If finalized, a draft report from a branch of the California Environmental Protection Agency will be the first to officially designate secondhand smoke as a cause of breast cancer.

Other health agencies, such as the World Health Organization (WHO) and the Centers for Disease Control and Prevention (CDC), have identified secondhand smoke as a cause of lung cancer, heart disease, and, in children, prematurity, low birth weight, SIDS, respiratory problems, and middle ear infections. The CDC estimates that secondhand smoke causes 3,000 lung cancer deaths and more than 35,000 heart disease deaths each year in the US.

However, a comprehensive review by the International Agency for Research on Cancer, the branch of WHO that evaluates cancer hazards, did not find the evidence linking secondhand smoke to breast cancer to be conclusive in 2002. A Surgeon General Report on secondhand smoke is currently reexamining the evidence.

The new report, by the California Air Resources Board, is sure to fuel debate about tobacco regulation and clean indoor air laws. It was prepared as part of a state effort to determine how big a health risk secondhand smoke actually is. That information will then be used to decide whether additional steps need to be taken in California to limit exposure to secondhand smoke.

"The issue of whether secondhand smoke causes breast cancer, as well as numerous other diseases, is difficult to study," says Dr. Michael Thun, vice president of epidemiology and surveillance research at ACS.

Active smoking is not proven to increase breast cancer risk, he said, so explaining why much lower doses of smoke would do so is very complicated. Furthermore, in Western countries, most women in the age range where breast cancer becomes common have been exposed to substantial amounts of secondhand smoke, so it is difficult to find an unexposed comparison group.

"The fact that mainstream health organizations have not yet found the evidence on breast cancer and passive smoke to be conclusive illustrates the high level of evidence they require as a basis for tobacco control policies," Thun said. "Mainstream health organizations are intentionally cautious in making claims about tobacco, reserving final judgment until the evidence cannot be dismissed by the tobacco industry."

Thun noted, however, that resolving whether secondhand smoke causes breast cancer is important and that the California group that developed this report was also the first to link secondhand smoke to heart attacks -- a link that has since been widely confirmed.

Whether or not the link to breast cancer is eventually established, Thun said there is overwhelming scientific evidence supporting the harmful effects of secondhand smoke. The real solution to this issue comes from communities that mobilize to fight for and win the right to clean air, he said.