NewsCarcinogenicity of welding, molybdenum trioxide, andindium tin oxide
In March 2017, 17 scientists from ten
countries met at the International
Agency for Research on Cancer(IARC; Lyon, France) to evaluate
the carcinogenicity of welding,molybdenum trioxide, and indium
tin oxide. These assessments will be
published in volume 118 of the IARC
Monographs.1Worldwide, an estimated 11 million
workers have a job title of welder,
and around 110 million additional
workers probably incur weldingrelated exposures. Welding caninvolve exposures to fumes, gases,
radiation (ultraviolet [UV] radiationand electromagnetic fields) and coexposures to asbestos and solvents.
Welding involves several processes(eg, oxyfuel [gas], arc, and resistancewelding) and materials (eg, mildand stainless steel). Exposure
determinants include the process,material welded, ventilation, degreeof enclosure, and use of personalprotection.
The carcinogenicity of welding
fumes was assessed by IARC in
19892 and classified as “possiblycarcinogenic to humans” (Group 2B),
based on ‘‘limited evidence in humanbeings’’ and ‘‘inadequate evidence’’ inexperimental animals.3 UV radiation
was classified in Group 1 in volume
100D of the IARC Monographs.
Substantial new evidence has sinceaccumulated from observationaland experimental studies. In thepresent evaluation, welding fumes
and UV radiation from welding wereclassified as “carcinogenic to humans”(Group 1).
Arc welding generates UV radiation,a risk factor for the rare cancer ocularmelanoma. Various ocular disorders(eg, cataracts or keratoconjunctivis)
occur in both welders and nearby
workers. Sufficient evidence for thecarcinogenicity of UV radiation fromwelding came from eight partly
www.thelancet.com/oncology Vol 18 May 2017

overlapping case–control studies andtwo census-based cohort studies thatreported on ocular melanoma. Most
case–control studies showed positive
associations, with risks of developing
ocular melanoma generally increased
by between two-fold and ten-fold.
Two of three studies that assessedrisk by duration of employment asa welder showed positive trends.3,4These studies also showed increased
ocular melanoma risk associated witheye burns—a proxy for UV exposure—and one reported a positive exposure–
response association for cumulative
occupational exposure to artificial UV
radiation, including welding.3,4 Risks
persisted after adjustment for sun
exposure, sun bed use, or both.4–6
Welding fumes are produced whenmetals heated above their melting
point vaporise and condense to fineparticles (mostly <1 µm in size).
Most studies, including more than
20 case–control studies and nearly
30 occupational or population-based
cohort studies, reported increased
risks of lung cancer in welders or otherworkers exposed to welding fumes.
Exposure–response
associations
with indices of longer or greater
cumulative exposure to welding
fumes were also reported in severalstudies, some of which were large,
high-quality studies.7–10 Exposure to
fumes was assessed indirectly throughwelding process or material, branch ofindustry, job title, expert assessment,or self-report.
Asbestos exposure and tobaccosmoking, which are important
potential confounders, could not
explain the observed excess lungcancer risk in welders. Positive
associations persisted after adjusting
directly or indirectly for smoking,asbestos co-exposure, or both;7–10
restricting to non-smokers or lowlevel smokers;11 and in cohorts withlow or minimal asbestos exposure.12

Positive associations for occupation
as a welder and kidney cancer werereported in nearly all relevant cohortand case–control studies. However, few
studies adjusted for solvents used forcleaning metal in tandem with welding,such as trichloroethylene (a risk factor
for kidney cancer). Increased risks wereconsistently reported across countries,
occupational settings, and study
designs. However, chance, bias, andconfounding could not be reasonably
ruled out because some findings
were not statistically significant,several studies had few exposed cases,
and there was little evidence of an
exposure–response association.
For all other cancers, the evidence
for carcinogenicity was inadequate
because of inconsistent findings
across studies, insufficient numbersof studies, or the potential for
confounding or selection bias. The
Working Group concluded that thereis ‘‘sufficient evidence in humans’’ thatwelding fumes cause lung cancer andlimited evidence for kidney cancer.
There is limited evidence in experi­mental animals for the carcinogenicity
of gas metal arc–stainless steel
welding fumes. In one oropharyngeal
aspiration study and one inhalation
study in male A/J mice, gas metal
arc–stainless steel welding fumes
promoted
3-methylcholanthreneinduced lung tumours.13,14
Absorption and excretion ofmetals (chromium, nickel, andmanganese) was shown in people
exposed to welding fumes, but datafor particle deposition and clearance
in welders were scarce. Strong
evidence suggests that welding fumes
induce chronic inflammation andare immunosuppressive. Lung andsystemic inflammation biomarkers
were increased in many panel studies(of cross-sectional and cohort design)of various arc welding fumes.15 Risk for
infection (pneumonia) was increased

