Dust - Formed by the breaking down of solid materials, normally when materials are altered. For example, sanding, cutting, grinding and brushing. In general, the smaller the dust particle, the greater the hazard that it presents. Fibres from materials should also be treated as dusts.
Mist - Formed by the processes that involve atomisation (such as spraying, cleaning and cutting/grinding using coolants) and consists of tiny liquid droplets rather like steam in a bathroom.
Fume - Formed by the vapourisation of a solid material by the application of intense heat. Extremely fine particulates are formed as the fume cools and condenses. Many processes form fume, such as smelting, pouring metals and many welding applications.
Vapour - A gaseous state formed by evaporation from substances that are normally solid or liquid at room temperature. Generally released at room temperature (petrol, methylated spirits), many industrial processes used in degreasing vapourise particularly quickly once heated.
Gas - An air-like substance at room temperature. Gases can travel far, very quickly.
Oxygen Deficiency/Enrichment - When an atmosphere is likely to contain less than 18% oxygen (or where conditions may exist in the future for this to happen); and in certain circumstances where the risk may be oxygen enrichment, which can lead to explosion and severe impairment of operatives. Note: Monitoring devices should always be used to check oxygen levels of an unknown environment. Conventional dust masks are not suitable for oxygen deficient situations.