DALLAS, Sept. 18 -- Airborne fine particles of metal, particularly zinc, may contribute to the development of lung cancer, according to an ecological study here.
In an ecological study comparing lung cancer rates with industrial release of metals, zinc and chromium were found to have positive associations with the disease, said Yvonne Coyle, M.D., of the University of Texas Southwestern Medical Center here.
On the other hand - for reasons that remain unclear - the combination of zinc and chromium appeared to have a protective effect, Dr. Coyle and colleagues reported in the September issue of the Journal of Thoracic Oncology.
The study isn't enough to show a causal link, Dr. Coyle and colleagues said, but it is "the first step to determining whether an association exists" between inhaled metal particles and lung cancer.
They noted that while the preponderance of lung cancer can be laid at the door of cigarette smoking, from 10% and 15% of the disease occurs in non-smokers, raising the possibility of an environmental influence.
"There is concern that other environmental carcinogens may be interacting with cigarette smoking or alone may be influencing the current trends for lung cancer incidence and mortality," Dr. Coyle said in a statement.
The researchers used an Environmental Protection Agency database, which records the industrial output of a range of toxic chemicals, including arsenic, cadmium, chromium, cobalt, copper, nickel, vanadium, and zinc.
The EPA information was compared with county-by-county average annual age-adjusted incidence rates of primary and non-small-cell lung cancer, the researcher said. They looked at 81,132 lung cancer cases from 1990 to 1995 in all 254 counties in Texas. In a univariate analysis, the study found:
- Zinc was significantly associated with the primary and non-small cell lung cancer rates, at P=0.02 and P<0.01 respectively.
- Chromium and copper were significantly associated with the non-small cell lung cancer rate, both at P=0.01.
In a multivariate analysis, zinc remained positively associated with both rates, although chromium and copper were no longer significantly associated. Dr. Coyle and colleagues said.
However, when they analyzed interactions among the metals, they found that the combination of zinc and chromium appeared to be significantly protective, at P=0.01.
"The reason for the protective effect of zinc and chromium on lung cancer incidence is unclear," the researchers said, noting that the EPA data do not distinguish between the different valence states of chromium released by industry. Some forms of chromium are known carcinogens, while others are essential for human health.
Other possible explanations are that co-exposure to chromium provided protection against the toxicity of zinc, or that the Texas counties that had both zinc and chromium released into the air had lower lung cancer rates for reasons completely unrelated to air pollution, the researchers said.
The study has several limitations, Dr. Coyle said, but "it provides new information suggesting that airborne metals, including those that are essential human nutrients, such as zinc and copper, play an important role in lung carcinogenesis."
She added that the study does not have data on the individual level, so it is impossible to tell whether patients with lung cancer have actually had previous high exposures to airborne metals. The researcher also did not have individual smoking data, although the rates for counties with and without airborne metal particles were similar.
The study covered the period from 1995 to 2000, the researchers said, which may not have been long enough for the true picture of lung cancer to develop in the 254 Texas counties examined.