A number of industrial processes produce combustible dust as a by-product. The most common being wood dust. Combustible dust has been defined as: "a solid material composed of distinct particles or pieces, regardless of size, shape, or chemical composition, which presents a fire or deflagration hazard when suspended in air or some other oxidizing medium over a range of concentrations." In addition to wood, combustible dusts include metals, especially magnesium, titanium and aluminum, as well as other carbon-based dusts. There are at least a 140 known substances that produce combustible dust. While the particles in a combustible dusts may be of any size, normally they have a diameter of less than 420 μm. , the United States Occupational Safety and Health Administration has yet to adopt a comprehensive set of rules on combustible dust.