N° 12 - March 2005


Lung Cancer and Electric Arc Welding

Prepared by Dr. G. H. Grant McMillan, MD, FRCP, FFOM
Consultant Occupational Health Physician, UK.

• Introduction

Electric arc welding of steel is at the centre of the industrialised and industrialising world. With an estimated three million workers around the world using it in some form or another and many more exposed at work to fumes arising from the processes, even a slight excess risk of a serious, commonly fatal disease such as lung cancer would result in a significant number of deaths globally. Welders have a 30 - 40% excess risk of developing lung cancer compared to the general population.1-6

Research to find a specific causal relationship is especially difficult because lung cancer is the commonest malignancy in the Western world. There is a need for more and better investigation but enough is known now to allow useful preventive action to be taken by applying occupational hygiene principles.

• Causative Agents - Prime Suspects

The agent or agents responsible for this apparent excess risk of lung cancer in welders must be identified before there can be confidence that the most effective protective actions are being taken. Meanwhile, there is sufficient information available to allow much to be done on a general or empirical basis.

The search for the cause or causes has focussed largely on the most commonly welded materials - mild steel and the less common but increasingly used stainless steel, the electric arc welding processes, described recently by Cunat7, and on incidental exposures to known carcinogens. The principal suspected agents that may contribute to the experience of lung cancer in welders include welding fumes, and its iron, chromium or nickel components, asbestos dust, tobacco smoke, and a social class health effect.

a) Welding fumes
“Welding fumes” is a commonly used generic term to describe the dynamic and often biologically active mixture of particulate matter and gases emitted from a welding process used to join metal components. Some regulatory authorities, however, use the term “fume” more specifically to describe only the particulate components, and that is the interpretation used in this paper.

In 1990, the International Agency for Research in Cancer (IARC) published its review of chromium and nickel in welding and concluded that there was inadequate evidence of welding fume carcinogenicity in animals. Epidemiological studies including a recent European Cohort Study variously showed an excess mortality from lung cancer in shipyard welders, mild steel welders and stainless steel welders. However, this did not appear to be related to duration of employment or cumulative exposure to total fumes, total chromium, hexavalent chromium or nickel. IARC concluded that there was only limited evidence of carcinogenicity in humans, and classified welding fumes as being “possibly carcinogenic to humans” (Group 2B).8 The IARC assessment raised all other forms of welding, including mild steel, to the level of concern which had previously been applied to welding stainless steel.9

b) Iron
Iron compounds are found in abundance in all steel welding fumes and may be inhaled and retained to cause siderosis. If, as has been suggested,10 any of its compounds were proven to be potent lung carcinogens, then an explanation might have been found for much of the excess risk among welders. Iron makers and workers have been said to have an elevated risk of respiratory tract malignancy11 but a recent study in a French iron refinery has shown no relationship between exposure to iron oxides and lung cancer mortality even among long-serving iron-exposed workers.12

Iron over-loading has been described in steel welders and has been associated with carcinogenesis,13 the risk increasing when individuals are exposed simultaneously to iron and chromium.14 To confound these observations, welders with siderosis, who one might assume are iron-loaded through heavy prolonged exposure to iron-rich welding fume, appear not to be at greater risk of developing lung cancer than other welders.15

Overall, the lack of definite evidence to the contrary suggests that iron and its compounds in welding fume are not potent carcinogens.

c) Chromium
Fume from welding stainless steel is rich in hexavalent chromium (Cr VI) and trivalent chromium (Cr III) compounds, the amounts varying within and between processes. This variation may provide some of the opportunities for fume emission risks to be controlled to some extent.16

Some hexavalent chromium compounds are known to be carcinogenic to the respiratory system by the inhalation route in workers in a range of industrial processes;8,17 chromium plating18,19 and in production of chromates20,21, chromate pigments22 and ferrochromium.23 Dose response relationships have been established for hexavalent chromium exposure and cancer and the possibility of a threshold effect has been suggested.21

There is coincident exposure in most circumstances to trivalent chromium compounds. These compounds are genotoxic but not carcinogenic in animals.8,24 IARC found there to be inadequate evidence regarding their carcinogenicity in humans.8 Metallic chromium is not known to be carcinogenic in humans.8

Where compounds containing hexavalent chromium are present in welding fume, they can be absorbed and excreted.8, 25-33 The extent to which retention, reduction, and excretion of hexavalent chromium, and their solubility act as an important determinant of carcinogenic potential is uncertain.24

d) Nickel
IARC has classified nickel compounds as carcinogenic to humans.8 The exact mechanisms of nickel-induced carcinogenesis are not known34, nor is it known with certainty which forms of nickel pose the threat.35 In highly nickel-polluted environments, the relationship appears to be strongest for nickel compounds classified as“water-soluble”.35-37

