IARC Evaluates Carcinogenic Risks Related to Welding Exposures
Three new monographs published by the International Agency for Research on Cancer address the carcinogenic risks to humans of welding, molybdenum trioxide, and indium tin oxide.  IARC classifies both welding fumes and ultraviolet radiation from welding as Group 1 carcinogens, the agency’s designation for agents that carry sufficient evidence of carcinogenicity in humans. According to the monograph, welding fumes cause lung cancer, and positive associations have been observed with kidney cancer. UV radiation from welding can cause ocular melanoma.  For all other cancers, studies considered by IARC presented inadequate evidence for the carcinogenicity of welding fumes. IARC had previously classified welding fumes as “possibly carcinogenic to humans,” or Group 2B, in 1989. The agency stated that the new classification is based on “substantial new evidence” from observational and experimental studies. Exposure to asbestos and tobacco smoking, which can confound associations with cancer, were determined to be insufficient to explain the excess risk of lung cancer for welders observed in the studies.
The hazards presented by welding fume depend on the welding process. Many different welding processes exist. Welding fume consists of fine solid particles with a diameter of less than 1 µm. Exposures can vary significantly among welders based on the type of welding process used, the presence of local exhaust ventilation, and the work practices of individual welders. According to IARC, studies suggest that novice welders or those with minimal training may have greater exposures than experienced welders. Approximately 11 million people worldwide have the title of welder, and an additional 110 million engage in welding activities, according to IARC.  Molybdenum trioxide, a related chemical that is mainly used in steel manufacture, is classified in Group 2B as a chemical that is possibly carcinogenic to humans. IARC’s classification is based on “sufficient evidence” of the chemical’s carcinogenicity in experimental animals. The agency notes that most occupational exposures to molybdenum trioxide occur in mining and metallurgy, steel foundries, welding, and other high-temperature processes using steel. IARC also classifies indium tin oxide in Group 2B. According to the agency, exposures to indium tin oxide occur mainly in its sintered form in occupational settings. Sintering is a process that uses heat and pressure to combine indium oxide and tin oxide powders. Workers can be exposed during the production, processing, and recycling of elemental indium. Exposures to indium in low- and middle-income countries where informal recycling of electronics occurs are also of concern because the chemical is used to produce transparent conductive films on glass or plastic panels used in electronic devices. Volume 118 of the IARC Monographs includes the evaluations of the carcinogenicity of welding and welding fumes, molybdenum trioxide, and indium tin oxide. A summary of IARC’s evaluations is available online in The Lancet Oncology. The full article is available free of charge to registered users (registration is also free). IARC monographs identify and evaluate environmental factors that can increase carcinogenic risks to humans. IARC is the specialized cancer agency of the World Health Organization, and government agencies worldwide use its monographs as scientific support for their actions to prevent exposure to potential carcinogens.
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The agency stated that the new classification is based on “substantial new evidence” from observational and experimental studies.