How to Read an NFPA 704 Diamond-Part 2



The Red square, at the top of the NFPA diamond, tells the reader how flammable the labeled substance is. As I mentioned in Part One, a rating of “0” indicates the lowest level of flammability, while a rating of “4” indicates the highest level of flammability. The ratings in this square are the most quantitative of all NFPA ratings and are fairly precisely defined.

To understand these ratings, it helps to understand the term “flash point”. Flash point is the lowest temperature at which a liquid can form an ignitable mixture in air near the surface of the liquid.

A rating of “0” means the material “will not burn under typical fire conditions”. This includes materials that are noncombustible by nature “such as concrete, stone, and sand.” Quantitatively, these materials “will not burn in air [even] when exposed to a temperature of 816C (1500F) for a period of 5 minutes.” Boric Acid, Mercury, and Chloroform are examples of chemicals with a flammability rating of 0 (but they have other hazards!)

A rating of “1” indicates a material that requires “considerable preheating . . . before ignition and combustion can occur.” This includes “most ordinary combustible materials”, like paper, wood, etc. Quantitatively, these are liquids, solids, and semisolids that have a flash point greater than 93.4C (200F) OR liquids with a flash point greater than 35C (95F) “that do not sustain combustion when tested” using a standard testing method. Antifreeze, Mineral Oil, and Grease have this rating.

A rating of “2” is given to materials that “must be moderately heated or exposed to relatively high ambient temperatures” before they will burn. These include liquids with a flash point at or above 37.8C (100F) and below 93.4C (200F) and solid materials “in a form that burns rapidly and creates a flash fire hazard, such as cotton, sisal, and hemp.” Examples include Acetic Acid, Cresol, and PGMEA.

A flammability rating of “3” is given to “liquids and solids that can be ignited under almost all ambient temperature conditions.” These include liquids with a flash point below 22.8C (73F) AND  boiling point at or above 37.8C (100F), as well as liquids with a flash point between 22.8C (73F) and 37.8C (100F), such as Acetone and Gasoline. It also includes solids that can easily produce an ignitable dust cloud. Aluminum, Titanium, and Silicon Powder fall into this category.

Finally, a rating of “4” is given to “materials that rapidly or completely vaporize at normal temperature and pressure” such as flammable gases, materials that ignite spontaneously when exposed to air, and any liquid with a flash point below 22.8C (73F) AND a boiling point below 37.8C (100F). Some examples are Acetylene Gas, Hydrogen, Diethyl Zinc, and Ethyl Ether.


DISCLAIMER: This article is designed to promote understanding of the NFPA rating system. It is not a complete explanation of the criteria involved.