cone shape, expanding as it moves away from the release.

hog-shaped cloud— a slowor nonmoving release with no particular shape, usually lying close to the surface. Normally it is visible, but the term may be used to describe the distribution of any release, such as from a nonvisible gas or vapor. Under certain weather conditions such as temperature inversion, a cone-shaped cloud may stop moving and spread out into a fog pattern.


Vapor— a dispersion of molecules from a liquid in air that increases in concentration as the temperature rises (for example, gasoline, water, acetone, isopropyl alcohol). Vapor concentrations in the flammable range of liquids that burn result from temperatures higher than the flash point of the liquid. At temperatures above 300° to 400° F vapors are also distilled from solids such as wood and plastics.

Fume— the characteristic smoky appearance and choking cloud resulting from the release of fuming materials such as highly reactive liquids, gases, or molten metal (for example, concentrated hydrochloric acid and sulfur monochloride). Fuming corrosive materials produce dense, choking, smoke-like emanations on contact with the moisture in air. Some liquefied gases that hydrolyze (react with water) when they evaporate are also said to fume (for example, anhydrous hydrogen fluoride and anhydrous hydrogen chlorides). Fumes from hot or molten metals may not have a dense, smoke-like appearance but are hazardous, usually by inhalation.

Mist — a dispersion of fine droplets from a liquid in air resulting from entrainment, spray atomization, or condensing of a material as its vapor cools. Mists also are referred to as aerosols.

Although a liquid may be cooler than its flash point, its mist still can be ignited because of its finely divided nature. Even mists from such high flash liquids as safety hydraulic fluid are ignitable when leaks occur in high-pressure lines.

Gas—a very even dispersion of molecules of a material above its boiling point (at ambient temperature) with the ability to occupy a space with uniformity. Typical gases are oxygen, nitrogen, chlorine, and carbon dioxide. (Note: Gases that form corrosive clouds on contact with the moisture in the air, such as hydrogen fluoride and hydrogen chloride, are said to fume.)

Dust—finely divided solid particles suspended in air or at rest. These may have damaging effects on the environment, may be dangerous by inhalation or skin contact, and frequently constitute an explosion hazard when dispersed in air. Settled dusts may be redistributed when disturbed by hose streams, collapsing structures, and wavefronts from fires, detonations, wind, or cleaning procedures.

Dust clouds, which at the least are irritating, behave much like vapor, gas, and fume clouds, except that they are heavier than air and will settle in still air. Fine particles of combustible dust such as this ink pigment are a double hazard. When dust is disturbed by hose streams, as happened at this fire, an explosion is very possible. In this case small puffs were noticed as water was applied to smoldering bags of pigment, and water application was halted until the dust settled. Extinguishment was completed with low-velocity sprays.The smoke in this photograph, which includes lighter-than-air gases, solid particles, and many heavier-than-air vapors, rises straight up in the still air of this winter morning. The heavier-than-air components eventually will settle but are carried up initially by the tremendous heat from the fire. (Fifty gallons of diesel fuel releases more than 500,000 Btu's.) Lighter-than-air gases such as methane, the principal component of natural gas, behave like the smoke, except that they are not driven by as much heat energy. (Photos and those that follow by author.)

Smoke—a mixture of gases, suspended solid particles, and vapors resulting from the combustion process. Color varies from thick black for hydrocarbon fires to light gray for wood smoke burning in an air-rich environment. Class A (wood/paper) fires release a rich yellow-to-brown smoke in air-lean environments such as basements or concealed spaces. Hazardous components may include varying percentages of HC1, oxides of nitrogen, sulfur compounds, acrolein, and free radicals.


Near-field effect: The effect of obstructions such as buildings, vehicles, and hills on the behavior of an airborne release. Near-field effect can cause a release to move 180° to the “far-field” wind direction or cause a stagnant condition resulting in a foglike cloud. Near-field winds are unpredictable and can shift drastically in speed and direction with only a slight shift in the far-field wind.

Strong breezes affect all releases the same way, even those driven by high heat energy, such as the smoke from this three-day dump fire. At 10 mph, smoke from a stack leaves at a 90-degree angle. Far-field plumes tend to rise and fall in an undulating fashion as they cross large spaces. Normal horizontal shifts in wind direction will affect the near-field cloud, but the far-field plume will track down the center of the variations. Only a complete change in the average dirertion, such as occurs with a weather front, will have significant influence on the far-field plume.

Scrubbing: The process of removing a contaminant from a gas (usually air) by passing the gas through a liquid spray, usually water. Scrubbers used in the chemical industry are long, vertical ducts partially filled with packing—generally a plastic material that resembles a tennis ball with a hole in it. The liquid is sprayed down while the gas flows up.

