Welding "smoke" is a mixture of very fine particles (fumes) and gases. Many of the substances in welding smoke, such as chromium, nickel, arsenic, asbestos, manganese, silica, beryllium, cadmium, nitrogen oxides, phosgene, acrolein, fluorine compounds, carbon monoxide, cobalt, copper, lead, ozone, selenium, and zinc can be extremely toxic. Welding fume is a mixture of airborne fine particles. Toxic gases may also be generated during welding and cutting. More than 90% of the fume arises from vaporisation of the consumable electrode, wire or rod as material is transferred across the arc or flame. The range of welding fume particle size is shown in relation to more familiar types of dust and fume. The respirable fraction of particles (especially less than 3μm) are potentially more harmful as they can penetrate to the innermost parts of the lung. Various gases and fumes can be generated during welding. Welding fumes are metal-containing aerosols consisting of particles formed through complex vaporisation-condensation-(oxidation) or vaporisation-(oxidation)-condensation processes during welding. The fumes are therefore complex in their composition and their structure.