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The NFPA Hazard Diamond

We all know how diamonds are formed. You take a lump of carbon and subject it to intense pressure and high temperatures, and magically those carbon atoms are pressed into a diamond. The diamonds we are discussing in this article are formed much more easily.

Vince McLeod, CIH

Vince McLeod is an American Board of Industrial Hygiene-certified industrial hygienist and the senior industrial hygienist with Ascend Environmental + Health Hygiene LLC in Winter Garden, Florida. He has more...

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Nothing Says We Care More

We all know how diamonds are formed. You take a lump of carbon and subject it to intense pressure and high temperatures, and magically those carbon atoms are pressed into a diamond. The diamonds we are discussing in this article are formed much more easily.

This month’s safety column is another in our series on safe laboratory chemical management. Loyal readers of our safety column might recall previous articles dealing with laboratory waste handling, chemical storage tips, safe flammable materials storage and, most recently, material safety data sheets (MSDSs). That last article was an indepth review of all the information contained in a typical MSDS and a tutorial on understanding material safety data sheets. This article will discuss the National Fire Protection Association (NFPA) hazard diamond, sometimes referred to as the fire diamond, and how to decipher the information it contains.

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Classes and the NFPA Hazard Diamond

Experienced laboratory managers know that there are four basic categories of chemicals: toxic, corrosive, flammable and reactive. However, in our chemical world there are many additional categories and subsets of these main four. We should also keep in mind that many chemicals exhibit a combination of properties and would fall into more than a single class or category. These four properties are the foundation of the NFPA hazard diamond. Coincidentally, these four categories are the main criteria used to define wastes as hazardous under the federal Resource Conservation and Recovery Act (RCRA). The hazard diamond has gained wide acceptance, and most manufacturers include it on their labels when appropriate. Figure 1 shows the layout of the different sections; our discussion will start at the top and work clockwise around the diamond.


The top of the diamond indicates the flammability hazard. The chemical is rated from zero to 4. A zero means the material will not burn under most common circumstances. Examples include hydrogen peroxide and sodium hydroxide. A rating of 1 indicates the material will ignite and burn at temperatures greater than 200°F. Materials that fall into this category are glycerin and propylene glycol. A 2 indicates substances that will burn at temperatures less than 200°F, such as naphthalene, octyl alcohol and nitrobenzene. A rating of 3 denotes materials with flashpoints below 100°F, such as xylene, amyl acetate and butyl alcohol. Finally, a 4 indicates extremely flammable substances. These include acetone, ethyl ether, acetylene and cyclohexane.

Flammability may be the single most hazardous characteristic, causing more injuries and damage than any of the other properties in the diamond. If there is anything other than a zero in this part of the diamond, make sure to use this material with adequate ventilation, clean up spills immediately, and above all, keep heat and flame well away from the area of use.


Moving clockwise, as promised, the next part of the hazard diamond designates the potential reactivity of the material, which is also rated from zero to 4. Zero indicates a stable chemical under nearly all conditions, even fire. Substances that are normally stable but can become unstable when heated, or may react with water, but not violently, are rated a 1. Chemicals that are rated a 2 are normally unstable and readily undergo violent decomposition. They may also react violently with water. Materials with a rating of 3 are capable of an explosive reaction or detonation if subjected to a strong initiating source such as heat or shock. A 4 indicates substances that are readily capable of explosive decomposition or detonation at normal temperatures.

For illustration of the reactivity, consider the following examples. Liquid nitrogen would receive a zero rating. It is stable, nonflammable and nonreactive with water. Phosphorus (red or white) is rated a 1, since it can become unstable at elevated temperatures. Calcium metal rates a 2. Less reactive than sodium, it reacts violently with water, alcohols and other materials and burns in air. Fluorine gas is an example of a reactive material rating of 3. It is the most reactive nonmetal; it decomposes in water, producing hydrofluoric acid and other hazardous compounds, and reacts vigorously with most oxidizable substances at room temperature, usually with ignition. An example of a class 4 reactive substance is trinitrotoluene or TNT. We are all familiar with its explosive properties.

