Welding Fume and Gas Exposure
Welding fume exposure tends to be highly variable due to several exposure factors.
- By Jerome E. Spear
- Jun 01, 2011
Welding fumes are very small particles that are formed when the vaporized metal rapidly condenses in air. They are typically too small to be seen by the naked eye but collectively form a visible plume. The health effects associated with metal fumes depend on the specific metals present in the fumes; they may range from short-term illnesses, such as metal fume fever (i.e., flu-like symptoms), to long-term lung damage and/or neurological disorders.
Gases are also generated from welding, which may include carbon monoxide (CO), ozone, and nitrogen oxides. CO is an odorless, colorless gas that may be formed by the incomplete combustion of the electrode covering or flux and by the use of carbon dioxide (CO2) as the shielding gas. Overexposure to CO inhibits the body's red blood cells to sufficiently carry oxygen to other tissues within the body, which subsequently results in asphyxiation. There is also a potential of an oxygen-deficient atmosphere if welding inside a confined or enclosed space if an inert gas (such as argon) is used as the shielding gas.
Ozone, nitrogen dioxide, and nitric oxide are produced by the interaction of ultraviolet light (from the welding arc) with the surrounding air. These compounds are irritating to the eyes, nose, and throat. High exposures also can cause fluid in the lungs and other long-term pulmonary illnesses.
If the metal has been degreased with a chlorinated solvent, other airborne gases (such as phosgene, hydrogen chloride, chlorine gas, etc.) may be produced. These gases generally cause irritation to the eyes, nose, and respiratory system, and symptoms may be delayed.
The first step in assessing potential exposures to welding fumes and gases is to understanding common welding processes, their relative fume generation rates (FGRs), and other potential exposure factors.
Common Welding Processes
Different welding processes have different FGRs. An overview of common welding processes and their FGRs is provided below:
- Shielded Metal Arc Welding (SMAW, "stick welding") is commonly used for mild steel, low-alloy steel, and stainless steel welding. In SMAW, the electrode is held manually, and the electric arc flows between the electrode and the base metal. The electrode is covered with a flux material, which provides a shielding gas for the weld to help minimize impurities. The electrode is consumed in the process, and the filler metal contributes to the weld. SMAW can produce high levels of metal fume and fluoride exposure; however, SMAW is considered to have little potential for generating ozone, nitric oxide, and nitrogen dioxide gases.
- Gas Metal Arc Welding (GMAW) is also known as metal inert gas (MIG) welding. GMAW is typically used for most types of metal and is faster than SMAW. This process involves the flow of an electric arc between the base metal and a continuously spool-fed solid-core consumable electrode. Shielding gas is supplied externally, and the electrode has no flux coating or core. Although GMAW requires a higher electrical current than SMAW, GMAW produces fewer fumes because the electrode has no fluxing agents.
- Fluxed Core Arc Welding (FCAW) is commonly used for mild steel, low-alloy steel, and stainless steel welding. This welding process has similarities to both SMAW and GMAW. The consumable electrode is continuously fed from a spool, and an electric arc flows between the electrode and base metal. The electrode wire has a central core containing fluxing agents, and additional shielding gas may be supplied externally. This welding process generates a substantial amount of fumes because of the high electrical currents and the flux-cored electrode. FCAW generates little ozone, nitric oxide, and nitrogen dioxide gases.
- Gas Tungsten Arc Welding (GTAW) is also known as tungsten inert gas (TIG) welding. GTAW is used on metals such as aluminum, magnesium, mild steel, stainless steel, brass, silver, and copper-nickel alloys. This technique uses a non-consumable tungsten electrode. The filler metal is fed manually, and the shielding gas is supplied externally. High electrical currents are used, which causes this process to produce significant levels of ozone, nitric oxide, and nitrogen dioxide gases. However, GTAW produces very little fume.
- Submerged Arc Welding (SAW) is another common welding process used to weld thick plates of mild steel and low-alloy steels. In this welding process, the electric arc flows between the base metal and a consumable wire electrode; however, the arc is not visible because it is submerged under flux material. This flux material keeps the fumes low. There are also little generation of ozone, nitric oxide and nitrogen dioxide gases. The major potential airborne hazard with SAW is the fluoride compounds generated from the flux material.
