A Flashy Topic: Combustible, Flammable and Ignitible Liquids | Help Center

A Flashy Topic – Combustible, Flammable and Ignitible Liquids

When it comes to liquids and fire, not all are created equal—some ignite in a heartbeat, others take their time, and a few need a sharp nudge. The terms flammable, combustible, and ignitable often get tossed around interchangeably, but they each have distinct meanings in the world of safety and science. These terms aren’t just jargon – they are used to determine how dangerous a liquid is in various situations, how should be stored, and how should be handled. Unfortunately, these terms may differ depending on which regulation you’re using. Understanding the difference could mean the difference between controlled safety and a fiery disaster. Let’s dive into what sets these terms apart and why they matter.

Transportation of Dangerous Goods

Under the current UN Recommendations for the Transport of Dangerous Goods (Model Regulations), there are only two types of liquids – flammable and non-flammable. After some development, the UN Recommendations (and regulations that are based on them, such as Canada’s “Transportation of Dangerous Goods Regulations,” the International Maritime Dangerous Goods Code and the ICAO Technical Instructions for the Safe Transport of Dangerous Goods by Air) came up with an easy-to-use dividing line – liquids with a closed cup flash point of 60°C or less would be flammable. Those with flash points exceeding 60°C would not. (The flash point represents “the temperature at which the vapour above a volatile liquid forms a combustible mixture with air” and can be measured by various testing devices, the most accurate being the “closed cup” system.)

Of course, it wouldn’t be a UN system without a handful of exceptions to complicate matters, so we have special rules for liquids with flash points below 60°C but with low combustibility, and liquids transported at temperatures above their flash points. But in general, the classification procedure in the UN Recommendations gives us a simple “yes or no” answer to the question, “does this liquid pose enough of a fire hazard to be transported as dangerous?”

One odd thing about the term “flammable” is that before the safety experts got involved, the term for easily-ignitable liquids was “inflammable.” However, for by the 1950s, safety studies showed that many workers thought the prefix “in” meant “non,” and got entirely the wrong idea. Grammar experts call this a “false antonym.” So, the word was changed to simply “flammable” in English when used for safety purposes. (However, you will see the word “inflammable” still used in other languages, such as French.)

U.S. “Hazardous Materials Regulations”

The United States modified the UN approach for their “Hazardous Materials Regulations” of Title 49 of the Code of Federal Regulations (49 CFR). Based on concepts developed by the U.S. National Fire Protection Association (NFPA) in the 1930s and 1940s, they broke liquids into three groups – flammable, non-flammable, and an intermediate classification of combustible. Flammable liquids would have flash points up to 100°F (37.8°C), while combustible ones would have flash points between 100°F and 200°F (93.3°C).

In the area of overlap between the UN system and the “Hazardous Materials Regulations” (100°F to 140°F), the user can choose to use either flammable or combustible. The advantage of combustible liquids is that they can be transported by road or rail as non-regulated when in non-bulk form, assuming they have no other hazard. This does create some disharmony with international regulations, but gives relief for smaller containers while maintaining a high level of protection for bulk shipments.

The GHS and Workplace Hazard Communication

The Globally Harmonized System for Chemical Classification and Labelling (GHS) used by both the hazard communication standard of the Occupational Safety and Health Act (OSHA) and Canada’s “Hazardous Products Regulations” (HPR) also uses the concepts of flammable and combustible liquids. These are aimed at safety when workers are handling the material (not transportation or storage specifically). Remember, the GHS also uses categories to indicate the degree of danger, starting with the most dangerous substances in Category 1. Flammable liquids (flash points not greater than 60°C) are divided into three categories, based on a combination of their flash points and boiling points. Combustible liquids (with flash points greater than 60°C but not greater than 93°C) are put into the combustible liquid category 4. Note that this means that a liquid with a flash point below 60°C might be classed as combustible under the “Hazardous Materials Regulations” for transport, but as a flammable liquid under OSHA hazard communication standard.

The National Fire Protection Association and Fire Codes

Today, most North American Fire Codes are based on standards created by the NFPA. Their concepts of what is “flammable” and what is “combustible” are laid out it the standard “NFPA 30, Flammable and Combustible Liquids Code.” This has, you guessed it, its own unique spin on flammable and combustible liquids, as well as introducing a new term, “ignitible.”

The NFPA uses its classification to determine storage requirements, so it’s a little more complex than what we see for transportation and workplace safety. It establishes three “classes,” which cover combustible and flammable liquids as follows:

Class	Name of Class	Flash Point	Boiling Point
I-A	Flammable liquid	Less than 22.8 °C (73 °F)	Less than 37.8 °C (100 °F)
I-B		Less than 22.8 °C (73 °F)	37.8 °C (100 °F) or greater
I-C		22.8 °C (73 °F) or more, but less than 37.8 °C (100 °F)	Not used
II	Combustible liquid	37.8 °C (100 °F) or more, but less than 60 °C (140 °F)	Not used
III-A		60 °C (140 °F) or more but less than 200 °F (93 °C)	Not used
III-B		200 °F (93 °C) or more	Not used

Then, both flammable and combustible liquids are grouped as “ignitible” for the purpose of the code. Yes, it’s not the normal spelling of the word “ignitable,” to indicate that the word has a specific meaning for the Code separate from normal English usage.

Then, to further complicate things, there is another NFPA standard, NFPA 704: Standard System for the Identification of the Hazards of Materials for Emergency Response, that has yet another classification system, this time for identifying hazardous materials on site for emergency responders, rather than workers. Fortunately, this system doesn’t use the terms combustible versus flammable or ignitible. Instead, it assigned substances to a designation of 0 to 4, depending on the risk of fire in emergency situations.

Conclusion

To properly classify your liquids as flammable, combustible, or the bane of spell checkers everywhere, ignitible, you need to know the regulation you’re dealing with. Transportation, workplace safety, storage and emergency response systems all have different definitions! But if you’re confused, just contact ICC Compliance Center here at 888-977-4834 (Canada) or 888-442-9628 (U.S.) and ask for one of our regulatory specialists. We can advise you on the various ways liquids can be classified.