What Makes Something Flammable? (Definitive Guide)

We can see how fire remains dangerous to humanity in all the wars we have memorized or read about. In fact, it is one of the horrible reactions we know that can force us to release our tears with sorrow.

Even living beings try to stay away from the fire for its heat, flames, ignition, and combustion. But most of us don’t know which staff will get fire in what condition. Likewise, we don’t know what makes something flammable.

Any material’s flammability refers to getting quick contact with oxygen. The strength of bonds between the molecules and the ease of oxidation controls the ability to ignite. Among many definitions of something being flammable, OSHA 29 CFR 1910.106 defines flammability as “burns in the presence of a spark at under 199.4 degrees Fahrenheit”.

If we talk about the flammability of any substance, we need to explain many things. Thus, it becomes a complex topic for discussion. This article will illustrate the flammability, ignition, and combustion of anything and some relevant issues.

What Makes Something Flammable

What is flammability?

Flammability is an innate idea of any material. It refers to the ability of the respective substance to burn or ignite, causing combustion or fire. Hence, chemical combustion requires a degree of difficulty, which is quantified through fire testing.

Generally, a material can be rated as extremely flammable, flammable, and non-flammable. When you read section 9 of a safety data sheet (SDS), you may find these ideas.

Another definition includes that a flammable material can readily catch fire or ignite at an ambient temperature. It refers to when you expose a material to a naked flame or spark; it will catch fire without any need for pre-heat.

Again, the material will not always be flammable at the same pattern, level, or condition. It depends on the different compositions of the same material, which respond differently to any naked flame.

Examples of Flammability:


It normally remains liquid at room temperature with very weak bonds between molecules. These bonds are easy to break so you can burn them quickly. When you burn alcohol, the burning of carbon and hydrogen is exothermic as it releases heat.

It refers to self-propagation of burning reaction meaning no need for help till the oxygen is present.


Sand is silicon dioxide and doesn’t burn. Silicon has a full bond with oxygen. In this sense, sand will catch fire when you burn the silicon. However, you can burn it with extremely high heat to break it into sole atoms. But the silicon of the sand doesn’t burn.

So, there is no chance of sand burning. Also, sand is not combustible.

Wood dust

Wood dust particles are very small. But they have a large surface area compared to their volume. When you blow the dust through the air, it will lead to extreme oxygen to surface area ratio.

As a result, wood dust can easily burn. It is so flammable that the wood dust will start burning with a simple strike of naked flame.

Wooden table

A wooden table has strong bonds between molecules. The bonds are very strong if you think of a large chunk of wood. This is the reason why the table will need more heat to burn. In terms of sawdust, the bonds perish so that it is highly flammable.


Paper is a result of mulched wood. If you light a naked flame, the paper will not explode. But it can burn the paper if the flame heat matches to ignition point of the paper.

Combustible vs. Flammable: What’s the difference?

Flammable and combustible materials (including liquids) can burn. But they have different flashpoint ranges. For easier understanding, I will discuss flammable and combustible liquid.

However, it is very important to know the physical properties of these liquids. Let me share them first.


Flashpoint is the base of defining a liquid (or any substance) as a combustible or a flammable one. It refers to the minimum temperature when vapor is released from the liquid and ignites in the presence of oxygen. Flammable objects have fewer flash points than combustible objects.

Boiling point:

The boiling point determines at which temperature the liquid boils. It happens when the vapor and atmospheric pressure of a liquid match together.

Generally, boiling substances produce more vapors, especially the hardness of boiling controls the vapor release.

Differences between Flammable and Combustible:

Flammable Objects (Liquids)

According to OSHA under 29 CFR 1926 and 29 CFR 1910, the definition of a flammable liquid is:

“[A]ny liquid having a flashpoint below 140 °F. and having a vapor pressure not exceeding 40 pounds per square inch (absolute) at 100 °F.” (Construction standard)

It refers to if a liquid can be flammable if it has a below-140°F flashpoint. Also, it must have a vapor pressure of 40lbs. per square inch or below it at 100°F. When the liquid exceeds the flashpoint, it will combust instead of igniting. However, this definition works for the liquids used in the construction areas.

Another definition:

“[A]ny liquid having a flashpoint below 100 °F. (37.8 °C.), except any mixture having components with flashpoints of 100 °F. (37.8 °C.) or higher, the total of which make up 99 percent or more of the total volume of the mixture.” (General industry)

This definition works for the liquids we use in general industries. These liquids may have a maximum 100°F flashpoint.

