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 going to form will be a tad easier.

This month’s safety column is the third in our series on safe laboratory chemical management. Readers of our Safety Guys column were introduced to safe chemical handling with our “Planning Chemical Management for the Forensic Laboratory” article describing how to develop and implement a program for proper management of laboratory chemicals.1 As that article stated, it all begins with a complete, up to date, and accurate chemical inventory. Our second article “Making Sense of MSDS” was a tutorial on understanding material safety data sheets.2 This issue we are going to discuss the National Fire Protection Association (NFPA) hazard diamond, sometimes referred to as the fire diamond, and how to decipher the information it contains.3

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 and our discussion will start at the top and work clockwise around the diamond.

Figure 1: NFPA Hazard DiamondFigure 1: NFPA Hazard Diamond

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

Flammability may be the single most hazardous characteristic causing more injuries and damage than any of the others 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 four. Zero indicates a stable chemical under most 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 one. Chemicals that are rated a two are normally unstable and readily undergo violent decomposition. They may also react violently with water. Materials with a rating of three are capable of an explosive reaction or detonation if subjected to a strong initiating source such as heat or shock. A four 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, non-flammable, and non-reactive with water. Phosphorus (red or white) is rated a one since it can become unstable at elevated temperatures. Calcium metal rates a two. Less reactive than sodium, it reacts violently with water, alcohols, and other materials and burns in air. Fluorine gas is an example of reactive material rating a three. It is the most reactive non-metal, 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 four 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, mentions only two approved symbols:

Some organizations and manufacturers use additional symbols to indicate hazards associated with the substance. Examples of a few of these are presented below.

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 four. A zero indicates no toxicity and no additional hazard beyond normal combustible materials under condition of fire. A one 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 two and may cause temporary incapacitation or injury with continued exposure unless medical treatment is given. A rating of three 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 four. Very short exposures could result in death or serious injury even with medical treatment.

I know you would be disappointed if I did not provide examples for the different health hazards as I have done for the flammables and reactives so here goes. Peanut oil is an example of a material that would rate a zero on the health scale. Turpentine would rate a one being irritating to skin and mucous membranes. Ammonia gas would rate a two 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 three 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 four 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.4

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. As Sergeant Esterhaus from Hill Street Blues says, “Let’s be careful out there.”


  1. Planning Chemical Management for the Forensic Laboratory, Vince McLeod and Glenn Ketcham, Forensic Magazine. Vol. 6, No. 1. 2009.
  2. Making Sense of MSDS, Vince McLeod and Glenn Ketcham, Forensic Magazine. Vol. 6, No. 3. 2009.
  3. Standard System for the Identification of the Hazards of Materials for Emergency Response, National Fire Protection Association, Publication 704.
  4. NIOSH Pocket Guide to Chemical Hazards. National Institute of Occupational Safety and Health. Publication 2005-149.

Vince McLeod is an American Board of Industrial Hygiene Certified Industrial Hygienist and the senior IH with the University of Florida’s Environmental Health and Safety Division. He has 24 years of experience in all facets of occupational health and safety and specializes in conducting exposure assessments and health hazard evaluations.