Dioxins

Although not well-quantified, there has long been a concern about risk associated with environmental estrogenic compounds, such as dioxins.

Light levels

Researchers at the National Cancer Institute and National Institute of Environmental Health Sciences have concluded a study that suggests that artificial light during the night can be a factor for breast cancer.

Viral pathogenesis research

Humans are not the only mammals susceptible to breast cancer. Some strains of mice, namely the house mouse (Mus domesticus) are prone to breast cancer which is caused by infection with the mouse mammary tumour virus (MMTV or "Bittner virus" for its discoverer Hans Bittner), by random insertional mutagenesis. This finding is taken to mean that a viral etiology of human breast cancer is at least possible, though there is no definitive evidence to support the claim that MMTV causes human breast cancer. For example, there may be critical differences between cancer pathogenesis in mice and people. The understanding of the role of MMTV or other viruses in human breast cancer is preliminary as of May 2007.