Doing a study and concluding that something doesn't exist is pretty easy, since all you have to do is do a poor job of looking for it. This legislation demanded independent studies be done, and more pointed inquiry so as to not miss what might be there. The study was broad reaching. And it looked at any disease, not just cancers. Most tampons are made of rayon.
Rayon is made from cellulose and fibers of wood.
The processing from raw materials includes chlorine bleaching which is known to produce dioxin. The makers say they wash it all away. Critics say they don't. Dioxin is a type of chlorine called an organochlorine. Organochlorines are similar in physical structure to sex hormones.