Leading medical experts debated whether exposure to environmental tobacco smoke increases the risk of breast cancer in young women at an event held at Dartmouth-Hitchcock Medical Center on Thursday. While presenters at the lecture, "Is Secondhand Smoke Exposure a Cause of Premenopausal Breast Cancer?" did not reach a consensus on the relationship between secondhand smoke and breast cancer, they all agreed that smoke exposure carries significant general risks for public health.
"The weight of evidence from epidemiological and toxicological studies suggests the association between secondhand smoke and premenopausal breast cancer risk is consistent with causality," Queen's University professor Kenneth Johnson said.
People are exposed to many more toxic chemicals through secondhand smoke than through direct smoking, because secondhand smoke is not filtered by the cigarette, Johnson said. He noted that secondhand smoke contains 50 times more formaldehyde than firsthand smoke.
As many as 20 carcinogens linked to breast cancer have been identified in secondhand smoke, he said, and 20 recent studies have found increased risk of premenopausal breast cancer as a result of environmental tobacco smoke.
Johnson acknowledged, though, that several past studies have found no correlation between secondhand smoke and breast cancer, citing a 2004 study by the International Agency for Research on Cancer as an example. But flaws in the measurement of secondhand smoke exposure for study participants undermined the validity of those studies, Johnson said.
Many older studies sorted participants based on whether they had been "exposed" or "not exposed" to secondhand smoke, he said. The "exposed" group consisted only of participants whose spouses were active smokers, so participants who had been exposed to secondhand smoke in other situations were placed in the "not exposed" group. This classification system skewed the results of the studies, he said.
"It's not surprising that you might not see a correlation with increased risk [in these older studies]," Johnson said.
Dartmouth Medical School professor John Baron, however, argued that current evidence is insufficient to suggest a causal relationship between environmental tobacco smoke exposure and breast cancer.
Smokers typically lead a very different lifestyle than non-smokers, Baron said. Smokers generally drink more alcohol and coffee than do non-smokers, and smokers often have a lower body weight and a less healthy diet, Baron said. These lifestyle differences, rather than the smoke itself, may increase the risk of breast cancer among smokers and those close to them, he said.
Baron also argued that correlation does not necessarily imply causation.
"What if I told you that people who carry cigarette lighters are more likely to develop lung cancer because the butane from the lighter damages their lungs?" Baron asked.
Baron also said that the relative risk of developing breast cancer is extremely small when compared with other risks associated with secondhand smoke. Individuals exposed to secondhand smoke were 1.4 times more likely to develop breast cancer, though they were 20 times more likely to develop lung cancer, he said.
Both speakers, however, agreed that there are significant overall risks associated with smoking and secondhand smoke. Baron said he believes that secondhand smoke should be minimized to improve public health, regardless of whether it is proven to cause breast cancer.
"This issue has been pervaded by public activism," he said. "I think if we're interested in science, we have to do science. If we're interested in public activism, we have to be clear it's not science."
