The ‘Big Five’

A Systematic Approach to Evaluating Product Hazards

CarolinaFireJournal - Glenn Clapp, CHMM, CFPS
Glenn Clapp, CHMM, CFPS
04/29/2011 -

Back in the 1980s, a concept emerged in the aviation world known as “Crew Resource Management” (CRM), which broadly applied to the study of human behavior in aviation — e.g. how flight crews react to and handle in-flight situations — and specifically related to a “Unified Command” type concept in managing flight operations through joint input of crew members. An additional component of this concept was the development of a “flow” in each crew member’s actions in each type of specific situation or operation — performing task A before task B, etc. This same idea of developing an operational “flow” is also evident in fire service operations, such as pumping operations on the fire scene. The prudent apparatus operator, or aspiring apparatus operator, will develop a distinctive “flow” in his or her actions, as is summarized by some sources in the Seven Steps of Pump Operation.


The concept of developing a “flow” in your fire service operations can also be applied to the hazmat arena. If I am dispatched to a hazmat call, I like to apply the “flow” concept to the hazards that may be presented by the products involved. Specifically, I like to consider the following five factors:

Radioactivity – Will the product present a radiological hazard? This was discussed in the previous issue’s article on radiological monitoring.

Corrosiveness – Will the product eat away metal or skin?

Flammability – Will the product ignite or explode — in non-technical terms, is it going to go “whoosh” or “boom”?

Vapor Production – How quickly will the product evaporate? Or, what is the level of vapor production?

Toxicity – Will the product present a hazard to our health?


As the factor of radioactivity was previously discussed, let us first take a look at the concept of corrosiveness, which we evaluate using the measure of pH. The pH of a substance delineates its relative corrosiveness and whether the substance is an acid or a base (bases are also known as alkalines or caustics). The pH scale runs from zero to 14, with neutral being seven. Substances with a pH less than seven are classified as acids, whereas substances with a pH greater than seven are said to be basic in nature. Specifically defined, pH is the negative logarithm of the concentration of hydronium ions (H3O+) in a solution (a low pH indicates a high hydronium ion concentration and vice-versa).


You may then be tempted to ask, “why are we concerned with corrosiveness?” The answer is two-fold, in that an acidic or basic product may degrade our personal protective equipment — inclusive of our suits, boots, gloves, and respiratory protection if the respiratory protection is externally worn. And, it may also negatively impact the operation of our air monitoring sensors, thereby rendering our air monitoring activities employed as part of a risk-based response approach null and void. Or, as I sometimes say, turning your meter into a wheel chock.

Although we may utilize a pH meter to determine corrosiveness in the field, the tried-and-true method that we normally use is the low-tech approach of pH paper. The pH paper can be taped to our PPE, can be mounted to a stick or pike pole to give us a forewarning of a corrosive environment, and can be wetted with distilled water to allow a more rapid detection of a corrosive vapor. Acids or bases will be displayed on the pH paper as distinctive color changes.


The third factor of the “big five” is that of flammability. One way in which flammability can be categorized is by flash point. As we recall from fire behavior class, flash point is the minimum temperature at which a liquid produces a sufficient quantity of vapor to form an ignitable mixture that will flash if an ignition source is present and then may go out. The lower the flash point, the more hazardous in terms of flammability the substance is. Flammability may also be categorized by ignition temperature, which is the minimum temperature at which a substance will ignite and continuously burn. It can also be thought of as how hot the ignition source needs to be to produce continuous combustion.

Flash point and ignition temperature oftentimes can be viewed as a “see-saw” type of arrangement. For example, gasoline has a relatively low flash point of -45o F and a relatively high ignition temperature of approximately 700 o F, whereas diesel fuel has a relatively high flash point of 124 o F and a relatively low ignition temperature of 494 o F.

An additional concept in flammability is that of flammable range. The flammable range of a substance is the range in concentration (expressed in percent concentration in air) of the substance within which it will burn or explode. The flammable range is bounded by the lower explosive limit (LEL) and the upper explosive limit (UEL) — also known as the lower flammable limit (LFL) and upper flammable limit (UFL).

Below the LEL, the mixture is too lean to burn or explode, and above the UEL it is too rich to burn or explode. As an example, gasoline has a flammable range of 1.4 percent to 7.6 percent, while acetylene has a flammable range of 2.0 percent to 99 percent. We regularly look at the percentage of LEL when we are monitoring an environment for flammability, as we are concerned with how close we are to our LEL. We are not as concerned with the UEL in the field, as when we are approaching a flammable product the concentration will be increasing and we will be approaching the LEL and hence the increased flammability risk.

