Welding fume from mild steels, zintec (zinc alloys), stainless steels, brass, aluminium and phospor bronze all carry a significant risk of long term health problems if exposure is not properly controlled. As occupational hygienists and qualified safety consultants we have many years experience of carrying out air sampling surveys to determine occupational exposure to welding fumes and similar substances.
If you would like to speak to one of our safety consultants or occupational hygienists about an air sampling survey for wood dusts please contact us on 01453 800100.
Guidance – Welding of Galvanized (zinc coated) Products
Welding of galvanized steel is completed in a very similar way to welding of the bare steel of the same composition. The same welding processes, volts, amps, travel speed, etc. can be used with little modification when the switch is made from uncoated steel to galvanized steel, unless the zinc coating is unusually thick.The difference between welding galvanized steel and welding uncoated steel is a result of the low vaporization temperature of the zinc coating. Zinc melts at about 480°C and vaporizes at about 900°C. Since steel melts at approximately 1,500 °C and the welding arc temperature is 8,300 to 11,000°C, the zinc that is near the weld is vaporized. By the time the weld pool freezes, the zinc is gone giving rise to two immediate consequences:
- The vaporized zinc increases the volume of welding smoke and fumes.
- The zinc at and near any welds is actually burned off by the heat of the arc, removing the protective zinc coating.
Sometimes a white dust can be seen following welds and this is typically zinc oxide, inhalation should be avoided.
Welding fume sampling method
A measured volume of air is drawn through a membrane filter mounted in a sampler, and the mass of fume collected is determined by weighing the filter before and after sampling subject to a period of stabilisation. The difference in weight reflects the mass of the fume collected and this, coupled with the flowrate and time period, enable the fume levels to be quoted as milligrams per cubic metre (mg.m-3).
Use of welding fume data for the assessment of exposure
Compliance with Regulation 6 of the COSHH Regulations will be ensured if the occupational exposure standard for particulate welding fume does not exceed 5 mg.m3, provided exposure to other toxic constituents of the fume which have lower occupational exposure limits are adequately controlled. It follows that where the fume contains one or more toxic constituents which have lower occupational exposure limits the OES of 5 mg.m3 may no longer apply.In these circumstances the exposure to individual constituents of the welding fume may have to be quantified separately. This procedure may be simplified for purposes of assessment and, where appropriate, monitoring under the COSHH Regulations. The total weight, in mg.m3, of welding fume at which each of the components of the fume will reach its occupational exposure limit can be calculated from the consumable suppliers’ fume analysis data.
Welding Fume – Background Information
Welding fume is a varying mixture of airborne gases and fine particles which if inhaled or swallowed may be a health risk. The degree of risk will depend on:
(a) the composition of the fume;
(b) the concentration of the fume; and
(c) the duration of exposure.
The main health effects are:
(a) IRRITATION OF THE RESPIRATORY TRACT
Gases or fine particles of fume can cause dryness of the throat, tickling, coughing, tightness of the chest and difficulty in breathing.
(b) METAL FUME FEVER
Inhaling many freshly formed metallic oxides, such as those of zinc, cadmium, copper etc., may lead to acute flu-like illness termed metal fume fever. With the exception of exposure to cadmium fume serious complications are rare. The most common cause of metal fume fever is welding galvanised steel.
(c) SYSTEMIC POISONING
Systemic poisoning can result from inhaling or swallowing substances contained in welding fume such as fluorides, hexavalent chromium, lead, barium and cadmium. The presence of these substances in the fume depends upon the welding process being used and the material being welded.
(d) LONG TERM OR CHRONIC EFFECTS
Inhaling welding fumes can lead to benign X-ray changes, referred to as siderosis. A subject of current concern is whether welders have an increased risk of developing respiratory cancer, as certain constituents of some welding fumes, such as hexavalent chromium and nickel, may be carcinogenic.
ORIGIN AND CONSTITUENTS OF FUME
To evaluate the risk to health from exposure, information is required on the sources of welding fume and gases. Usually more than 90% of particulate welding fume arises from the vaporisation of the consumable electrode or rod. The metal being welded usually dictates the welding process and the consumable used, but it does not itself contribute significantly to the particulate fume composition except at certain operations which include:
(a) welding through metallic coatings, e.g. zinc and cadmium plated materials;
(b) welding through painted surfaces such as those which contain lead compounds;
(c) removal of base metal, e.g. cutting or arc gouging.