Special hazards

At the bottom of the diamond is the white section. This section is used to denote special hazards. NFPA 704, Standard System for the Identification of the Hazards of Materials for Emergency Response,1 mentions only two approved symbols:

Some organizations and manufacturers use additional symbols to indicate hazards associated with the substance. One example is the Hazardous Materials Emergency Response Guidebook2 a few of these are presented above.


The final section of the diamond is the blue section on the left-hand side. This area denotes the health hazard of the compound, and is also rated from zero to 4. A zero indicates no toxicity and no additional hazard beyond that of normal combustible materials under condition of fire. A 1 means the material is slightly toxic and usually considered innocuous when used responsibly. It may cause some irritation, but only minor, even without treatment. Moderately toxic materials are rated a 2 and may cause temporary incapacitation or injury with continued exposure unless medical treatment is given. A rating of 3 indicates a serious toxic material that can cause injury upon short exposures, even if medical attention is given. Deadly or extremely toxic materials rate a 4. Very short exposures could result in death or serious injury, even with medical treatment.

I know you would be disappointed if we did not provide examples of the different health hazards as we did for the flammable and reactive chemicals above, so here goes. Peanut oil is an example of a material that would rate a zero on the health scale. Turpentine would rate a 1, being irritating to skin and mucous membranes. Ammonia gas would rate a 2, since it is definitely irritating and corrosive but generally regarded as nonflammable unless mixed just right in air. Also, ammonia has an exposure limit of 50 ppm and is immediately dangerous to life and health (IDLH) at 300 ppm. Extremely corrosive chlorine gas ranks a 3 for health hazard. It can form explosive mixtures and cause fatal pulmonary edema. The OSHA permissible exposure limit (PEL) is 1 ppm and the IDLH limit is only 10 ppm. An example of a health hazard 4 substance is arsine gas. A colorless gas with a mild garlic odor, it is extremely poisonous. For comparison, the OSHA PEL is a diminutive 0.05 ppm, and arsine is IDLH at only 3 ppm.

Now you understand diamonds, those that provide colorful gems of knowledge. Be on the lookout for them and pay attention to what they are telling you. Hill Street Blues fans will remember that Sergeant Esterhaus said it best: “Let’s be careful out there.”

Vince McLeod is an American Board of Industrial Hygiene– certified industrial hygienist and the senior industrial hygienist in the University of Florida’s Environmental Health and Safety Division. He has 22 years of occupational health and safety experience at the University of Florida, and he specializes in conducting exposure assessments and health-hazard evaluations for the university’s 2,200-plus research laboratories.

Use Warning Signs to Designate Particular Hazards
By James A. Kaufman
The use of warning signs to designate particular hazards is not just a good idea. It’s the law. The OSHA Laboratory Standard 29CFR1910.1450 requires that those areas in which particularly hazardous substances (select carcinogens, reproductive toxins, and highly toxic substances) are used be clearly designated. The OSHA Hazard Communication Standard requires the labeling of hazardous chemicals in the workplace...Click here to Read More


  1. Standard System for the Identification of the Hazards of Materials for Emergency Response, National Fire Protection Association, Publication 704. http:// asp?DocNum=704
  2. Hazardous Materials Emergency Response Guidebook, U.S. Department of Transportation. Washington, D.C. 2008. hazmat/2004-emergency-response-guidebook.htm

Additional Resources

NIOSH Pocket Guide to Chemical Hazards. National Institute of Occupational Safety and Health. Publication 2005-149.

The Merck Index, an encyclopedia of chemicals, drugs and biologicals. 14th edition. Merck & Company, Inc. Rahway, N.J. 2006

OSHA Hazard Communication Standard. http://www. table=STANDARDS&p_id=10099