Do Combustible Liquids Matter Anymore?

Do Combustible Liquids Matter Anymore?

Learn about one professional's experience with the terms flammable and combustible liquids.

It has been over 10 years since OSHA adopted the Globally Harmonized System of Classification and Labelling of Chemicals (GHS) into its Hazard Communication Standard (HazCom), and yet, there is some misunderstanding about the standard that still persists. Despite years of advance notice and the efforts of several manufacturers, importers, distributors and the companies who use the chemicals to update labels and SDSs, HazCom continues to not only be on OSHA’s list of Top 10 Most Frequently Cited Standards but to be towards the top of that list, even a decade later. To be fair, the HazCom standard has proven difficult for many businesses to comply with even before the adoption of GHS. While most of the changes to how chemical hazards are communicated in the workplace are arguably better, some of them have caused confusion and misunderstanding.

One significant area of change is in how flammable and combustible liquids are defined. Not only were the criteria for the different categories changed but the terminology was changed as well. Shortly after the changes went into effect, I had several discussions with colleagues claiming that the term combustible liquids was no longer in use. There were even articles in safety journals that stated the same. Even today, conducting a web search will find safety websites claiming that combustible liquids are no more. But is this claim accurate?

Personally, I was notably surprised by this change. Being actively involved in safety at my workplace, I decided to improve my knowledge and skillset by going back to school and getting a degree in occupational safety and health sciences. In my very first safety course, I learned the difference between flammable and combustible liquids, and this was an ‘Aha!’ moment for me because despite being involved in safety, I never really understood the difference between the two. In my mind, I had somehow decided that a combustible liquid must have something to do with the compression force that resulted when the liquid was ignited. (I was thinking along the lines of a combustion engine, though this line of reasoning doesn't really make sense for several reasons, not the least of which is the fact that gasoline is used in combustion engines but is considered a flammable, not combustible, liquid.)

So to learn that it was something as simple as a flashpoint—a flashpoint below 100 degrees Fahrenheit was flammable and everything at or above it was combustible (see figure 1)—was eye-opening for me; but it was also empowering, as if I had gained some secret special knowledge. It wasn’t that I wanted to keep the secret to myself. I readily shared it with others. I waited for that moment when somebody would ask me, “What is the difference between flammable and combustible?” (Of course, nobody actually ever did.) But I learned something that gave me a better understanding of safety in the workplace. It was moments like these that made me feel that all the effort to get my degree as a working adult was worth it. So when I was told that combustible liquids were no more, it was as if something had been stolen, a small triumph of knowledge that had been taken away.

However, there was a bigger issue at stake than my bruised ego. This claim that combustible liquids no longer exist simply isn’t true. Just because GHS switched the terminology for OSHA to four categories of flammable liquids rather than delineating differing classes of flammable and combustible liquids doesn’t mean that the term combustible isn’t used elsewhere in the world of safety (compare figures 1 and 2). OSHA is not the only voice on safety; and, after all, OSHA standards are simply the bare minimums that any company should consider for the safety of their workforce.

There are several official and standard-setting organizations that still use the term combustible liquids. For example, the National Fire Protection Association (NFPA) still uses the same classes of flammable and combustible liquids in its NFPA 30 Standard, upon which the previous HazCom Standard definitions were based. However, it is important to note that they also have recently changed some of the terminology. To reduce confusion caused by the various definitions and the perception that combustible liquids are somehow less hazardous, the NFPA has switched to the term ignitible liquids. Even so, the full term is ignitible (flammable and combustible) liquid, as the categories within the ignitible umbrella include delineation between flammable and combustible liquids.

The Department of Transportation under 49 CFR 173.120 still defines liquids as either combustible or flammable (except using 140 degrees Fahrenheit as the dividing flashpoint—see figure 3). The Environmental Protection Agency (EPA) has used the term ignitable liquid for decades to describe any liquid with a flashpoint below 140 degrees Fahrenheit. The term ignitable (with a slightly different spelling than NFPA) is used because it also describes other hazardous wastes such as solids and gases. The point is, just because GHS utilizes four categories of flammable liquids does not mean that the term combustible—or other terms such as ignitable—no longer exists or shouldn't be understood by a safety professional.

To be sure, there are definite advantages to simply having different categories of flammable liquids under the GHS. Just as the NFPA recognized, the terms combustible and flammable have caused confusion and misunderstanding. Obviously, I wasn’t the only individual to have a hazy (or completely wrong) understanding of the differences between flammable and combustible liquids. In addition, having different terms for the liquids may give the wrong impression that somehow combustible liquids are safer than flammable liquids. As an example of how wrong this notion can be, kerosene would be considered a combustible liquid under the older definition. But with a flashpoint barely over that 100 degrees Fahrenheit dividing line, it can easily be heated—just by the weather in some cases—to a point that it is more dangerous and ignitable than gasoline is at room temperature. So, defining all these liquids as varying categories of flammable liquids may communicate the hazards more clearly, which after all is the whole point of the HazCom Standard.

So why make an issue about the use of the term combustible liquids when it isn't even used by OSHA? First, as a safety professional, you are likely to have to reference other standards, such as those used by the DOT, NFPA and EPA. And second, if you believe that the HazCom Standard under GHS doesn't use the term combustible liquid, you'd be wrong again. Yes, they have reclassified their definitions into four categories of flammable liquids, but if you dig a little deeper, you'll find an interesting little tidbit. Under 29 CFR 1910.1200 Appendix C.4.19, Category 4 flammable liquids must carry a Hazard Statement that reads: “Combustible liquid”!*(This change of definition actually puts OSHA's terminology more in sync with the DOT—placing combustible liquids above the 140-degree-Fahrenheit flashpoint level, see figure 3—which interestingly enough, was changed years earlier from the NFPA 30 Standard to align with UN definitions to facilitate international trade.) So OSHA not only still uses the term combustible liquid, the regulations require it to still be used on labels under the newer HazCom Standard. Contrary to the claims, combustible liquids not only still exist—it appears they are here to stay.


The new GHS definitions of flammable liquids (found at 29 CFR 1910.106) are similar, but simpler and easier to understand than the old HazCom definitions of flammable and combustible liquids based upon the NFPA 30 Standard. Class IA and IB liquids become Category 1 and 2 liquids respectively (the temperatures are based upon degrees Celsius rather than Fahrenheit in GHS, but are essentially unchanged). Class 1C and Class II liquids are combined into Category 3 liquids. Class IIIA liquids are reclassified as Category 4 liquids. There is no obvious delineation between flammable and combustible liquids anymore. Also, Class IIIB combustible liquids aren't categorized under the new system, although any flammable liquid with a flashpoint greater than 200 degrees Fahrenheit still must be cared with the same precautions as a Category 4 flammable liquid if it is heated to within 30 degrees Fahrenheit of its flashpoint.

Figure 1: NFPA 30 Ignitable Liquids

Figure 2: OSHA 29 CFR 1910.1200 (GHS) Flammable Liquids

Figure 3: DOT 49 CFR 173.120 Flammable and Combustible Liquids


  1. Hazard Communication Standard: Labels and Pictograms. Retrieved from
  2. Asfahl, C. R., Rieske, D. W. (2010). Industrial safety and health management, sixth edition.
  3. Ignitible Liquids and NFPA 30. Retrieved from
  4. Haight, J. M. (2013) Hazardous Material management and hazard communication, second edition.

This article originally appeared in the August 1, 2023 issue of Occupational Health & Safety.

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