Depending on the welding process, gases encountered during welding may be:
(a) fuel gases which are used in gas welding and cutting which on combustion will produce carbon dioxide and in some circumstances carbon monoxide;
(b) shielding gases such as argon, helium, carbon dioxide or mixtures of these gases. These gases may be toxic or asphyxiant;
(c) gases produced by the action of heat upon the welding flux or slag, such as carbon dioxide and monoxide;
(d) gases produced by the action of heat or ultraviolet radiation upon the atmosphere surrounding the welding arc. These may include nitric oxide, nitrogen dioxide, and ozone. Ozone may be formed at some distance from the arc, depending upon the welding process being used and the metal being welded.
FACTORS THAT INFLUENCE THE COMPOSITION AND CONCENTRATION OF FUME AND GASES
The quantity and composition of welding fume and gases are influenced by a number of variables, usually dictated by the job requirements. The most important variable is the type of process: however, it does not necessarily follow that exposure to welding fume will be the same for all welders using a similar process. Therefore each welder should be assessed individually in relation to the job that is being carried out. To ensure an adequate assessment of exposure is made it is necessary to consider each of the factors which are relevant to the particular welding operation..
Gas shielded welding
Gas shielded welding uses a continuous solid wire consumable to provide filler metal and form the arc which is protected by an inert gas shield such as argon or helium (MIG (metal inert gas) welding), or an active gas shield such as carbon dioxide or mixtures of gases containing carbon dioxide or oxygen and an inert gas (MAG (metal active gas) welding). Process variables are important: the arc length increases with the current and the mode of metal transfer changes from globular to spray, with a consequent increase in emission of particles and pollutant gases. Another mode of transfer is obtained by using pulsed current conditions, the fume emission rate will depend upon the welding parameters.
THE EFFECT OF WELDING CONSUMABLE ON CONCENTRATION AND COMPOSITION
The type of consumable used, and its chemical composition, will be dictated by the technical demands of the welding process. Various types are available. MIG consumables may consist of a solid bare wire, or copper coated wire and, in the case of FCW, a tubular wire containing flux in-fill. The type of consumable will affect not only the quantity of particulate fume produced, but also its composition.
An adequate assessment of the risk to health from exposure to welding fume needs information on the chemical constituents and their concentration in the fume. Welding fume will usually contain all the chemical elements present in the consumable, although the proportion and toxic nature will have changed as a result of physical and chemical processes which occur during welding. The most important changes concern consumables that contain chromium, such as those used in hardfacing and welding of stainless steel. Chromium metal in arc welding processes oxidises to trivalent chromium compounds but also some conversion to hexavalent chromium may occur. This is important because trivalent and hexavalent chromium have different occupational exposure limits. Trivalent chromium compounds have an OES of 0.5 mg.m3 whereas the ‘guidance value’ given in Table 4 of Guidance Note EH 40 for hexavalent chromium is ten times lower at 0.05 mg.m3. Where hexavalent chromium is present in welding fume it will therefore be the principal substance of hygiene interest.
Stainless steel MIG welding fume usually contains up to 18% chromium but only a small percentage is likely to be present as hexavalent chromium. An adequate assessment of health risk requires information on the chemical constituents and their concentrations produced from a given consumable during a specified process.
The Welding Manufacturers Association has produced a standard format for hazard data sheets for welding consumables to enable their members to comply with their legal obligations under the Health and Safety at Work etc. Act 1974 Section 6. Most UK manufacturers and suppliers now provide information using this format. The hazard data sheets should include information on chemical analysis of substances of hygiene interest present in the fume produced by the consumable, the appropriate OELs, and an indication of the measures necessary to ensure adequate control.
THE EFFECT OF SURFACE TREATMENT AND PARENT METAL ON COMPOSITION
The composition of the welding consumable is of primary importance in assessment of exposure of welders to fume. In certain circumstances, however, and at specific types of welding operation the surface treatment and composition of the parent metal also need to be considered. Fume from oxygen arc cutting, and flame gouging processes and flame cutting will consist of particulates which are generally similar in composition to the parent metal. Information on the composition of the metal or alloy is important to establish the fume composition and the relevant OEL which will apply. It is likely, although no information is available, that chromium in fume from arc gouging of alloy steels will be present in the hexavalent form and the guidance value of 0.05 mg.m3 for hexavalent chromium will apply.
