An Internet search on bras and their relationship with breast cancer reveals a very unusual result. Despite enough specialistic studies and hard proof that establishes the causality between wearing bras and the cancer condition, the two seem tightly linked with most sources pointing to a book named Dressed to Kill by Singer and Grismaijer.
Bras do not cause it themselves, but wearing sizes that are too tight may help cancer growth and the reason for that is tight fitting bras can prevent women's bodies from excreting toxic and dangerous carcinogenic chemicals. Statistics show that eighty percent of women wear the wrong-sized bra.
The tight bras restrict lymph flow in the breasts. There are several nodes and pathways in the arm pits and in between the breasts. The task of the lymph nodes is to flush out waste materials and toxic material away from the breasts.In theory, tight fitting bras restrict this same flow thereby causing toxins to accumulate in the breast area and may help cancer to develop. Toxic materials include DDT, dioxin, and benzene all of which cling to fatty human tissue such as the breast.
The book does present some startling statistics:
75% of women who wore bras everyday for 24 hours developed breast cancer
14% of women wearing bras more than 12 hours per day (not in bed) developed breast cancer
0.6% of women who wore bras for less than 12 hours a day developed the cancer
It is very important to note here that the numbers may explain a relationship between wearing bras for a long period of time and breast cancer but they do not necessarily prove a causal relationship between the two. More research needs to be done on lymphatic system and breast cancer to illuminate this issue.