Emergency scrubbing of airborne releases involves the application of water sprays from fixed or portable nozzles to reduce the impact of the release on the community by removing some of the harmful contents.

Near-field effed is very evident at this liquefied petroleum gas incident in Eureka, Missouri, where a fanner damaged an eight-inch pipe while working his field. As the liquid escapes, some of it flashes to a gas, absorbing heat from the atmosphere. This cold environment keeps some of the material in the liquid state and also condenses water from the air. These vapors form the visible cloud, but most of the liquefied petroleum is now an invisible explosive gas cloud extending much farther. The release is in an open field just beyond the tree line. The wind is steady from left to right. As some of the cloud drifts into the trees, the near-field effed causes the gas to reverse diredion 180 degrees and move upwind!

CAUTION: The overspray and runoff will contain some of the scrubbed material and may be corrosive, toxic, and so on. Also keep in mind that many materials are water reactive— so exercise great care that scrubbing water does not enter containers or pooled material.


The following are some examples of how to incorporate these definitions in verbal and written reports.

  • The drum of 37 percent hydrochloric acid was leaking slowly at the bottom chime. The resulting fumes formed a fog around the rear of the trailer. As the fog rose, a crosswind pushed the cloud over the motel in a 15-degree cone.
  • The gasoline spill in the basement involved approximately SO square feet of surface area. Vapors were detectable above the lower explosive limit (EEL) out to HO feet in a fog-like pattern.
  • The tank car is sitting on its side. A fog-like cloud has formed around the dome. The dense, smoky cloud appears to be from a fuming material such as sulfur monochloride, which is known to be on the train.
  • The leak in the sulfuric acid line was spraying onto the platform. An acid mist was drifting from under the platform downwind toward the highway.
Water sprays can be used to modify a release, but there will always be side effects that must be considered. If the material is water-soluble, up to 90 percent of the cloud can be removed by scrubbing. Even if the material is only partially soluble, the overspray and the runoff will be corrosive, toxic, and so forth, depending on the type of release.Tests at the St. Louis site of the Mallinckrodt Specialty Chemical Company show that one of the most effective scrubbing methods is to place a wide fog pattern nozzle operating at 100 psi immediately downwind of the source. When using this technique, personnel placing the nozzle must be fully protected against the cloud. Care must be taken to ensure that back spray does not enter the container or spill area of sensitive or reactive materials. Water sprays also can be used to dilute the concentration of insoluble vapors and gases, but it must be understood that this does not reduce the amount of material, just its concentration. This technique may be effective in reducing a flammable vapor below the lower explosive limit, but the static electricity charge induced by the spray has been known to cause ignition in the vapor space above a spill.The training assembly in the photo was constructed by Mallinckrodt firefighters. It consists of a theatrical fog generator in the wood box and an open top drum. The box and drum are bolted to a pallet, and the fog passes through a two-inch pipe from the box to the drum. It is very effective in simulating fuming-type releases.Two typical characteristics of water-reactive fuming corrosives are shown in the photograph taken at the Transportation Test Center in Pueblo, Colorado during the 1986 test of special hazardous-materials foams by Chubb National Foam. The aggressive cloud rising in the center is the result of a segment of the tests in which the foam was intentionally plunged into the test pan, which contained 65 percent oleum (all types of foam perform best when rolled quietly onto spills).Direct application of water in a straight stream is absolutely explosive! When the application team, consisting of the author and a factory technical representative, gently rolled the foam on, the release was controlled in less than five minutes. Behind the reactive cloud is the typical fume cloud that results from the hydrolysis of moisture in the atmosphere as air passes over a spill. Oleum fume clouds consist of submicron-size particles of sulfuric acid.


Understanding and describing an airborne release depends greatly on your ability to visualize its shape, size, and movement This is particularly true of material such as sulfur dioxide gas, which is not visible, and also for lower concentrations of fuming materials such as nitric acid.

Experienced firefighters have no difficulty visualizing fire situations. They know what the second floor hall will look like in a specific type of working fire. As they approach the building, firefighters visualize the smoke and tire conditions that they will encounter at the top of the stairs.

Knowledge of smoke behavior can help firefighters understand airborne chemical releases The airborne cloud of a chemical release is much like a trash or leaf fire. Heavy concentrations are in the plume. However, as with a trash fire that has an odor far outside the visible smoke, a chemical release will have potentially harmful concentrations to the side and upwind. Remember that near-field effect can move airborne materials 180 degrees to the wind. Two other aids in visualizing releases are steam or smoke from nearby industrial sources and the behavior of a short piece of barricade tape attached to a pike pole or pipe close to the release.

An accurate and complete description of a release involves recognition, definition, and visualization.

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