A liquid must not have more than 140°F flashpoint from the definitions. Below this temperature, the material’s vapor will start igniting. You should know that the lowest flashpoint of liquid makes it the most volatile.

Examples: Ethyl chloride, pentane, acetone, gasoline, turpentine, mineral spirits

Combustible Objects (Liquids)

According to OSHA under 29 CFR 1926 and 29 CFR 1910, the definition of a combustible liquid is:

“[A]ny liquid having a flashpoint at or above 140 °F. (60 °C.) and below 200 °F. (93.4 °C.).” (Construction standard)

The definition refers to the substances that have flashpoints ranging from 140°-200°F. When the liquid (or the substance) reaches this temperature, its vapor will start igniting.

Comparatively, this flashpoint level is more than flammable objects. In other words, any liquid having a flashpoint more than a flammable one and a fire point less than its boiling point is known as a combustible liquid.

“[A]ny liquid having a flashpoint at or above 100 °F. (37.8 °C.).” (General Industry)

In the general industry, combustible liquids have a flashpoint starting from 100°F.

Both definitions clarify that flammable liquids can ignite and burn quickly at regular working temperatures. But combustible liquids can be a bit hard to ignite or burn at the same temperature.

For instance, a particular flammable liquid may burn below 60°F, and if it is a combustible liquid, it may have a flashpoint above 60°F.

Examples: Engine oil, diesel, kerosene, linseed oil

Is combustible the same as flammable?

No, they are not the same. A combustible material has a flashpoint that is more than flammable material. Consequently, a flammable object can easily burn at working temperature, whereas combustible needs heat to burn.

Again, the flammable thing can be explosive at the same temperature, while the latter one will still remain safe for us.

Is flammable more dangerous than combustible?

It is not easy to say which one is safer or more dangerous. It mostly depends on how easily an object can catch fire or come close to fire.  

Think about alcohol; it has a flashpoint of 55°F, which is close to regular room temperature. Alternatively, TNT (dynamite) has a 176° flashpoint.

Theoretically, alcohol is flammable, and TNT is combustible. But it is tough to say which one we would prefer to stay with it in a room.

Another example can be a rubber tire. It has a higher flashpoint. But it is still dangerous as it emits toxic fumes that are absolutely dangerous. Also, you won’t love to breathe in hydrogen cyanide.

Flammable vs Ignitable – What’s the difference?

Usually, we define something flammable as it burns readily at an ambient temperature. In that sense, any flammable thing is ignitable. But it does not refer to anything ignitable that can be surely flammable.

Ignitable means it can catch fire. But we need not mention any given temperature or condition set to the ignitable things. Moreover, almost anything can be ignitable if under the right condition. However, it may not withstand a flame.

According to FMG DS 7-29, ignitable liquid has a measurable fire point. We cannot use the terms combustible or flammable liquid onto them. Again, FMG DS 7-29 categorizes ignitable liquids by their flashpoints.

The safety measures are also taken depending on the liquid’s flashpoint and the container (type and size) to store the liquid.

Some hazardous wastes are categorized as ignitable. Here are some of them:

  • Liquids less than 140°F flashpoint
  • Solids that burn impulsively
  • Oxidizers and flammable compressed gas
  • Materials having 3 or 4 flammability hazard ratings according to NFPA and HMIS

In general, ignite means the process of starting a fire to any material. It is the beginning of burning something. Hence, it is like you are describing the process of starting an argument with others.

Again, it suggests something that happens faster yet with an impactful result. After the ignition, an object can burn spontaneously.

So, the main difference between these two is that an object ignites first, then burns while maintaining its respective flashpoints.

What Makes Something Flammable?

Flammability is a characteristic of any substance. It suggests whether it will burn easily or require more fuel to burn. According to OSHA and NFPA of the United States, we can consider a liquid as flammable that can catch fire below 100°F.

Hence, the temperature at which the liquid burns is called the flashpoint. If it is oil, then it will be the smoke point. On the other hand, the United Nations identifies the liquids as flammable, igniting at 0-140°F.

So, there is a difference between the two theories of the USA and the UN. The USA defines those liquids as flammable, which will burn at room temperature. Alternatively, UN theory requires hotter than regular room temperature.

The definition of a flammable solid is based on the temperature when it burns immediately on exposure to a naked flame.

Overall, it is a bit tough to define or other people to understand the definition of “flammable.”

What makes something burnable/combustible?

