What Is The Difference Between Flammable and Combustible?

What Is The Difference Between Flammable and Combustible?

A flammable material easily catches on fire when in contact with air and a source of ignition (a lit match is an example of a source of ignition. A combustible material can catch on fire, but the required conditions are much less restricted than for a flammable material.

Specific and technical definitions for flammable and combustible are not universal, they differ depending on a series of factors:

  • Country
  • Regulating agency
  • Physical state of the material
  • Purpose of the material
  • If the material is being stored, transported or consumed in some way

Although there are different definitions, some properties are equal by most of the definitions. Below there are three questions appointed to flammable and combustible materials.

Can it Catch on Fire?Yes.Yes.
Does It Need Air to Catch on Fire?Yes.Not necessarily. Although oxygen in the air is by far the most common oxidizing agent a combustible material may or may not need air to ignite.
Does It Need An Ignition Source to Catch on FIre?Yes.Not necessarily, a combustible material may ignite by other means.
Does it need to be preheated to become ignitable?Up to a certain temperature, the specific value varies by country.A combustible material may need up to an extremely high temperature to become ignitable.

Furthermore, tests are essential to determine if a given material is flammable or not and if it is combustible or not. There can be different methods of testing for the same material. Some materials are relatively new and have never been properly evaluated in regards to their flammability.

What Is Required For Something To Catch On Fire?

Most fires start when substances in the gaseous state undergo a combustion reaction which releases enough heat to make molecules within the gas mixture to ionize. This gas ionization is what produces the flames.

The heat of the fire mainly comes from the difference in energy between the substances being consumed and formed in the combustion reaction. For example when methane and oxygen react with each to form carbon dioxide and water heat is produced.

It so happens that molecules of carbon dioxide and water are molecules with less energy in their chemical bonds in comparison with the chemical bonds in the molecules of the initial material (methane and oxygen).

When a substance is oxidized in a combustion reaction that produces heat, the oxidized substance can be called the fuel, while the substance that oxidizes the fuel in the same reaction can be called the oxidizing agent.

Is Every Flammable Material Considered a Fuel?

No, a fuel does not need to be a flammable material. Fuel is a more general term which includes not only substances that are consumed (oxidized) in combustions but also includes any material that is consumed by other means.

For example, in nuclear plants, radioactive materials are used to generate energy. In these cases the radioactive material does not undergo a combustion reaction (or any chemical reaction whatsoever) but a nuclear reaction.

Does Every Combustion Reaction Produce Fire?

A combustion reaction is a chemical reaction in which a starting material or substance is oxidized and another is reduced. In the above example, methane was oxidized to carbon dioxide and oxygen was reduced to water.

Not every combustion reaction generates enough heat to produce a fire. In fact, there are countless reactions happening in every living organism that are technically combustion reactions. 

Therefore, when a reaction is referred to as a combustion reaction it is implied that the reaction generates a relatively large amount of heat, which sometimes is enough to produce flames. Oxidation-reducing chemical reactions that don’t generate a lot of heat are simply called oxidation or reducing reactions.

What Is The Ionization Of A Gas?

When an electrically neutral molecule gains or loses an electron the overall charge of the molecule changes. Ionization can happen by different means, high temperatures is one of them.

What Are Examples Of Ignition Sources?

What every source of ignition has in common is that they provide energy to the combination of a combustible material and the oxidizing Sources of ignition include:

  • Naked flames (a flame that is exposed).
  • Electrical sparks.
  • Static electricity.
  • Hot surfaces.
  • Mechanical shock.
  • Friction.

What Are Examples Of Oxidizing Agents?

The most common is the oxygen (O2) in the air. Ozone (O3) higher up in the atmosphere is another example. Potassium permanganate (KMnO4) is a very strong oxidizing agent which is a chemical used in many processes in the chemistry industry.

The main ingredient in bleach, sodium hypochlorite (NaClO) is also a very strong oxidizing agent. Strong acids such as sulfuric acid (H2SO4) and nitric acid (HNO3) can also act as strong oxidizing agents.

Oxidizing agents don’t need to have an oxygen atom in them, examples of oxidizing agents without oxygen include: fluorine gas (F2), chlorine gas (Cl2)

What Is The Flammability Limit?

When a flammable material is dispersed in the air there limits of concentration below and above which the material will not catch on fire, these limits are referred to as flammability limits.

What Is The Relation Between Flammability and Flash Point?