Vapor Production

The fourth factor of the “big five” is vapor production. Although vapor production closely ties in with flammability, we will look at the concept as a separate factor due to the fact that vapor production also affects other areas in terms of hazard evaluation. Vapor production is indicated by the vapor pressure of a substance. Vapor pressure is defined as the pressure the vapor above a liquid imparts on an enclosed container. The higher the vapor pressure of a substance, the greater the vapor production, the more volatile the substance is, and the quicker it will evaporate. Vapor pressure is temperature dependent, meaning as the temperature increases, the greater the vapor production and therefore the higher the vapor pressure.

Vapor pressure is normally expressed in millimeters of mercury (abbreviated mmHg). This unit of measure simply is the distance in millimeters the vapor pressure will force a column of mercury vertically upwards. As a reference, the standard atmospheric pressure of 14.7 psi at sea level equates to 760 mmHg. We can look at the vapor pressure of three substances of which we readily know the relative evaporation rates to generate benchmarks to be applied in the field. At 70o F, diesel fuel has a vapor pressure of 2.17 mmHg, water has a vapor pressure of 25 mmHg, and gasoline has a vapor pressure of 200 mmHg. We can use these figures (rounded off) to derive the “Rule of 2, 20, and 200,” where the vapor pressures of diesel fuel, water, and gasoline are expressed as 2 mmHg, 20 mmHg, and 200 mmHg respectively.

Since we know from experience the relative evaporation rates of these three substances, we can then use any vapor pressure data obtained in the field to compare the quantitative data given to the figures expressed for the three aforementioned substances. For example, if a substance has a vapor pressure of 190 mmHg, we can infer that it will evaporate at approximately the same rate as gasoline.


The fifth and final factor of the “big five” is toxicity. The toxicity of a substance is expressed in various levels of concern, which are simply the minimum concentrations of the substance that will produce specified toxicological or health effects. The particular level of concern utilized depends on the particular situation and setting involved. One such level of concern is known as immediately dangerous to life or health (IDLH). IDLH is defined as the maximum concentration of a chemical that one can be exposed to without any irreversible health effects or escape-impairing symptoms over a 30 minute time frame. IDLH values are generally expressed in either parts per million (ppm) or milligrams per cubic meter (mg/m3) and are used in respirator selection and as a level of concern for emergency responders.

Other toxicological levels of concern have been developed by various agencies and serve as valuable benchmarks in our “toxicological toolbox”. The National Institute for Occupational Safety and Health (NIOSH) has developed recommended exposure limits (REL’s) for various substances. NIOSH recommends REL’s to the Occupational Safety and Health Administration (OSHA), and REL’s are not legal standards but are rather recommendations based in scientific research. REL’s are generally expressed in ppm or mg/m3 and may be categorized in terms of time weighted average (TWA) values averaged over a ten hour workday during a forty hour workweek, ceiling values (C) that should never be exceeded, or short-term exposure limit (STEL) values that should not be exceeded over a fifteen minute time frame.

OSHA has developed the level of concern known as the permissible exposure limit (PEL). PEL’s-unlike NIOSH REL’s-are regulatory limits (legal standards) applied to occupational exposure and are expressed in ppm or mg/m3. PEL’s normally reference an eight-hour time weighted average exposure. An additional level of concern is the threshold limit value (TLV) developed by the American Conference of Governmental Industrial Hygienists (ACGIH). TLV’s are-like NIOSH REL’s-recommended exposure limits applied to occupational exposure and not legal standards. TLV’s are usually expressed in ppm and may be classified as an eight hour workday, forty hour workweek TWA, a ceiling (C), or fifteen minute STEL. Other levels of concern also exist that can be stored in our “toxicological toolbox” and utilized in making informed decisions in our development of the proper toxicological level of concern for the particular situation at hand.

In conclusion, the “big five” factors — radioactivity, corrosiveness, flammability, vapor production, and toxicity — related to determining the hazards presented by the products involved serve as a pertinent template for organizing our thought patterns and “flow” of hazard assessment actions when responding to and operating at hazmat incidents. By organizing our approach we can ensure that we respond to each hazmat incident both safely and effectively.

As always, be safe out there and visit the North Carolina Association of Hazardous Materials Responders website at

Glenn Clapp is President of the North Carolina Association of Hazardous Materials Responders and is a Fire Training Commander (Special Operations) for the High Point Fire Department. He is a Technician-Level Hazmat Instructor, a Law Enforcement Hazmat Instructor, and is a Certified Hazardous Materials Manager and Certified Fire Protection Specialist.
Comments & Ratings

  10/8/2011 10:18:38 AM
Anonymous Lee Dorey 

MD, CDL, trainee 
New Comment Very clear for thinking

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