Published Online
April 10, 2017
http://dx.doi.org/10.1016/
S1470-2045(17)30255-3
For more on the IARC
Monographs see
http://monographs.iarc.fr/
Upcoming meetings
June 6–13, 2017, Volume 119:
Some chemicals in food andconsumer products
Oct 10–17, 2017, Volume 120:
Benzene
IARC Monograph Working
Group Members
J Hansen (Denmark)—Meeting
Chair; A K Nersesyan (Austria);
J Lavoué (Canada); D Luce(France); W Ahrens (Germany);
S Fukushima (Japan);
H Kromhout, S Peters(Netherlands); A ‘t Mannetje(New Zealand); M Albin(Sweden); M G Baker,
D C Christiani, J M Fritz,
W M Gwinn, R M Lunn, E J Tokar,
P C Zeidler-Erdely (USA)
Declaration of interests
SP declares that her previous
research unit at the University ofWestern Australia received
research support (ceased July,2014) from WesTrac Ltd (WesternAustralia, Australia)—
a construction equipment
supplier—to investigate theadverse health effects ofenvironmental asbestos exposure.
All other working group members
declare no competing interests.
Invited Specialists
None
Representatives
Y Chen, for the Health and Safety
Executive (HSE), UK; P Guillou,
for the French Agency for Food,
Environmental and Occupational
Health and Safety (ANSES),France; F Pega, for WHO, Geneva,
Switzerland; A Ullrich, for WHO,Geneva, Switzerland
Declaration of interests
All representatives and World
Health Organization members
declare no competing interests.
Observers
A Hall, for the University ofBritish Columbia, Canada;
S Hubbard, for the International
Molybdenum Association(IMOA), UK; L Levy, for theInternational Molybdenum581

NewsAssociation (IMOA); A Oller, forEurometaux, Belgium; V Thomas,
for the University ParisDauphine, France; W Zschiesche,
for the Institute for Prevention
and Occupational Medicine of
the German Social Accident
Insurance, Germany
Declaration of interests
SH is a consultant for theInternational Molybdenum
Association (IMOA), was a recent
employee of RioTinto Ltd (until
the end of 2015), and hasreceived IMOA funding to travelto the IARC meeting. LL provides
consulting expertise inoccupational health toxicology
to the IMOA. AO is employed byNiPERA Inc. All other observers
declare no competing interests.
IARC Secretariat
L Benbrahim-Tallaa; V Bouvard;
R Carel; F El Ghissassi; Y Grosse;
N Guha; KZ Guyton; D Hashim;
MK Honaryar; R Khlifi; D Loomis;
K Muller; L Stayner; K Straif;
N Vilahur
Declaration of interests
All secretariat declare nocompeting interests.
For the Preamble to the IARC
Monographs see
http://monographs.iarc.fr/ENG/Preamble/index.php
For IARC declarations ofinterests see http://monographs.
iarc.fr/ENG/Meetings/vol118participants.pdf

582

in epidemiological studies ofdifferent designs. In rodents, welding
fumes induced chronic pulmonary
inflammation and, in fewer studies,
impaired resolution of pulmonary
infection.
Molybdenum trioxide (MoO3) isa high-production-volume chemicalwith rare natural occurrence. Of
the >100 000 tons of MoO3 produced
annually, most is used in steel
production, with other uses in bio­cides and increasingly in photo­voltaic technology. Environmental
exposures to MoO3 are negligible, but
occupational exposures can occur—
mainly in mining and metallurgy, steel
foundries, welding, and other hightemperature processes using steel. No
epidemiological and few toxicokinetic
and mechanistic data for MoO3 wereavailable. In one inhalation study inmice, MoO3 increased the incidences ofbronchioloalveolar carcinoma in malemice and bronchioloalveolar adenoma
or carcinoma (combined) in female
mice, and caused a positive trend in
the incidence of bronchioloalveolar
carcinoma in female mice.16 Molyb­
denum trioxide was classified as“possibly carcinogenic to humans”(Group 2B) based on sufficientevidence in experimental animals.
Indium tin oxide (ITO) is a lowproduction-volume chemical, notnaturally occurring, which is a mixtureof indium oxide (In2O3) and stannicoxide (SnO2). It is mainly used inproducing transparent conductive
films on glass or plastic panels usedin electronic devices. Exposure to ITO
occurs mainly in occupational settings,

during ITO production and processing,or during elemental indium recycling.
Indium has been detected in exposedworkers, but toxicokinetic data for
ITO were otherwise sparse. Long-term
studies in rats, mice, and hamsters
provided strong evidence that ITO
induces chronic inflammation. In
2-year inhalation studies, ITO increased
the incidences of bronchioloalveolar
carcinoma in male and female rats, andcaused a positive trend in the incidenceof bronchioloalveolar adenoma or
carcinoma (combined) in female
mice.17 Indium tin oxide was classified
as “possibly carcinogenic to humans”(Group 2B) based on sufficientevidence in experimental animals.
We declare no competing interests.

Neela Guha, Dana Loomis,
Kathryn Z Guyton, Yann Grosse,
Fatiha El Ghissassi, Véronique Bouvard,
Lamia Benbrahim-Tallaa,
Nadia Vilahur, Karen Muller,
Kurt Straif, on behalf of theInternational Agency for Research onCancer Monograph Working Group

567891011121314

International Agency for Research onCancer, Lyon, FranceReferences
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www.thelancet.com/oncology Vol 18 May 2017