Introduction
Welding and cutting activities can generate gases that are hazardous to health. The gas or gases generated and their concentrations depend on the process used and the gas formation mechanisms. Gases, some of which may be hazardous, are inherent in some processes, either as a shielding gas to protect the molten weld pool against atmospheric contamination or, for flame processes, as a consumable that is burnt.
Many of the gases fall within the scope of the Control of Substances Hazardous to Health Regulations (COSHH) 2002 (Amendment) 2004, which require that exposure to them is maintained below concentrations known as Workplace Exposure Limits (WELs). WEL values are given in Guidance Note EH40 from the Health and Safety Executive (HSE), which is usually updated annually.
Exposure to gases may be measured according to the methodology defined in BS EN ISO 10882-2: 2000. Health and safety in welding and allied processes - Sampling of airborne particles and gases in the operator's breathing zone - Part 2: Sampling of gases. Exposure measurement may be used to verify compliance with regulations, identify a need for exposure control or to identify faults with existing control systems.
Information on the gases generated, their origin, their health effects and their expected concentrations relative to exposure limits is given in this document.
Guidance on the gases generated and their expected concentrations relative to exposure limits is given in Guidance Note EH54 from the Health and Safety Executive, Assessment of exposure to fume from welding and allied processes. Further details are supplied in this document.
Hazards, health effects and risks
Shielding gases
For gas shielded welding processes such as TIG, MIG/MAG, FCAW, shielding gases may be inert gases, such as argon, helium and nitrogen, or argon-based mixtures containing carbon dioxide, oxygen or both. Helium may be added to argon/carbon dioxide mixtures to improve productivity. Carbon dioxide (CO2) may be used, on its own, in MAG and FCAW. With the exception of CO2 , these gases are not defined as hazardous to health under the COSHH Regulations but they are asphyxiants. CO2 has a long-term exposure limit of 5000ppm (8-hour TWA reference period) and 15000ppm short-term exposure limit (15-minute reference period). None of the gases can be seen and none have a smell - so their presence in hazardous concentrations is difficult to detect without prior knowledge or measuring equipment.
The main hazard arising from exposure to shielding gases is asphyxiation, usually stemming from accumulation of the gases in confined spaces. Shielding gases are supplied at a flow rate of around 15l/min in gas shielded welding processes and the gases may leak from connections in gas supply lines if these are not properly tightened. Argon is heavier than air, so argon and gases comprised mainly of argon tend to collect in low areas such as pits. Inhaling a gas, such as pure argon, which contains no oxygen can cause loss of consciousness in seconds. Workers should not enter an atmosphere that contains less than 18% oxygen.
Gases generated by the process
Carbon monoxide and carbon dioxide
Carbon monoxide (CO) and CO2 may be generated in fluxed welding processes by the action of heat on flux materials such as carbonates and cellulose. In MAG welding they can both originate from CO2 in the shielding gas, CO2 undergoing reaction in the vicinity of the arc to form CO. Flame processes also generate CO and CO2 . The relative amounts depend on whether the flame is oxidising or reducing, with CO present in higher concentrations when the flame is reducing.
CO is by far the more hazardous of the two gases. It can cause a reduction in the oxygen carrying capacity of the blood that can be fatal. In lower concentrations it causes headache and dizziness, nausea and weakness. CO2 acts mainly as an asphyxiant, as indicated above. CO has a short-term exposure limit (15-minute reference period) of 200ppm and a long-term limit (8-hour reference period) of 30ppm. From above, the values for CO2 are 15000 and 5000ppm for the short-and long-term reference periods respectively.
The amounts CO and CO2 generated by fluxed processes are small and, generally, they do not present an exposure problem. The amounts of CO and CO2 generated by flame processes are also small, so the risk of over-exposure is usually low. In special cases, such as high velocity oxy-fuel gas cutting, where large quantities of gas are consumed in a short period of time, the risk of over-exposure to CO may be a problem.
MAG welding with a carbon dioxide shielding gas or shielding gases containing high proportions of carbon dioxide, e.g. 80%Ar/20%CO2 does not usually present an exposure problem to CO or to any CO2 generated by the process. However, a CO2 asphyxiation problem could arise as indicated above. Similar comment can be made about gas shielded FCAW.
