New developments in advanced welding

Recent and ongoing research

10.3.1 Fundamental difficulties

Despite the labour figures indicating that around 400000 people in the USA are directly engaged in welding, it is difficult to research health effects and make positive associations between causative factors and those effects. Working environments are complex, and each of us is exposed to a wide range of both physical and chemical agents at work. We have different lifestyles, different diets and different susceptibilities to disease. All of these factors make it difficult to pinpoint patterns of exposure and associate them with particular health effects - especially if the association is only weak. It is rarely acceptable to embark on experimentation with human subjects.

NIOSH recently published a comprehensive review23 noting that past research discovered that various respiratory disorders are found in large numbers of welders. It is known that nickel and chromium (VI) are classified as carcinogens and that chronic exposure to manganese has been associated with a disease similar to Parkinson’s. However, we do not have data to indicate whether welders are exposed to these substances in such quantities that they could trigger these effects, or how such exposure can lead to serious long-term effects.

Many past studies have involved comparing large groups of people to try to associate patterns of disease or causes of death with differences in their exposures and lifestyles. This is not always a successful strategy, due to many problems. Such studies often use death certificates to identify the diseases that people suffered. However, death certificates generally only record the immediate cause of death and may not record the underlying cause(s) of death, or they may not mention a condition that was present, but would have been of interest to the research study.

In retrospect, it is difficult to quantify exposure of individuals to different agents unless measurements have been taken during their working life. Making such measurements presupposes that the things that must be measured are already known to be the key factors in the development of the disease. Subsequent assumptions about exposures may be wrong - and an example of the problems with making assumptions using generic job titles is evident in the more recent research in electric and magnetic fields, outlined in Section 10.3.4.

Research into health effects therefore tends to be an iterative process where a prevalence of a particular disease is noted to be associated with a particular exposure or a particular trade. Subsequent research attempts to show that it is specific to a certain agent and attempts to rule out other influences, such as bias in the choice of age of the subjects, coincidence, the presence of another agent that has not been taken into account, etc. The data we have at present are too limited and further research is necessary.24 We need a continuation of epidemiological studies24 - investigating the patterns of disease among populations of interest. This is needed to gain a better understanding of the role that welding fume may play in the suppression of the immune system, the development of lung cancers, neurotoxicity, skin damage, reproductive disorders and the other effects that prior studies have associated with the components of welding fume. The second strand of research is at the molecular level24 to gain an insight into the ways that changes in cells or in genetic material can lead to tumour formation, nerve damage or other adverse changes.

10.3.2 Fume measurements

Welding, by its nature, has many variables - among them are the process itself, the consumable (where applicable), the parent metal, flux and/or shielding gas (where applicable), voltage, current and standoff. Fume emissions depend on all the variables. While there are many fume emission measurements relating to the various arc welding processes, research continues. Continued research is important in order to produce high quality, reliable and reproducible data against which one can formulate control measures to safeguard health and which can be used to verify mathematical models.

Recently a research group has devised an improved design of welding chamber for the capture of fume for analysis,25 which has allowed more reproducible and accurate measurements to be made. This enabled them to map out the fume emission rates for GMAW, as a function of voltage and current, with much greater precision than was previously achievable. They found a complex relationship between fume emission rate and welding parameters. Fume emissions rose as the current, voltage and wire feed speed increased in globular transfer mode, only to drop suddenly when the mode changed to spray transfer. At even higher voltages the fume emission rates increased once more.

Fume emission measurements are required wherever new materials are welded, or a new consumable is developed. They are also needed whenever a significant change in working practice is made that might have a bearing on the quantity or identity of the emissions - for instance when welding through coatings.

It is common practice in the automotive industry to weld materials that have been treated with sealants, or have adhesives on their surfaces. Resistance welds are routinely made through these materials and fume is generated as a result. A recent research project was carried out at The Welding Institute (TWI)26 in which resistance welds were made on pieces of metal coated with a range of typical sealants and adhesives, representing the most widely used types in the industry. Emissions included benzene, 1,3-butadiene and several other compounds, but the concentrations were low in comparison with the total welding fume.

10.3.3 Dangers of explosion

It has long been known that hot work on tanks that have contained flammable liquids requires special measures to ensure that explosion will not occur. However, a less well-known cause of explosion, the ignition of unburnt gas during preheating of weld preparations, has recently been the subject of research.27 Preheating is commonly carried out using a propane torch, where the flame contains regions of unburnt gas. Under certain circumstances unburnt propane can pass through the welding gap. If the space behind is confined, subsequent explosions are possible and fatal accidents have occurred.

The research indicated that unburnt gas passing through the welding gap collects in the space behind where there is, during the time of preheating, insufficient oxygen to cause ignition. However, when the flame is removed the weld preparation begins to cool and air is drawn into the space. If subsequently an ignition source is brought to the gap the unburnt gas behind it may explode. Both large confined spaces, such as legs for oil platforms, and small confined spaces are susceptible. To avoid this sequence of events the recommendation is, for preference, to avoid the fabrication of an enclosure by welding. Alternatively, other forms of preheating are recommended in place of the use of a fuel gas. Possibilities include induction heating or radiant gas burners.