The definition of “flammable” is also complex to define “combustible.” Ironically, there is no absolute definition of both flammability and combustibility.

It is more understandable if we know the differences between flammable and combustible. It can help to understand that a combustible material can catch fire at its flash point and how it is not a flammable material.

There are differences in measuring combustibility in different parts of the world. Even many areas only give value to combustibility. They may ignore flammability at all.

The core concept of combustibility is the reason why they consider it more. It is a simple measurement of how faster a material can burn. This is where the confusion arises between combustibility and flammability.

But the most considerable way is selecting an authority who defines the theories. There are lots of authorities around like Canadian Building Code, the OSHA regulations, UN transport definitions.

Also, you can pick other definitions that are sincere to the local laws. For instance, the British Standard for combustibility stands as 76-4:1970. It needs to heat three samples of the same material in a heating furnace.

What flashpoint is considered flammable?

A flammable liquid is considered a combustible liquid only when you can easily ignite it in the air at room temperature. Many national and international organizations have set standards for the flashpoint of this liquid.

The OSHA considers a liquid flammable when it has a flash point less equal to 199.4°F. The same liquid is flammable according to the UN GHS standard of 2012 when it has a flashpoint below 100°F.

They also consider the liquid with 100-200°F flashpoint to be a combustible liquid. Experts suggest that the flashpoint is the determiner of a liquid’s flammability. Also, it depends on the altitude.

Liquids with closed cup flash points under 100°F and vapor pressures below 40 PSI at 100°F are flammable liquids. They are referred to as Class 1 liquids.

1.   Class IA liquids – flashpoints below 73°F (22.8°C) and boiling points below 100°F (37.8°C).

2.   Class IB liquids – flashpoints below 73°F (22.8°C) and boiling points at or above 100°F (37.8°C).

3.   Class IC liquids – flashpoints at or above 73°F (22.8°C) and below 100°F (37.8°C).

(Source: https://www.northeastern.edu/ehs/ehs-programs/laboratory-safety/general-information/flammable-and-combustible-liquid-definitions/)

Flashpoint points to the lowest temperature when any chemical vaporizes and ignites in the air. If a material has a lower flashpoint, it is considered the higher flammable one. The vapor may vanish to start burning if you remove the ignition source.

Additionally, flash point is different from auto-ignition temperature. It means a chemical can burn without any need for an ignition source.

How do you know if a liquid is flammable?

After looking at it, it is fully driveling if you want to say something flammable or combustible. In most cases, you don’t need to look at those chemicals. If you see a thing that burns easily, it may have a warning label on its package, like household things.

Through this, you may learn how quickly it will burn. Also, you will have a way to prevent the burning.

You may also check out online if the material is flammable or not. In most cases, things we use with high water relatively take more time or heat to burn as the water doesn’t catch fire. But when we evaporate the water from the object, the leftover may burn.

The expert suggestion will be that you can consider it flammable if you cannot figure out the selected material’s flammability. Also, try to keep it far from any naked flame or ignition sources.

What are common household items flammable?

People are becoming aware of flame safety within their residences day by day. Some household materials are extremely prone to catch fire, whereas others are least risky.

However, there are many household things or items that are flammable:

Rubbing alcohol:

It is already considered flammable, yet sometimes we are not aware of how much damage it can do. So, it is better to store far from open flame.

Nail polish and remover:

Flammable acetone is a core nail polish and nail polish remover ingredient. Also, the vapor a nail polish or remover can produce is ignitable near a flame.

Shoe polish:

Shoe polish contains natural and synthetic ingredients. Some of these are flammable, and even many manufacturers use combustible charcoal dust in these products.

Plastic containers or bottles:

Plastics are extremely flammable. Yet some manufacturers try to add additives to make the containers less likely to burn. Still, they can burn if you keep them near high heat.

Aerosol cans:

are unsafe as they contain flammable liquids that can easily catch fire. So, you should keep them far from any heat-related objects like air vents and furnaces.

Hand sanitizer:

Most sanitizers can make life hell if they come close to heat or an open flame. You should not use them or place them when carrying a lighter or smoking.

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So, what do you think, “what makes something flammable”? A flammable thing or object means that its vapor will start igniting once it reaches its flashpoint.

However, you can confuse combustibility and flammability. But I have made it clear in this article. I hope you will easily understand the facts and maintain safety.


OSHA = Occupational Safety and Health Administration

GHS = Globally Harmonized System of Classification and Labeling of Chemicals

Flashpoint = at which temperature a material’s vapor ignites.

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