95+% of the products sold today are anti perspirant and as the name clearly indicates, prevents us from perspiring.
Directly under the skin of the underarm, we have lymph nodes which are a part of the immune system fighting infections, diseases and removing foreign particles and toxins. These nodes accumulate "dirt" through the blood stream which is then secreted with the perspiration.
If we stop the perspiration the nodes swell and toxins stay in our body for a longer time and at a greater concentration.
We absorb and store deodorant chemicals applied to the skin. In a recent study 18 of 20 breast tumors contained parabens (synthetic preservatives) found in underarm products. Parabens mimic estrogen, and estrogen can drive tumor growth.
Source: Journal of Applied Toxicology, February, 2004
The FDA which authorizes selling of cosmetics and drugs in USA classifies antiperspirant as a drug, since this product changes a natural body act (blocks perspiration from the underarm).
While a deodorant which only fight the odor of perspiration but does not block it is classified as cosmetic.
The FDA says in an interview to CBS news on 05/12/05 that it is aware of the concern that using antiperspirant together with shaving the underarm is connected to the increase of the risk of breast cancer.
The underarm area is a main gate of perspiration. The perspiration process is a part of our body's immune system.
The chemicals from deodorants and antiperspirants stay in the underarm area for a long time during the day and might penetrate the thin skin layer (exactly like the stickers sold in order to make our body temperature lower), mix with the blood stream and so reach every place in our body.