According to the American Cancer Society, there is little scientific evidence and "no strong epidemiologic studies" linking breast cancer risk and the use of underarm deodorants. Still, other experts point to disturbing studies that show a link between the two, and consider two chemicals — aluminum and parabens — as the likely culprits.

"Aluminum poisons our brains and is a known carcinogen," says Dr. David Brownstein, author of Dr. David Brownstein's Natural Way to Health newsletter. "There is no therapeutic use for aluminum in our bodies, and there is certainly no reason we should be smearing aluminum on our underarms and stopping lymph glands close to the breast from draining.

"The site for most breast cancers has changed over the years, from other areas of the breast to the upper outer quadrant which is close to where deodorant is applied," Brownstein tells Newsmax Health. "There are also more cysts in that area."

Research published in 2013 in the Journal of Inorganic Chemistry measured aluminum in higher amounts in human breast tissue, and evidence suggested that the levels were high enough to adversely affect cells, including increasing inflammation.

Previous research indicated that aluminum salts used in some deodorants — which can be as much as a quarter of the volume of some antiperspirants — could break through the skin, and mimic estrogen once inside the body. Estrogens are known to foster the development and growth of breast cancer.

Another study, also published in the Journal of Inorganic Chemistry, noted that clinical studies found disproportionately high numbers of breast cancers in the upper outer quadrant of the breast, the area closest to the underarm area where deodorant is applied. The researchers speculated as to whether long-term, low-level absorption of aluminum salts found in deodorants could increase the incidence of breast cancer.

Kris McGrath of Northwestern University found a link between breast cancer and how often women shaved and applied deodorant. The study, which was published in the European Journal of Cancer Prevention, found that women who shaved and used deodorant at least three times a week were diagnosed with breast cancer 15 years earlier than those who did neither.

A 2016 study published in Morphologie found that antiperspirants expose the breast to high amounts of aluminum close to the area where there is a high incidence of breast cysts and breast cancer. "Gross cystic breast disease is the most common benign disorder of the breast and evidence is presented that aluminum may be a causative factor in formation of breast cysts," the researchers wrote. They also noted that aluminum can cause breast cells to multiply "inappropriately" and can " increase migration and invasion of human breast cancer cells."

Parabens, the other questionable ingredient in deodorants, are chemicals used as a preservative in many cosmetics, pharmaceuticals, and food products. Scientists at Britain's University of Reading examined 160 tissue samples collected from the tumors of 40 women undergoing mastectomies for breast cancer — four from each woman. They discovered that 99 percent of the samples contained at least one paraben and 60 percent had five.

Scientists have known since 1998 that parabens are estrogenic — that is, they act like estrogens in the body.

Parabens are found in underarm deodorants, although many U.S. companies have removed them from their products. However, they are still commonly found in moisturizers, makeup, and toothpaste, as well as in some processed foods, and in numerous over-the-counter and prescription drugs. 

The most common forms of parabens used in cosmetics are methylparaben, propylparaben, and butylparaben.

If you want to use a deodorant that contains neither aluminum nor parabens, read labels — these ingredients must be listed, and you can find natural products that are free of both. 

Brownstein believes the FDA should do more to remove questionable products from store shelves. "If the FDA was working for us, they would pull deodorants containing these ingredients off the market," he says.

About 12 percent of American women will develop breast cancer during their lifetime. In 2017, 252,710 new cases of invasive cancer are projected to be diagnosed, along with 63,410 new cases of non-invasive (in situ) breast cancer.