Surface treatment may include zinc galvanising, cadmium plating or applications of paint primers and sealers. When welding or cutting operations are carried out on coated steels additional constituents of the fume will be formed by the effect of heat on the surface coating. These may include oxides of the metal used for coating, or thermal degradation products from the primer application. Suppliers of coated steels and primer formulations have duties under HSW Act Section 6 to provide information on the composition of the material , the risks and precautions which should be taken during welding. Similarly welding directly on to steel which is coated with oil, to prevent corrosion, can give rise to smoke containing polycyclic aromatic hydrocarbons.
Welding or flame cutting existing steel structures or cutting metal scrap presents particular problems, as the composition of the metal alloy and any surface coating will not be known. Old structures and plant are frequently coated with paint that may contain lead, zinc, chromate or cadmium pigments, which will increase both the quantity and toxicity of the fume emission. Surfaces treated with PVC and/or chlorinated rubber coatings decompose with heat to give fume and gases containing hydrochloric acid and phosgene. In all cases of cutting and welding it is necessary as part of the assessment to determine the composition of any surface treatment, and the metal where appropriate, before work starts, to prevent or control exposure to toxic substances.
Welding certain metals can produce high concentrations of ozone. The predominant pollutant during MIG/MAG welding of aluminium and aluminium alloys is ozone, similarly significant concentrations of ozone are produced during TIG and MIG welding of stainless steel. Ozone is formed by the effect of ultraviolet radiation from the arc on atmospheric oxygen and can be produced some distance from the arc. Effective control of particulate fume emission in certain circumstances can result in significant increase of ozone generation. This is because particulate welding fume may reduce or inhibit emission of UV radiation from the arc.
JOB ASPECTS THAT AFFECT EXPOSURE TO WELDING FUME
The type of process, size and composition of any consumable used will influence the amount of fume generated and its composition. However, the extent of exposure to welding fume is considerably influenced by the skill of the welder. Changes in current, voltage, welding angle and arc gap can significantly increase or decrease the quantity of fume generated in a given time.
The principal welding positions are flat (downhand) horizontal, vertical and overhead. The downhand position is most commonly used and also induces the highest fume levels in the welder’s breathing zone. The welder’s posture in relation to the welding position is also important: exposures of welders in a crouching position are significantly higher than exposures of welders working in a sitting position, and exposure to fume when standing is intermediate between the crouching and sitting position. These differences reflect the proximity of the welder to the welding plume, and every effort should be made to prevent head and shoulder contact with the plume, by changes where practicable to the working position.
Equally important is the location of the welding process. In a large workshop and welding on an open structure welding fume and gases will be partially dispersed and diluted by air movement, and although exposure of the welder may be high during arcing the fume and gases do not accumulate in the working area. In a small workroom, or in a space with restricted air movement, fume from welding processes will not disperse so readily, with the resultant increase in average exposure. Work in confined spaces, such as internal welding of process plant or in ship construction, can lead to accumulation of high concentrations of particulate fume, by-product and shield gases, which do not disperse and require the use of efficient ventilation systems to ensure that exposure is adequately controlled and there is no depletion of oxygen of the working atmosphere.
Duration of exposure
Both long term and short term limits relate to the concentration averaged over a specific
reference period. For most substances contained in particulate welding fume the averaging period for the occupational exposure limit is eight hours. Exposure to welding fume will be intermittent, the highest exposures occurring during the welding operation, i.e. during arcing or flame cutting. The periods between the actual welding operation should give rise to minimal exposure to fume, although this will depend upon the size of the workshop, the number of welders, their work patterns and effectiveness of control measures and general ventilation. The pattern of work, the arcing time and down time for any individual welder will vary from day to day and similarly duration of exposure and pattern of work will vary significantly between welders although they may be doing similar work. Exposure (to substances hazardous to health) should be calculated according to the approved method, which is reproduced in Appendix 1 of Guidance Note EH 40. Assessment of average exposure becomes very difficult and will require frequent sampling unless the welding operation is of a routine nature, for example production line welding of domestic boilers.