Posted By Pat Hannaway Hi, the definitions (according to IOSH Principles of Health and Safety at Work) are as follows: Fumes are solid particles formed by condensation from the gaseous state, e.g. metal oxides from volatised metals. THe can flocculate and coalesce. Their particle size is between o.1 and 1 micron. Gases are any substances in the physical condition having no definite volume or shape, but tending to expand to fill any container into which they are introduced. Vapours are the gaseous forms of substances which are normally in the solid or liquid state e.g. sodium vapour in luminaires. They are generated by decrease from normal pressure or temperature. Hope these are of use. PatH