From Pam Stephan,
Your Guide to Breast Cancer.
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About.com Health's Disease and Condition content is reviewed by V.K. Gadi, MD

Smoking is a controversial lifestyle factor that has been tied to increased breast cancer risk. Studies have yet to conclusively prove a link between tobacco smoke and breast cancer, but evidence is piling up.

Studies do show some suggestive links between smoking and breast cancer:

• Secondhand (passive) smoke may increase risk in younger, premenopausal women
• Teens who smoke are more likely to develop breast cancer before menopause
• Active smoking is linked to aggressive, hormone receptor-negative (HR-) type breast cancer
• Smoking may promote the spread of breast cancer to your lungs

No Amount of Smoking is Safe
The link between smoking and cancers, in general, is undeniable. Cigarette smoke contains cancer-causing chemicals that are absorbed into your body and affect your present and future health. Here are just a few of the 3,000 chemicals in tobacco smoke that are related to cancer:

• Tar – a sticky substance that is created as tobacco burns. Inhaling while smoking pulls tar into your lungs, where it accumulates over time and causes tissue destruction.
• Nicotine – an extremely addictive drug that helps cancers grow. It does not cause cancer, but it can promote cancer cell growth.
• Nitrosamine – a carcinogenic compound that occurs in tobacco; it has been used in cosmetics, processed meats, pesticides, and latex products.

Teens, Smoking, and Breast Cancer
During puberty, estrogen produced in the ovaries causes breast tissue to start developing. Lobes, ducts and supportive fatty tissue are growing and preparing to supply milk for breastfeeding. That developing tissue is more vulnerable and affected by the chemicals in tobacco smoke than some other tissues. Cancer cells may have estrogen receptors; when high levels of estrogen are present, they will be urged to grow. Although nicotine can suppress estrogen, estrogen levels during puberty are so high that smoking will not lower those levels. A survey of studies by the Public Health Agency of Ottawa, Ontario Canada, even showed that secondhand smoke increases risk of developing premenopausal breast cancer by 14 to 119%, depending on the amount of exposure.

Adult Women, Smoking, and Breast Cancer Risk
Premenopausal women who smoke have an increased risk of breast cancer as well as lung cancer. Women who stop smoking can decrease their risk of breast cancer to average that of nonsmokers' after 10 years. After menopause, when estrogen levels decline, an active or long-term female smoker -- who started smoking before age 65 and before her first child was born -- has a 30 to 40% increased risk of breast cancer. If a postmenopausal woman smoked for 20 years or more and used hormone replacement therapy (HRT), which boosts estrogen levels, her risk is 50% greater than average.

Evidence That Relates Smoking to Breast Cancer