Occupational medical examinations on the thorium substituent exposure of welders during use of thorium-free tungsten inert-gas electrodes and on their significance for health

Status:

completed 01/2004

Aims:

Tungsten inert-gas welding (WIG) using non-melting tungsten electrodes is employed in areas with particularly high quality requirements, e.g. in chemical plant construction, in pressure vessel construction and in power station construction. As a matter of principle, this procedure is considered a lowemission procedure as, due to low current and voltage values, both the fume and the ozone emissions are much lower than in other welding procedures. One of the particularities of this procedure - similar to micro-plasma-arc welding - is the fact that the electrode generating the arc does not melt (tungsten melts at a temperature of 3410°C). In order to enhance ignitability and to stabilize the welding arc, these tungsten electrodes are doped with 1-4 percent thorium dioxide. Studies have shown that WIG welding using thorium dioxide-containing tungsten electrodes, in particular alternating current welding, may generate thorium dioxide-containing fumes. Inhalation of these fumes may cause the hazard of an internal radiation exposure. Therefore, the use of thorium oxide-free tungsten electrodes is recommended as a primary protective measure. There are thorium dioxide-free tungsten electrodes with other oxide additions, e.g. cerium-containing and lanthanum-containing electrodes.

Up to now, no data on the Rare Earth exposure (cerium, lanthanum, yttrium, zirconium) of welders were available. Therefore, there was no information about whether this may result in a particular hazard to the health of the welders. The objective of the campaign was to obtain data on the health-specific issues.

Activities/Methods:

Within the framework of the campaign, both pre-liminary examinations and field studies were carried out. The preliminary examinations were intended to ensure the suitability of the analytical methodology performed on various filter types to detect Rare Earths. Two analytical procedures were used: inductively coupled plasma spectrometry (ICP) and neutron activation analysis (NA). The field studies were carried out on five WIG welders employed in a large chemical enterprise. An occupational medical evaluation of the significance of thorium substitutes for health was performed on the basis of the results obtained and of the available scientific literature.

Results:

The result report shows that even when assuming the least favourable conditions (highest filter content of the doping elements, lowest air through-put) an air concentration of only 146,4 ng/m³ for cerium and of 68 ng/m³ for lanthanum could be detected within the respiration area of the welders. No zirconium could be detected. Even if there is only a limited occupational medical basis for assessing the Rare Earth exposure of welders, the results of this study and the derived medical evaluation as a whole do not give rise to the assumption that Rare Earths cause a hazard to the health of welders. Thus, from the occupational medical point of view, the use of Rare Earths for doping tungsten electrodes can be considered harmless.

Last Update: