Flammability vs. Combustibility — What’s the Difference?

If you frequently come into contact with chemicals at your home or workplace, you may notice labels that say flammable or combustible. While flammability and combustibility both describe the ability of these chemicals to catch flame, there are slight differences between these two terms. The level of risk a material provides is directly affected by whether it is flammable or combustible. Understanding these two properties will allow you to handle and safely store these materials to keep yourself and your surroundings safe.

What Is Flammability?

Flammability can describe a solid, liquid or gas that burns with a flame. A flammable object does not have to reach a specific temperature threshold before it can ignite. Flammable objects can burn with a flame at ambient temperatures.

Flammable and combustible materials have flashpoints, which are temperatures that produce enough vapor near the surface to flash or briefly ignite after exposure to an open flame. For flammable liquids, the flashpoint is close to or lower than normal working temperatures, making them more likely to ignite. A lower flashpoint means the liquid is more volatile.

An ignition source can take the form of static electricity, open flame sparks or a hot surface. A flammable material will only burn with an external ignition source. Some common examples of flammable liquids include acetone, benzene and gasoline.

When a flammable material ignites, it is not the material that burns. Rather, the vapor and air around it burn. For a flammable vapor to catch fire, it requires an ignition source, such as static electricity, open flames, hot surfaces or sparks. To keep your home or business safe, keep any possible ignition sources away from flammable liquids, materials and vapors.

What Is Combustibility?

The term combustible can also describe the burning ability of a solid, liquid or gas. However, unlike a flammable material, you must raise the temperature of a combustible material for it to burn. Combustible materials have a higher flashpoint than flammable materials. Combustible liquids have a flashpoint above at or above 37.8°C (100°F) and below 93.3°C (200°F). If a material has a flashpoint higher than the usual temperatures in the climate zone or work environment where the material will be used or stored, it is combustible rather than flammable. In other words, the material would require heating before it becomes a combustion risk. In comparison, flammable materials can burn spontaneously at these ambient temperatures.

A combustible material can also burn spontaneously. The process of spontaneous combustion occurs when the temperature of the material rises to its flashpoint through oxidation. If the heat generated during the oxidation process cannot dissipate, it will continue building until ignition begins. Generally, spontaneous combustion occurs when the material is in a pile when the heat generated cannot escape.

For spontaneous combustion to occur, the air must be moving at a rate to create an oxidation reaction but not so fast to stop the material from reaching its flashpoint. Since this balance of oxygen and heat loss requires a careful balance, spontaneous combustion is not common. It is also simple to prevent spontaneous combustion by placing combustible materials in sealed metal containers to prevent excess oxygen from entering the can.

Since combustible materials require a higher temperature before releasing an easily ignitable vapor, they are less volatile than flammable materials. Some common examples of combustible materials include kerosene and mineral spirits.

Physical Properties of Flammable and Combustible Liquids

While flammable and combustible liquids have some key differences, they also have crucial similarities. Some physical properties these two materials share include:

• Flashpoint: The flashpoint is the main characteristic that defines flammable and combustible liquids. A flashpoint is the minimum temperature at which a liquid releases vapors that can ignite when combined with air near the surface of a material.
• Boiling point: The boiling point is the specific temperature at which the vapor pressure of the liquid is equal to the pressure of the atmosphere. The harder a liquid boils, the more vapor it will release.
• Flammable or explosive range: The flammable or explosive range of a liquid is the amount of concentrated vapor that must be present for an explosion to occur. If there is not enough fuel for ignition, the vapor concentration is “too lean.” A vapor concentration that is “too rich” does not have enough oxygen for ignition.

Gasoline is a flammable liquid most people are familiar with. The flashpoint of gasoline is approximately -36 degrees Fahrenheit, which is lower than the usual working temperatures we use gasoline in. It’s a hazard to smoke at a gas station because the temperature is well above the -36 degree flashpoint. Some common examples of combustible liquids include kerosene, carbolic acid, formic acid and many oils.

Many chemical providers will supply businesses with a Safety Data Sheet (SDS) to accompany the chemicals they deliver. These data sheets contain safety information such as the chemical and physical properties and data on whether the material is flammable or combustible.

Industries ranging from hair salons to industrial manufacturing plants use flammable and combustible liquids. These materials are also present in many homes in materials ranging from cleaners to food products. With so many uses, the probability of using or being around flammable or combustible materials is high. Understanding the differences between these two properties will allow you to properly use and store these materials and prevent fires and other dangerous situations.

Where Do I Store Flammable Materials?

If you are storing, transporting or using flammable materials in an environment with ambient temperatures above their flashpoints, you must use extreme caution to prevent combustion or ignition. Flammable liquids release a vapor that can ignite at standard working temperatures, especially in a poorly ventilated area. Similarly, a combustible material can release a flammable vapor when its temperature rises above its flashpoint. These vapors are invisible and dangerous.

Since the vapor released from a flammable liquid is gas, it can easily absorb into other materials. These materials that absorb the vapors can then release this vapor back into the air, even after you clean up the primary source of flammability. Flammable and combustible vapors are often denser than air and can spread quickly. If these vapors ignite, the flame can travel back to the liquid source by following the vapor fumes. This occurrence is known as a flashback.

To reduce significant fire hazards, here are some storage tips for flammable and combustible materials.

• Correct ventilation: Vapors are denser than air, causing them to ignite easily in areas with poor ventilation.
• Separate materials from ignition sources: Store flammable and combustible materials away from open flames or sparks to prevent fire hazards.
• Keep incompatible materials away from one another: When incompatible flammable and combustible materials mix, it can cause dangerous chemical reactions such as oxidation and spontaneous combustion. Consult a dangerous goods segregation table to determine the correct distance for spacing these materials.
• Contain spills: Proper cleanup is vital when working with flammable and combustible materials to prevent cross-contamination.
• Reduce human error: Frequently check on your storage units and the hazardous materials inside to ensure they do not have any signs of damage and are without leaks or punctures. Use all storage units properly for maximum safety.

Contact Hazmat School Today

Proper training for handling and storing flammable and combustible materials allows you to keep your business safe. At Hazmat School, our online training courses are a convenient and affordable way for your employees to learn about flammable and combustible materials safety. Our engaging online courses provide useful links and reference books as well as 24/7 access to instructors. To learn more about our OSHA HAZMAT courses and certification opportunities, please contact us today!