Fume Generation Rates
The primary sources of information when determining the components likely to be in the fume are the material safety data sheet and/or the manufacturer's technical data sheet of the consumable electrode/wire. About 90 to 95 percent of the fumes are generated from the filler metal and flux coating/core of consumable electrodes (Lyttle, 2004). Because the base metal weld pool is much cooler than the electrode tip, the base metal contributes only a minor amount of the total fumes. However, the base metal may be a significant factor of the fume exposure if the metal or surface residue contains a highly toxic substance (such as chromate-containing coatings, lead-based paint, etc.).
In addition to the welding process, the FGR is also influenced by the following factors (Spear, 2010):
- Electrical current: In general, the FGR is exponentially proportional to the current.
- Arc voltage: The FGR generally increases when the arc voltage increases.
- Electrode diameter: The electrode diameter has a modest effect on the fume generation rate because of the differences in voltage and current. In general, a small-diameter electrode has a higher FGR than a large-diameter electrode, all else remaining equal. However, there is usually a step up in electrical current when using larger-diameter electrodes.
- Electrode angle: The angle of the electrode to the work piece has a slight (but unpredictable) affect on the FGR.
- Shielding gas: In gas-shielding arc welding, the FGR tends to be greater when 100 percent carbon dioxide (CO2), as compared to argon, is used as the shielding gas.
- Speed of welding: As the welding rate increases, the fume generation rate obviously increases.
- Steady current/pulsed current welding: Technology has advanced to power sources that have pulsing capabilities. Studies (Wallace et al., 2001) have shown that utilizing a pulsing current during welding generates fewer fumes than under steady current welding process.
In general, FCAW produces the greatest fume generation rate (for low-alloy welding), closely followed by SMAW. However, when welding chromium-containing steel, Cr(VI) contained in the fumes generated from SMAW tends to be greater than Cr(VI) generated from FCAW. Alkali metals, such as sodium and potassium, stabilize Cr(VI) and are often SMAW electrode coatings and may also be present in FCAW flux (Fiore, 2006), which may explain why Cr(VI) concentrations from SMAW operations are often higher than Cr(VI) concentrations from FCAW. GMAW tends to have a moderate relative FGR. GTAW and SAW are inherently low fume-generating processes.
Other ancillary process (such as air arc gouging and plasma arc cutting) also can generate a significant amount of fumes because of the high electrical current and arc voltage associated with these processes. Potential exposures to not only the operator, but also other personnel in the work area can be significant from such processes, especially in enclosed and confined spaces.
Hexavalent Chromium Exposure Factors
Pursuant to a court order, OSHA issued a final rule on Feb. 28, 2006, that addresses occupational exposure to Cr(VI) (OSHA, 2006). OSHA determined that the Cr(VI) rule is necessary to reduce significant health risks due to Cr(VI) exposure.
Chromium metal is found in stainless steel and many low-alloy materials, electrodes, and filler materials. The chromium present in electrodes, welding wires, and base materials is in the form of Cr(0), so welders do not ordinarily work with materials containing Cr(VI). It is the high temperatures created by welding that oxidize the chromium in steel to the hexavalent state.
Welding fume exposure tends to be highly variable due to several exposure factors. These factors should be considered when assessing potential exposures to Cr(VI). The primary Cr(VI) exposure factors are as follows:
1. Welding process (as summarized above)
2. Chromium content and flux ingredients in the consumable
3. Chromate coatings on base material
4. Welding rate
5. Relative welding position (e.g., down-flat, horizontal, vertical, and/or overhead welding positions)
6. Local exhaust ventilation (LEV)
7. Welding environment (inside or enclosed space)
8. General/dilution ventilation and natural air currents
9. Other welding (or ancillary/allied processes) performed in the area
Assessing Exposures to Welding Fumes and Gases
The above information should be considered when conducting exposure monitoring during welding operations. The welding process and composition of the material (primarily the ingredients in the electrode, unless the steel is coated) should be the basis of categorizing similar exposure groups (SEGs).
The SEGs can be further defined by the specific task, position of the work piece (in relation to the welder's breathing zone), presence or absence of LEV, and/or other work-related factors.
This article originally appeared in the June 2011 issue of Occupational Health & Safety.