20 SUSPECTED OCCUPATIONAL CARCINOGENS

July 21, 2010 — A new international report identifies 20 suspected "high-priority" carcinogens for which the potential to cause cancer is unclear and in need of further study.

The report focuses on possible carcinogens in the workplace, such as metals and solvents, but the list also includes shift work.

The report is a collaboration of the International Agency for Research on Cancer (IARC), in Lyon, France, and the National Occupational Research Agenda, a program of the National Institute for Occupational Safety and Health (NIOSH) in the United States.

The main purpose of the report is to identify research efforts that might close gaps in knowledge about potential occupational carcinogens.

The full report is available online; another version was published online June 18 in Environmental Health Perspectives.

The report, entitled Identification of Research Needs to Resolve the Carcinogenicity of High-Priority IARC Carcinogens, lists 19 substances and 1 occupational circumstance:

* Lead and lead compounds
* Indium phosphide
* Cobalt with tungsten carbide
* Titanium dioxide
* Welding fumes
* Refractory ceramic fibers
* Diesel exhaust
* Carbon black
* Styrene-7,8-oxide and styrene
* Propylene oxide
* Formaldehyde
* Acetaldehyde
* Dichloromethane, methylene chloride (DCM)
* Trichloroethylene (TCE)
* Tetrachloroethylene (perc, tetra, PCE)
* Chloroform
* Polychlorinated biphenyls (PCBs)
* Di(2-ethylhexyl) phthalate (DEHP)
* Atrazine
* Shift work

The IARC classifies substances and circumstances in 5 groups: carcinogenic to humans (group 1), probably carcinogenic to humans (group 2A), possibly carcinogenic to humans (group 2B), not classifiable as to its carcinogenicity to humans (group 3), and probably not carcinogenic to humans (group 4).

The 20 selected agents in the report, and listed above, belong to the IARC categories of the most uncertainty: groups 2A, 2B, and 3.

"There is significant concern among the public about substances or exposures in the environment that may cause cancer, and there are some common occupational agents and exposure circumstances where evidence of carcinogenicity is substantial but not yet conclusive for humans," said lead author Elizabeth Ward, PhD, vice president of surveillance and health policy research at the American Cancer Society, in a press statement.

Shift Work Is a Possible Carcinogen

Some of these occupational hazards, including shift work, have received considerable media coverage. For instance, many news outlets, including Medscape Medical News, reported last year that Denmark paid government compensation to women (including nurses) who developed breast cancer after long spells of working at night.

The employment sectors with the highest percentage of workers on a nonday shift include healthcare, hotels and restaurants, agriculture, and transport and communication, according to the report.

At least one potentially carcinogenic class of agents, the PCBs, has had diminished exposures in recent decades. The report notes that the production of PCBs was halted worldwide in 1993, and that new occupational exposure has been confined to 4 groups of workers: personnel replacing or repairing transformers and capacitors still containing PCB dielectric fluid; first responders to incidents where a transformer has exploded; construction workers removing old paint, plaster, caulk, or floor finishes containing PCBs; and workers at hazardous waste disposal sites.

Research Needs, Research Recommendations

The report aims to identify research needs and to make research recommendations about the suspected carcinogens to "resolve classification uncertainties," writes one of the authors.

"This is a concerted effort to identify means of reducing the insufficiency of available data for classifying particular agents in the [IARC] system," writes Paul A. Schulte, PhD, director of the Education and Information Division of NIOSH, in Cincinnati, Ohio, in an introduction to the full report.

The report group tasks itself with making 2 kinds of recommendations: where epidemiologic studies might yield sufficient evidence in humans, human cancer studies are to be included in the research plan; and where mechanistic studies might yield "strong evidence in exposed humans," mechanistic studies are to be included.

Lead and Lead Compounds

For example, in considering lead and lead compounds, the report authors say that previous research has categorized these as probably carcinogenic to humans (group 2A).

The report authors, in their section on research needs and recommendations for lead, say that "additional epidemiologic information can be provided by large cohorts with established exposure above background environmental levels."

The authors say that "it appears likely that there will be a mortality study of approximately 50,000 workers with past measured blood lead levels above 25 μg/dL in the United States," referring to an existing NIOSH surveillance program. The authors note that the probable study "should prove informative."

"There are also an additional 50,000 subjects with lower blood leads," the authors continue. "Subsequent possible case–control studies of cancers of interest (stomach, brain, kidney, lung) could also provide more information."

The authors then make a recommendation: "This study could be strengthened by the addition of 2 components: 1) measurement of a sample of subjects for bone lead to determine the correlation of the blood lead measurements with cumulative exposure as measured by bone lead; and 2) assessment of whether Helicobacter pylori infection has been more common among those with higher blood leads. If so, such infection could either be a mechanism by which lead caused higher rates of stomach cancer, or it could be a confounder," they write.

The report was supported by a grant from the American Cancer Society to the IARC. Support was also provided by NOISH.

Environ Health Perspect. Published online June 18, 2010. Abstract