Is
it possible that secondhand smoke causes breast cancer?
In
2005, the well-respected state Environmental Protection Agency in California
reviewed the scientific literature and decided that women were more likely to
get breast cancer at a young age if they regularly inhaled someone else's
cigarette smoke.
But
now, preliminary findings from the British Million Women Study, described as the
largest study of its kind in the world, suggest there is no link.
So
should women get more-frequent mammograms if they worked in smoke-filled
restaurants or bars or had parents or a spouse who smoked? There are passionate
scientists on both sides of the debate, and the answer may come down to the
details of the research.
The
British work looked at 1.3 million women ages 50 to 64. The preliminary results,
presented at a conference in September, showed absolutely no link between
secondhand smoke and breast cancer, according to Dr. Michael Thun, who saw a
copy of the presentation and heads up epidemiological research for the American
Cancer Society.
"These
data are strong, and they seriously weaken the evidence that there is in fact
any connection," said Thun, adding that he is now reconsidering whether
it's worth the American Cancer Society's resources to study the subject further,
as it had planned to do. The study "seems like a major challenge to the
hypothesis."
The
California study agreed that there is no association between older secondhand
smokers and breast cancer. But it did find that premenopausal women exposed to
secondhand smoke were at a 70 percent higher risk for developing breast cancer
than those who weren't exposed.
"According
to our calculation, a third of the breast cancer in young women who worked as
waitresses is attributable to secondhand smoke," said Stanton Glantz, a
professor of medicine at the University of California-San Francisco who was on
the scientific panel that reviewed the study.
Melanie
Marty, the California EPA scientist who oversaw the study, said she can't
explain why the finding applies only to younger women. One theory is that breast
cancer in older women is caused by different factors than in younger women.
Another explanation is that a woman's breast tissue is most susceptible to
carcinogens from puberty until her first child is born, making younger women
more vulnerable to tobacco smoke.
But
Dr. Valerie Beral, the coordinator of the British study, doesn't buy it. She
said that when she looked at premenopausal women in her group, she found no
association between breast cancer and passive smoking.
Some
researchers are skeptical of the California finding, Thun said, because many
previous studies have failed to established a link between active smoking and
breast cancer -- so there should be even less of a connection with the smaller
doses of carcinogens in secondhand smoke.
"There
is little effect of either active smoking or passive smoking on breast
cancer," said Sir Richard Peto, an epidemiologist at Oxford University in
England who collaborated on the forthcoming British study.
Marty
and Glantz disagreed, saying that some studies do, in fact, find an association
between active smoking and breast cancer. There's also a theory that smoking
actually mutes the carcinogenic effect of cigarettes by suppressing estrogen
production, which is associated with some breast cancer development.
The
risk of getting breast cancer from secondhand smoke is even higher than the risk
of getting lung cancer, the California report said.
Toxicology
studies also support the California conclusion. Scientists identified 20
chemicals in tobacco smoke that cause tumors in the breast tissue of rodents,
giving the connection "biological plausibility," Marty said.
Even
if the evidence for a link between breast cancer and secondhand smoke isn't
there, people should still be concerned about exposure, said Gary Giovino, a
public health professor at the State University of New York in Buffalo who was
formerly chief of epidemiology at CDC's Office on Smoking and Health.
"We
know there are tons of things wrong with secondhand smoke," Giovino said.
"If I had a restaurant, I wouldn't want to be exposing my waitresses to it
for this reason and many other reasons."
At
a minimum, Thun said, women should know about the scientific debate.
"We
don't have a final answer, but it's one additional reason to avoid secondhand
smoke," he said.
© Copyright 2007 Globe Newspaper Company.