The flash point of a material is the temperature in which ignition can start right above the given material at normal pressure and air once an ignition source is applied. Most everyday materials have their respective flash points established after tests and can be consulted in the literature or in safety data sheets.

There are two main types of methods to determine the flash point of something, they are called open cup and closed cup. And within these types there are many variations. For example, a well known example of open cup method is the Cleveland open cup.

The lower the flash point of something the more likely it is for its vapors to be able to catch on fire. However, the temperature of the fire (that is its strength or its destructive power) is not directly related to its flash point.

For instance, cooking oils produce extremely high temperature fires once ignited but their flash point is normally above 200 ºC (392 ºF).

By the GHS regulations a liquid is considered flammable if its flash point is equal or below 93 ºC (199 ºF)

What Is The Fire Point?

The fire point of a fuel is: “the lowest temperature at which the vapor of that fuel will continue to burn for at least five seconds after ignition by an open flame of standard dimension”.

The fire point is equal to or above the flash point, since at the flash point the ignition may not have enough energy to self-sustain the flames for more than an instant.

The fire point is usually applied to common fuels, many flammable substances do not have their respective fire points well established given the high cost and risks associated with properly measuring the fire point.

What Is The Autoignition Temperature?

Also known as, kindling point, autoignition temperature of a substance is the lowest temperature in which it spontaneously ignites without going in contact with an ignition source at pressure of 1 atm and atmospheric air.

If the autoignition temperature of a gas is 54 ºC (129 ºF) or below it is considered a pyrophoric substance.

Liquids and solids that spontaneously ignite within five minutes after going in contact with air are also considered pyrophoric (they do not necessarily react with oxygen, the water in the air may be the oxidizing agents for some pyrophoric liquids or solids).

What Are Fumes?

It is common for fumes to be produced in  fire. Fumes are considered any particle, solid, liquid or gas, that goes into the air during heating or a fire. Commonly the use of the term fume refers to something that is harmful or dangerous in some way.

Sometimes, fumes can also refer to vapors liberated by materials at regular temperatures.


In this article it was attempted to give some insights into the meaning of flammable and combustible materials. The main outtake is that these classifications are not strictly objective since it is almost impossible to develop definitions of these nature to be equally applicable in every region of the world and in every work or academic field.

Frequently Asked Questions (FAQ): What Is The Difference Between Flammable And Combustible?

What is the difference between flammability and combustion?

The flammability of something indicates how easily it can burn in the air. The flammability can be attributed by degrees or classifications which aim to categorize materials that are extremely unlikely to catch on fire, materials that will catch on fire after subjection to intense heat, materials that will catch on fire by simply having an ignition source applied to them and to materials that can spontaneously burn at room temperatures. Combustion is a chemical process by which substances catch on fire by going through a chemical transformation which requires both an oxidizer and a reductor. Not every combustion leads to fire.

Is gasoline flammable or combustible?

Automobile gasoline is flammable given how its flash point temperature, but it is also combustible. Some aircraft fuels (also called aircraft gas) are not considered flammable given the high value of their flash point. Ultimately it will depend on the local standard for flammability. By some definitions a flammable substance must have a flash point of up to 93 ºC (199 ºF) while by other definitions the flash point has to be of up to 38 ºC. Kerosene, a commonly used hydrocarbon in aircraft fuels has a flash point between 38 ºC and 52 ºC.

Is oil flammable or combustible?

Oil can burn therefore it is combustible, if it burns easily it is also flammable if not it is only combustible.

Is every flammable material also combustible?

By some definitions yes. In the end it will depend on the competent local authority.

Is oxygen combustible or flammable?

Oxygen (O2) is neither flammable or combustible, but oxygen is present in the vast majority of fires as the oxidizing agent. If a given room or compartment has a high concentration of oxygen, materials that otherwise are stable can turn into fire hazards for ambients enriched with oxygen can cause cloth or hair to easily catch on fire.

Is diesel flammable or combustible?

Diesel has a flash point between 53 ºC (127 ºF) and 93 ºC (194 ºF) which means that by some standards it is considered flammable and by other standards it is considered non-flammable.



Treese, S. A.; Pujado, P. R.; Jones, D. S. J. Handbook of Petroleum Processing (2 ed.). Springer. 2015. p. 1736.

Laurendeau, N. M.; Glassman, I. Combustion Science and Technology. 1971. 3, 77. doi:10.1080/00102207108952274 https://doi.org/10.1080/00102207108952274

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