Oily rags left in closed containers can present a serious risk of fire. Many people do not believe that oily rags left undisturbed could ignite all by themselves without the presence of an ignition source, such as a spark or lighted match. However, oily rags stored in a waste can or a in pile on the floor, can definitely ignite, even without any help from a separate ignition source. This is known as spontaneous combustion.

Spontaneous combustion occurs when a combustible material is heated to its ignition temperature by a chemical reaction involving the oxygen in the air around the material. This heating process is known as self-heating. In the case of oily rags, it’s a relatively simple process of oil oxidizing generating heat, which if not dissipated, will build up until combustion occurs. Generally, this can happen when the materials are left in piles, which provide a source of insulation, trapping the heat that is generated.

A number of materials are moderately or highly prone to self-heating and spontaneous combustion. Those materials can include rags, cotton or other fibrous combustible material, that have come into contact with oil based paint; rags that are damp with any one of a number of different types of oils, including vegetable oils; and oily uniforms or work clothes.

The possibility of spontaneous combustion increases when the surrounding air is also warm and dry. Also, heat radiating from nearby sources, such as machinery or a non-insulated steam line, can accelerate the self-heating process by heating the combustible materials and the surrounding air.

It is simple to prevent spontaneous combustion. Materials subject to spontaneous combustion should always be stored in metal containers with covers in place. A metal safety can with a self-closing lid manufactured specifically to hold oily waste is recommended for this purpose. Admittedly, the container will contain oxygen at first. However, the oxidation process will use up the limited oxygen and the reaction will stop, thus preventing a fire. These safety containers should be placed at key locations (away from ignition sources) and they should be removed from the site as soon as possible.

Oil-soaked rags should never be disposed of in trash compactors, trash dumpsters, or ordinary trash cans. When removing rags from your property, consider whether there is a local hazardous waste disposal center nearby that will accept them, or a company who may be contracted with to remove them.

In our next post, we will review examples of what can happen if oil-soaked rags are not disposed of properly.