If a gas torch is to be used welders are cautioned:

• to light the torch correctly by pointing it downwards towards a horizontal surface to trap the vapour and to light it quickly;

• to avoid damaging the nozzle, and check that there is no leakage of fuel from the rear of the heads;

• to choose a welding gap that is 5 mm or more;

• to use a standoff distance that is as large as practicable - at least 150 mm;

• wherever reasonably practicable, the space behind the preparation should be checked with a flammable gas detector after preheating and before bringing another ignition source up to it;

• ventilation behind the gap should be maintained where reasonably

practicable to keep the unburnt gas to below 10% of its lower explosive

limit;

• routinely to check hoses, regulators, flame arrestors for integrity.

The British Compressed Gases Association (BCGA, United Kingdom)28 and the Compressed Gas Association (CGA, United States of America)29 both publish documents that give advice on the selection, use and maintenance of hoses, regulators, gas torches and other such equipment.

10.3.4 Electric and magnetic fields (EMF)

Public concern continues to grow over exposure to electric and magnetic fields. This has at least in part been fuelled by the rapid increase in mobile communications, with its associated transmitters, and hand-held telephony equipment. We are all exposed to both electric and magnetic fields. Magnetic fields are generated by the passage of an electric current and are therefore larger close to electrical equipment drawing relatively large currents such as sewing machines, magnetic resonance imaging machines, computers and can openers. Magnetic fields generally decay very rapidly with distance from electrical appliances, but are difficult to shield. They are only present when the equipment is actually energised and working. The electric field is generated by the voltage between an appliance or a cable and earth. Electric fields do not decay, but are easily shielded by objects that conduct electricity, which includes buildings and trees.

There has been much research into the effects of both electric and magnetic fields. The subject is complex, because the effects, if any, may depend on the frequency of the field, the strength, whether it is electric, magnetic or both, and the peak exposures. People in specific age groups may have increased susceptibility - for example children. The research results have so far shown no clear unequivocal evidence for a link between exposure to EMF and adverse health effects. Some of the research has been confounded by a lack of measurements of exposure - early research used job titles to assign workers to low or high electric and magnetic fields. Actual measurements show that this is not likely to have been a reliable indicator. Some typical measurements are given in Table 10.2, showing that the designation ‘electrical worker’ does not necessarily indicate that the exposure to that individual is greater than in other occupations.30 Further research is clearly needed. At the time of writing there are around 200 research projects in at least 27 countries. A review of the current status can be found in a document produced by the National Institute of Environmental Health Sciences30 and there is a great deal of information available from the National Radiological Protection Board (NRPB).31

The welder is potentially exposed to both magnetic and electric fields, but the magnetic field is believed to be the more significant as it is slightly

elevated compared to that in many other occupations, as shown by the figures in Table 10.2. At the current level of knowledge there is no proven link between exposures at the levels experienced by welders to adverse health effects. However, since the research is inconclusive, in line with the precautionary principle it is suggested that welders do not expose themselves unnecessarily to magnetic fields. This can be done by welders avoiding wrapping the cable around their bodies and by keeping the welding cable and the return cable close together.

Workers with medical prostheses are a special group. There is a possibility that workers wearing certain types of pacemaker, for certain heart conditions, may be adversely affected by the rather large fields generated by a resistance welding machine. It is recommended that the advice of the consultant physician who is managing the worker’s heart condition is sought if they are worried, or if their job brings them close to high magnetic fields.

The legislative timetable relating to electromagnetic fields is not yet fixed. The fall-back position in the UK is that there is the expectation that employers will apply general health and safety legislation to this topic, and refer to the guidelines of the NRPB.31 There is a European proposal for a Directive on the exposure of workers to electromagnetic fields and waves.32 The Directive is concerned with the acute effects of electromagnetic fields, which are apparent at relatively high fields. The proposal states that there is as yet no conclusive evidence linking these fields to cancer. Proposed ‘action values’ and exposure limits are in the document. The action values are the same as those listed in Table 6 of the International Commission on Non-Ionizing Radiation Protection (ICNIRP) guidelines document.33 The proposed exposure limit values are the same as those listed in Table 4 in the ICNIRP document, for occupational exposure. If the proposed directive is adopted, new regulations will be made. Where the exposure ‘action values’ are exceeded, employers will be required to put into place an action plan to reduce exposure to a minimum. This will include a consideration of adopting different working methods that entail less exposure, the choice of appropriate equipment, technical measures to reduce the emission of fields, appropriate maintenance, the design and layout of workplaces and workstations, administrative measures, information and training, the limitation of exposure and the availability of adequate personal protective equipment.