By CHRISTINE DELL'AMORE UPI Consumer Health Correspondent WASHINGTON,
Sept. 15 (UPI) -- People who inhale tiny metal particles contained in
air pollutants may be more likely to develop lung cancer, an
epidemiological study has shown. In a study of lung-cancer cases in
Texas, researchers found an association between exposure to metals in
airborne particulate matter and incidence of adenocarcinoma lung cancer,
a type of cancer that often afflicts non-smoking women. "This type of
research gives us an idea of what might be causing lung cancer in women,
particularly those who don't smoke," said senior author Dr. Yvonne
Coyle, an associate professor of internal medicine at UT Southwestern
Medical Center in Dallas. The large-scale study, the first of its kind,
was published in the Journal of Thoracic Oncology. Coyle and colleagues
identified more than 80,000 lung-cancer cases from 1995 to 2000 in 254
Texas counties and compared them with data of industrial air pollution
reported to the U.S. Environmental Protection Agency between 1988 and
2000. The cancer rates were more prevalent in counties with the highest
levels of pollution; conversely, cancer rates were lower in counties
with less pollution. However, Coyle and colleagues' research is still
preliminary, and the link between airborne metals and cancer is not yet
conclusive. Particulate matter, tiny particles of dirt, dust and water,
settle into the atmosphere from fossil-fuel burning usually related to
industrial processes or automobile driving. The smallest type of
particles, called PM2.5 (which refers to 2.5 micrometers, a size 100
times thinner than a human hair) contain heavy metals, and previous
research has shown they may contribute to lung cancer. The EPA sets
limits on how much PM can be emitted as one of its six criteria
pollutants under the Clean Air Act. Every five years the agency updates
the standards; the next revision is due in 2009. Although PM2.5's
deleterious effects are well known, Coyle wanted to look at the tiny
bits of metals inside the particles -- namely, arsenic, cadmium,
chromium, cobalt, copper, zinc, nickel and vanadium. Coyle selected
these for analysis because of their ability to mimic estrogen, their
relatively high concentration in PM and their inclusion into EPA data.
After she conducted a statistical analysis, Coyle found strong
associations between copper, chromium and zinc and lung cancer. After
further analyses, zinc, an essential nutrient in the body, came out on
top as the greatest risk factor. It may sound strange than an essential
nutrient such as zinc -- popular in multivitamin tablets -- could be so
detrimental. But Coyle said some nutrients, when excessive, can have
toxic consequences. It could be the Texans in the study were inhaling
too much of the metal from outside air, which in turn led to their
cancer. Coyle speculates the metals are especially carcinogenic in
women because of their estrogenic effects, or ability to bind to
estrogen receptors in the lung tissue. Many other substances, both
natural and manmade, trick the body into believing it is processing
estrogen, a hormone that can be carcinogenic. Furthermore, women often
do not have the ability to metabolize carcinogens as well as men. Some
studies suggest women can combat this deficiency by eating fruits and
vegetables and exercising to reduce the amount of metals in the body.
The research team adjusted for sex, race and ethnicity and urbanization
to ensure these factors did not complicate their results. Although
smoking is the overriding cause of lung cancer, Coyle notes smoking
prevalence was similar for all the counties regardless of their rates of
pollution. However, the research team was not able to determine how
much the lung-cancer patients were exposed to PM, a weakness inherent in
observational studies, Coyle said. She plans to measure the amount of
metals in individuals in future studies. Lung cancer is the No. 1 cause
of cancer deaths in American men and women, claiming about 160,000 lives
a year. Smoking causes 87 percent of cases, and radon, a gas present in
many homes and office buildings, is thought to make up 12 percent. Genes
also play a part: Some people may have a genetic makeup more or less
susceptible to lung cancer. Yet scientists believe other environmental
carcinogens, in addition to cigarette smoke, may be influencing lung
cancer as well. Dr. Jonathan Samet, chair of epidemiology at the Johns
Hopkins University School of Public Health, said metals and particulate
matter is an area with promising leads for follow-up, although the early
studies, such as Coyle and colleagues', are exploratory and not
conclusive. "It's another one of those small steps along the way in
understanding the specific carcinogens of lung cancer," said Samet, a
leading expert in tobacco smoke and its effect on lung cancer. Even so,
Coyle believes the EPA should consider the research as additional
evidence of how PM can damage human health. The study should also
reinforce the trend of eliminating secondhand smoke -- also a carcinogen
-- from public places. "Cleaning up our air and water is what we should
be striving for," Coyle said. "It's not enough to ask people to exercise
and eat a certain diet (when) our environment is dirty." -- To see
how much pollution is in your county, go to: www.epa.gov/tri