Flammables and combustibles explained

There are various factors that people should consider when working with flammable and combustible liquids. Kevin Murphy, MD of environment, health, and safety provider Spill Doctor, gives us the rundown on these dangerous fluids.

A liquid is classified as either flammable or combustible depending on its flashpoints. Generally speaking, flammable liquids will ignite and burn easily at normal working temperatures. Combustible liquids can burn at temperatures that are usually above working temperatures.

There are several specific technical criteria and test methods for identifying flammable and combustible liquids. Under the Workplace Hazardous Materials Information System (WHMIS), flammable liquids have a flashpoint below 37.8 °C, while combustible liquids have a flashpoint of between
37.8 °C and 93.3 °C.

Flammable and combustible liquids are present in almost every workplace, including fuels and many common products like solvents, thinners, cleaners, adhesives, paints, waxes, and polishes. Everyone who works with these liquids must be aware of the hazards involved and be taught how to work safely with them.

What is a flashpoint?

The flashpoint of a liquid is the lowest temperature at which it gives off enough vapour to ignite and start burning at the surface of the liquid. Sometimes more than one flashpoint is reported for a chemical; since methods and liquid purity during testing may vary, flashpoints are intended as guides only, not to draw fine lines between safe and unsafe.

Does the liquid itself burn?

Flammable and combustible liquids themselves do not burn. Rather, it is a mixture of their vapours and air that burns. Even at temperatures as low as -40 °C, petrol gives off enough vapour to form a burnable mixture in the air. The combustible liquid Phenol, on the other hand, has a flashpoint of 79 °C, so it must be heated above that temperature before it can ignite in air.

How can flammable and combustible liquids be a fire or explosion hazard?

At normal room temperatures, flammable liquids can give off enough vapour to form burnable mixtures with air. As a result, they can pose a serious fire hazard. Flammable liquid fires burn very fast and give off a lot of heat, often accompanied by thick clouds of black, toxic smoke.

Combustible liquids also release enough vapour to form burnable mixtures with air at temperatures above their flashpoint; hot combustible liquids can pose a fire hazard just as serious as flammable liquids.

Spray mists of flammable and combustible liquids in the air may burn at any temperature if an ignition source is present, while the vapours of these liquids are usually invisible and can therefore be hard to detect unless special instruments are used.

Most flammable and combustible liquids flow easily, so a small spill can cover a large area of workbench or floor. Burning liquids can flow under doors, down flights of stairs, or even into neighbouring buildings to quickly spread the fire to a wide area. Materials like wood, cardboard, and cloth can also easily absorb flammable and combustible liquids, meaning that even after a spill has been cleaned up, a dangerous amount of liquid may still remain in surrounding materials or clothing, giving off hazardous vapours.

What is the danger of flashbacks?

Open containers of nearly all flammable and combustible liquids may give off vapours that are heavier than air; when ventilation is inadequate, vapours may settle and collect in low areas like sumps, sewers, pits, trenches, and basements. The vapour trail can spread far from the liquid and, if it contacts an ignition source, the fire produced can flash back to the liquid source. Flashback and fire can happen even if the liquid giving off the vapour and the ignition source are very far – even several floors – apart.

Can flammable or combustible liquids be hazardous to my body?

The most obvious potential bodily harm comes from the danger of a fire or explosion, but even after the immediate danger of a fire is gone, these liquids sometimes have other properties that may be hazardous to the body. Certain liquids may, for example, cause health problems depending on the specific route of exposure (including breathing the vapour/mist; eye or skin contact; or swallowing/ingestion).

Some flammable and combustible liquids are corrosive, and many undergo dangerous chemical reactions if they contact incompatible chemicals such as oxidising materials, or if they are improperly stored. An example is 2-propanol (also known as dimethylcarbinol, isopropanol, or isopropyl alcohol). This colourless liquid has a sharp odour similar to rubbing alcohol or a mixture of ethanol and acetone. It is flammable in both liquid and vapour form and because the vapour is heavier than air it can spread long distances, meaning distant ignition and flashback are possible. It is also considered to be a mild central nervous system depressant. Dense vapour may cause headaches, nausea, dizziness, drowsiness, incoordination, and confusion, while 2-propanol may also irritate the respiratory tract and/or eyes.

The material safety data sheet and the supplier’s labels on their containers should tell you everything you need to know about the hazards for specific flammable and combustible liquids with which you work. But you can always contact a knowledgeable health and safety provider, like Spill Doctor, if you need any further assistance.