In order to understand how the impacts of chemical spills can be predicted, some science and chemistry concepts must be examined.


Vapor pressure is the measurement of a particular liquid chemical's tendency to vaporize. All liquids will evaporate, even thick liquids. Everyone is familiar with the evaporation of liquids like water, gasoline, and the solvents used in paints. When the paint dries, its solvents have all evaporated away, leaving the dry finish. These individual chemicals all evaporate at different rates, and they all have different vapor pressures.

And the vapor pressure for each increases as the temperature goes up. When water is heated closer to its boiling point of 212 F, it evaporates at a faster rate than water at room temperature. The vapor pressure for the water increases as the water gets hotter. Those same paint solvents will dry faster if the paint is warmer. It takes longer for paint to dry in a cold house than a warm house.

Responders should beware of liquids with substantial (large) vapor pressures, which is the primary measure of a chemical's tendency to vaporize. Some chemicals evaporate faster than others.

Vapor pressure is also the measurement of the pressure exerted on the walls of a container which is partially full of the liquid chemical and free of any other vapor or gas. Higher temperatures cause increases in the vapor pressure. Lower temperatures cause a decrease, and there is a direct relationship between the temperature of any given substance and its vapor pressure. The warmer a liquid is, the more it tends to evaporate.

Vapor pressure is most often expressed in units of millimeters of mercury (mm Hg).

As a rule of thumb, the higher the vapor pressure, the further the distance the vapor will disperse. There is a direct relationship between the vapor pressure of an evaporating substance and the maximum concentration that its vapor or gas may achieve when mixed with air in the open environment. In other words, the concentration of the vapors of a spilled chemical will only be so much in the air, and this concentration depends on the vapor pressure of the chemical.

Higher vapor pressures above the surface of a substance require that more molecules of the substance be physically present. Thus, if the vapor pressure of the substance is known, the approximate maximum airborne contaminant concentration it may attain can be calculated. Such concentrations are most commonly expressed in units of percent in air by volume, parts per million parts of air (ppm) by volume, or in milligrams of chemical per cubic meter (mg/m3) of air.

No matter what the vapor pressure of a spilled chemical, it can only evaporate into the air if it is exposed to the air. So limiting the surface of a spilled liquid is a common strategy in responding to a spill of a liquid that is evaporating. Firefighters will dike (push dirt up to form a pool) the area around a tank that is leaking liquid to limit the surface area of the growing pool of liquid. The amount of surface area of a diked pool of liquid is much less than an uncontained spill.

To illustrate this, look at a filled glass of water, and note the small portion of the water that is in direct contact with the air. It is just a fraction of the total amount of the water in the glass that is exposed. If the glass of water is spilled, almost all of the water is exposed to the air, and the water spreads out into a much larger pool. And obviously, it would take longer for the water in the glass to evaporate than the spilled water, because more of the water molecules are exposed to the air.


Any liquid will boil at the temperature at which its vapor pressure equals the pressure being exerted by the environment onto the surface of the liquid. This is why water boils at a lower temperature in high elevations. So boiling points are related to the pressure exerted on the chemical.

Liquids in sealed containers (barrels, tanks) will remain as liquids when heated above their normal boiling points although their vapor pressures may become very dangerously high. If heating continues and the pressure is not adequately relieved by a safety device, the pressure and temperature within the tank may eventually rise to the point that some part or all of the container may burst or rupture. Water will boil when heated to the boiling point in an open pan on a stove, but if water is sealed inside a container and heated to its boiling point, the water will cause the container to burst.

This is why firefighters will take steps to cool a tank near a fire--to prevent a violent tank failure. When a tank bursts under this scenario, the liquid tends to vaporize almost instantly, and if flammable, burn almost instantly. One of the dangers associated with a tanker of propane is a BLEVE, Boiling Liquid Expanding Vapor Explosion. A BLEVE is a catastrophic explosion of a flammable fuel.

If a spilled liquid is in an environment above its boiling point, it will rapidly boil and expand, sometimes explosively. An example of this is Ammonia. Ammonia boils at -28.17 degrees Fahrenheit. If Ammonia spills, there may be a second phase reaction, which can cause a cloud of Ammonia to expand and move very quickly. Ammonia is lighter than air, and once it warms up, it tends to move up and way into the atmosphere.

Often, the available data about a chemical will indicate its boiling point, but, just like water, a chemical in liquid form will evaporate at temperatures far below the boiling point.


The weight of the air is on everything, and the atmospheric pressure at sea level is more than at the top of a mountain. The weight of the air at sea level has been measured. The atmospheric pressure at sea level is 14.7 pounds per square inch, expressed as psi. This means that for every square inch of surface an object has, there are 14.7 pounds of pressure pushing down.

This sea level pressure is also expressed as one atmosphere. In the United States, the most common relative scale of measurement is in terms of gauge pressure, where a reading of zero matches an absolute pressure of one standard atmosphere. In this system, an absolute pressure of 15.7 psi would be expressed as 1.0 pound per square inch - gauge, or 1.0 psig for short. [15.7 - 14.7 = 1.0] The pressure inside a tank of chemical will be usually expressed in these terms.

Any gas or vapor entering the atmosphere will quickly adjust its volume to achieve a total pressure of the atmosphere it is released in. The vapors from a spill of a chemical in a higher elevation will travel further than one at sea level, all other factors being the same.

A tank of a liquefied gas, when opened to the atmosphere, will spew out many times the original volume of the tank. Unless the tank is sealed, the liquefied gas will escape as a gas until the pressure of the atmosphere outside the tank is the same as inside the tank. A tank of propane, when emptied directly into the air, will still contain propane, but the pressure of the the remaining propane will be at the same atmospheric pressure as the outside air.

To illustrate how gases behave when they escape from opened containers, open a bottle of a carbonated beverage. When one is opened, bubbles of carbon dioxide are seen forming and rising to the surface. Before opening the bottle, the carbon dioxide was dissolved into the liquid under pressure. When the bottle is opened, the pressure is released, and the carbon dioxide is seen escaping. If a person shook the bottle before opening, the carbon dioxide will escape more rapidly, spraying the liquid quite a distance. There is no way to look at the bottle of carbonated beverage and tell if it has been shook up or not. When modeling what a plume from a tank of chemicals might be, these same uncertainties are present.

When a tank of liquids vents gases into the outside atmosphere, the tank of liquids itself is cooled. This lowers the vapor pressure of the liquid in the tank, and may significantly slow the release of the gases from the tank. It also creates a special hazard because the leak may be more difficult to detect.


Chemicals have different molecular formulas and makeup. Vapor density is the ratio of the density of a pure gas or vapor to the density of air. Vapors or gases with a vapor density less than air tend to float above the air, moving up and away from the spilled chemical and the ground. This is just like oil floating on water because oil is less dense than water. Vapors or gases with a vapor density higher than air tend to sink and move along the ground. When the vapor density is close to that of air, the behavior can be more unpredictable, but is treated as if the gas is heavier than air.


The molecular weight of a chemical depends on what elements make up its chemical formula. The molecular weight will vastly influence the behavior of a spilled chemical, including vapor pressure. The heavier the molecule of a chemical that is evaporating, the slower it will be to evaporate and float away. The heavier the molecules, the more likely they will be to move along the ground rather than upwards.