Factors affecting composition and quantity of fume and gases
The quantity and composition of fume and gases generated during welding is influenced by the following:
- type of process
- welding consumable
- presence of any material coatings
- nature of MMA flux coating, or core of FCA consumable
- welding parameters
- shielding gas composition
The two most important are the welding process and the choice of consumable.
Welding processes
Gas welding
Gas welding fume contains pollutants formed by combustion of the fuel gas. When an oxidising flame is used, these will be carbon dioxide with oxides of nitrogen but, for a slightly reducing flame, carbon monoxide will also be present.
Safe practice and accident avoidance
Providing there is good ventilation, the levels of fume and gases generated when welding mild steel are normally well below any workplace exposure limits (WELs). No special precautions are required.
Manual metal arc (MMA)
A MMA rod has a suitable composition for the weld metal deposited but the flux covering provides gases for the arc, additional alloying elements and slag for protecting the weld pool. The composition and quantity of the fume generated will depend on the process variables, for example, type of consumable, polarity, voltage and current. The toxic elements in the fume will be similar to those in the consumable, although the proportions are likely to be different.
Safe practice and accident avoidance
For normal welding operations, the exposure levels of fume constituents will generally exceed relevant WELs. Local exhaust ventilation (LEV) should be used to remove fume at source. As stainless steel fume, in particular, causes respiratory tract sensitisation (the welder becomes more susceptible to occupational asthma), special care should be taken to control exposure to as low a level as is reasonably practicable.
Flux cored arc (FCA)
Continuously fed wire in self-shielded FCA welding contains a flux which produces a large amount of gas for the arc and protection/deoxidisation of the weld pool. Self-shielded wires are normally used for welding outdoors. With gas-shielded wires, which are only used in the welding shop, an additional shield (CO2 or argon-CO2) is needed to protect the weld pool. As FCA is normally employed at high welding current levels and higher duty cycle, more fume will be produced compared with MMA.
Safe practice and accident avoidance
Providing sensible precautions are taken, self-shielded FCA welding, taking place outside, will not need any fume removal measures. For gas-shielded welding inside a building, similar precautions to MMA welding should be used, such as LEV.
Metal inert gas (MIG/MAG)
MIG/MAG uses a solid wire and a separate gas to form the arc and shield the weld pool. The shielding gas is normally CO2 or a mixture of argon-O2 -CO2 but argon can be partly replaced with helium. As well as the effect of the welding parameters, the mode of metal transfer has a significant effect on fume levels.
Dip transfer mode operates at a low welding current level and has a characteristic short arc length. Fume levels are low. Spray transfer mode which operates at much higher current levels and at a greater arc length can generate higher fume levels, but a spray condition at minimum voltage generates low levels of fume.
Pulsed transfer mode operates at similar low current levels to dip transfer but with a longer arc length and generates intermediate fume levels depending upon the welding parameters.
Special mention must be made of the presence of ozone which is generated by the effect of ultraviolet radiation on the air immediately surrounding the gas shield as shown in the diagram. Welding with an aluminium-silicon wire, in particular, produces high ozone exposures. Over exposure to ozone may also occur during MIG/MAG welding of aluminium, mild and stainless steels.
Safe practice and accident avoidance
Use LEV to remove fume and ozone at source. If ozone is not adequately controlled by this method, the welder must wear Respiratory Protective Equipment (RPE).
Tungsten inert gas (TIG) and plasma
The arc is formed between the tungsten electrode and the workpiece with an inert gas. As the filler metal is added directly to the weld pool, metal does not pass through the arc, so there is hardly any fume generated. In open workshop conditions, exposures will not normally exceed WELs.
Welding of stainless steel may generate unacceptable levels of ozone.
Ozone is generated by the effect of ultraviolet radiation
on the air immediately surrounding the gas shield.
Safe practice and accident avoidance
Good ventilation does not require fume extraction but when welding stainless steel, control of ozone using LEV or LEV plus RPE may be needed.
Submerged arc
Submerged arc welding uses a bed of granulated flux to cover the arc. As the arc is not exposed, there is negligible emission of fume and gaseous pollutants, although exposure to fluorides should be considered when using highly basic fluxes.
Safe practice and accident avoidance
Special precautions are not usually needed but it should be remembered that dust may be produced when filling the hopper with flux.
Exposure to welding fume and gases
The effect of process, and consumable type on exposure assessment is summarised in the Table. The Indication of exposure levels assumes that no control systems are being used.
The generic term Workplace Exposure Limit (WEL) is used and may apply to one or all of the individual constituents of the fume. Assessment indicators are the fume and gas components most likely to exceed their respective WELs during welding.
The Welding Manufacturers Association has produced a standard format for hazard data sheets to enable manufacturers to comply with their legal obligations under the Health and Safety at Work Act 1974 Section 6. The hazard sheet contains the following information:
- chemical analysis of substances present in the fume produced by the consumable
- the appropriate WELs
For safe use of consumables, reference should always be made to the manufacturers' or suppliers' hazard data sheets.
It should be noted that in addition to fume, where appropriate, a risk assessment must also consider exposure to shielding gases, which may be toxic or asphyxiant. Gases are also produced by the action of heat on the welding flux or slag (carbon dioxide and carbon monoxide) and heat or ultraviolet radiation on the atmosphere surrounding the welding arc (nitric oxide, nitrogen dioxide and ozone).
Effect of process, and consumable composition on the assessment of exposure
| Process | Material | Indication of exposure levels | Assessment indicators |
|---|
| Gas welding |
Mild steel |
Below WELSs except in poorly ventilated or confined space |
Carbon dioxide, carbon monoxide (reducing flame) nitrogen dioxide |
| Manual metal arc (MMA) |
Mild steel and low alloy steels |
Above WELs |
Iron oxide, Manganese |
| Stainless steel |
Above WELs |
Hexavalent chromium or nickel |
| Aluminium, copper, nickel alloys, cast iron, hardfacing |
Generally in excess of WELs |
Exposure to aluminium, copper, nickel, chromium barium cobalt, depending on the alloy welded. |
| TIG and plasma arc |
Mild and low alloy steels |
Less than WELs |
Shielding gas can constitute a hazard in confined spaces by reducing the available oxygen to a level which will not support life |
| Stainless steel alloys |
Less than WELs for fume, greater than WEL for ozone |
Ozone, shielding gases in confined spaces |
| Metal inert gas/metal active gas (MIG/MAG) |
Mild and low alloy steels |
Generally in excess of any WELs |
Iron oxide, manganese, ozone, shield gases. |
| Stainless steel |
Greater than WELs |
Exposure to chromium, nickel, ozone and shielding gases. |
| Aluminium and aluminium alloys |
Greater than WELs for ozone and fume |
Aluminium oxide fume. Ozone levels can be very high, particularly with aluminium/silicon alloys. |
| Flux-cored arc |
Mild and low alloy steels |
Greater than WELs |
Iron oxide, manganese. Some self shielded consumables may give rise to soluble barium in fume |
| Stainless steel |
Greater than WELs |
Exposure to Hexavalent chromium, chromium, nickel |
Publications and relevant standards
- COSHH
- Essentials for welding cutting and allied tasks
- EH 40 (updated annually)
-
Workplace Exposure Limits, HSE Books.
Welding Manufacturers Association, Leaflet 236, Hazards from welding fume.
Note: When assessing fume risk, the suppliers' data sheets and hazard information must be fully consulted.
For more information, please contact us.
This Job Knowledge article was originally published in Connect, October 1997. It has been updated so the web page no longer reflects exactly the printed version.