When individual nickel-containing particles are present in inhaled welding fume, they are of a size which enables them to reach as far as the terminal bronchioles and alveoli of the lung. Those retained there clear slowly. Whereas nickel may be present in welding fume only in forms which are insoluble in water, the presence of higher concentrations of nickel in the blood and urine of manual metal arc stainless steel welders compared to occupationally unexposed individuals8 leads one to the conclusion that the nickel compound particles in fume from that source were at least sparingly soluble in biological fluids. Leaching into local tissues and systemic absorption can be expected to continue while particles remain in the lung.

Smoking habits of employees further complicate the interpretation of cancer mortality data in the nickel industry. The combined effect of exposure to nickel and smoking has been said to suggest a multiplicative risk for lung cancer.36 Cigarette smoking may not only increase the risk of lung cancer directly, but also indirectly by way of impairing mucociliary clearance of toxic particles from the bronchial mucosa40 and consequently prolonging the availability of particles for leaching and absorption.

e) Asbestos dust
Exposure to respirable asbestos dust is associated with an increased risk of lung cancer.41 One study from the Netherlands has indicated that, after adjustment for smoking and diet, about 11.6% of cases of lung cancer in men is attributable to a lifetime occupational exposure to asbestos.42 It has been a suspect as the cause of the excess risk of lung cancer in welders, at least in part, this risk varying with the type of asbestos, extent and duration of exposure, and smoking habit, and being greatest in smokers.5, 41-48

Many welders have been exposed to this dust in their present workplace or from previous exposure from work in shipyards,49 boiler shops, locomotive and rail carriage works, and manufacturing and repair facilities. That many of these welders were exposed to significant levels of asbestos dust at work is borne out by the scattering of cases of the malignant tumour mesothelioma in welders found in several epidemiological studies50 and in the European Cohort Study.6

f) Tobacco smoking
There is no doubt that smoking tobacco increases the risk of lung cancer in users. In 1980, Beaumont and Weiss speculated that cigarette smoking may be a contributing factor to the excess of lung cancer in welders as welders tend to smoke more cigarettes than do other occupational groups.2 Others have expressed that opinion over the last 24 years.45-47, 50, 51

If there was convincing evidence of more welders smoking or welders smoking more heavily than the control groups, then tobacco smoke could offer an explanation for the excess risk of lung cancer in welders at least in part. It is thought to be insufficient to explain it entirely on its own. Unfortunately, where evidence about the smoking habits of welders is available, it is inconsistent, some surveys showing more smoking than in the general population and others not showing that excess.

The effect of exposure to asbestos and tobacco smoking together is greater than that of the single effects acting separately. There is argument as to the nature of the relationship; multiplicative, additive or more complicated and nebulous.48, 52-56 In the most recent of these authoritative reviews, the excess relative risk of lung cancer from asbestos exposure is estimated to be two to three times higher in non-smokers (who otherwise would have a low risk) than in smokers (who already have a high risk).56

g) Social class-linked health package
Studies of the health of workers show that manual workers have a greater risk of ill health in general and lung cancer in particular than “white collar" workers. This may well be relevant to welders. There is a difference in lung cancer risk between social classes in addition to the effects of smoking.57 This can be explained by social deprivation and poor socioeconomic conditions throughout life.

h) Other factors
It is also possible that there may be an as-yet-unappreciated carcinogen at work in welding fume or some other aspect of the welders' environment. At present, there is nothing to suggest the presence of a mystery carcinogen in the fume.

• Conclusions and Recommendations

It is concluded that:

• Electric arc welders have an excess risk of 30-40% for lung cancer.

• The work environment is likely to play a part in the causation of that excess.

• Asbestos dust and tobacco smoke are likely to be major contributors to causation in those so exposed.

• Welding fume from some processes contains compounds of elements such as chromium and nickel - which are proven to be carcinogens in other work processes.

• The evidence for welding fume or any of its constituents acting as a carcinogen is not strong.

• Industry-wide education and positive action campaigns should be mounted to further control exposure to recognised risk factors such as asbestos and tobacco smoking, and to reduce worker exposure to welding fumes.

• Consideration should be given to improving life style factors, as poor life style seems to be reflected in poor health - including lung cancer.

• There is a need for further research.

• References

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