Nitrogen monoxide and nitrogen dioxide
Nitric oxide (NO) and nitrogen dioxide (NO2 ) are known collectively as nitrous gases (NOx). They can be generated by oxidation of nitrogen in the air by heat from an arc or flame. Chemical Hazard Alert Notices (CHANs) were issued in UK in 2003, withdrawing the exposure limits of NO and NO2 because they were not considered to be adequate to protect occupational health. New exposure limits of 1ppm 8-hour TWA have been recommended for each gas.
Nitric oxide is a severe eye, skin and mucous membrane irritant. Nitrogen dioxide is a highly toxic, irritating gas. After inhalation, nitrous gases act more on the deeper rather than the upper (nose, trachea, large bronchi) respiratory tract. The following symptoms are an indication of the primary stage of poisoning by nitrous gases:
- Irritation of the eyes, nose and trachea
- Intensive cough
- Narrowness in breathing
- Dizzines and headache
- Sickness and fatigue
The symptoms of over-exposure may not be apparent for several hours after the cutting activity has ceased. Severe over-exposure may lead to an accumulation of water in the lungs which impairs oxygen supply to the blood and may lead to death.
Welding generates only small amounts of nitrous gases so exposure to nitrous gases during welding does not present a problem. Exposure problems may arise during cutting activities, particularly if the cutting is hand-held, as this places the operator closer to the emissions. Hotter flames generate higher concentrations of nitrous gases, so using acetylene generates more nitrous gases than using propane or natural gas.
The risk of over-exposure to nitrous gases is considered to be low for oxy-fuel gas cutting, unless the work conditions are unfavourable e.g. hand held cutting in a confined space with a high duty-cycle.
Plasma cutting with air or nitrogen generates higher levels of nitrous gases than oxy-fuel gas cutting and there is considerable risk of over-exposure.
Free-burning flames generate the highest concentrations of NO and NO2, and the risk of over-exposure is also highest. Caution should be exercised during activities such as flame heating, flame straightening, flame brazing, flame spraying, etc - particularly as emissions from these processes are difficult to control. The flame should be extinguished when not in use.
Ozone
Ozone can be generated by reaction between UV light from the arc and oxygen in the air. The exposure limit for ozone is 0.2ppm for a 15-minute reference period.
At the levels of exposure to ozone found in welding the main concern is irritation of the upper airways, characterised by coughing and tightness in the chest, but uncontrolled exposure may lead to more severe effects, including lung damage.
MIG welding of aluminium alloys with an aluminium/silicon filler wire generates by far the highest concentrations of ozone. Using an aluminium filler wire generates substantially less ozone, and using an aluminium/magnesium filler wire generates the least ozone when MIG welding aluminium alloys. Other process/material combinations that may generate hygienically significant concentrations of ozone are MAG/mild steel, MAG/Stainless steel and TIG/stainless steel.
Ozone is only generated during arcing and decays quickly on arc extinction. Therefore, exposure to ozone is very dependent on the duty cycle employed. Although research in the laboratory has shown that ozone concentrations at points around a welding arc can exceed 0.2ppm, it is uncommon to find that average exposure to ozone, in a real work situation, exceeds the ozone exposure limit. An exception to this statement is exposure to ozone during MIG welding with an aluminium/silicon consumable.
Organic gases
It is becoming increasingly common, particularly with resistance welding in the auto and white goods industries, to weld through, or close to, organic materials such as shop primers, organic coatings, adhesives, sealants, oils, etc. Decommissioning of plant can involve cutting through many coating materials, including paint.
During welding or cutting, a wide range of degradation products may be generated, the composition of which is difficult to predict, even with knowledge of the composition of the product welded or cut through. Further, it is uncommon to find information on the degradation products within Material Safety Datasheets. Research has shown that a wide range of toxic degradation products may be generated but that their concentrations are usually low. Many degradation products do not have prescribed exposure limits but this does not mean that they are safe. Control to levels that allow exposure without harm to health must be exercised.
Degreasing solvent gases
Chlorinated hydrocarbons, such as trichloroethylene, may be used for degreasing. The radiation from welding arcs causes trichloroethylene vapour to decompose to products that are readily detected by smell. The primary decomposition products are dichloroacetyl chloride and hydrogen chloride but phosgene, which has very low exposure limits (long-term limit 0.02ppm, short-term limit 0.06ppm), is also formed. Fortunately, the smell and lachrymatory properties of the initial breakdown products are sufficient to warn the welder of a problem and welding is likely to be stopped before harmful levels of any product are achieved.