Many common pesticides (such as ant, roach and mice poisons) have been linked to a variety of human diseases, including breast cancer. It probably shouldnt surprise us that these compounds can be dangerous; they are designed to kill insects with which we share many common biological systems!
Several pesticides are known endocrine disruptors (they disrupt natural hormone-signaling pathways), and through these mechanisms have been implicated in increased risk for breast cancer. In addition to effects by themselves, these chemicals have been shown to have additive effects with other kinds of endocrine disruptors. In other words, exposures to small doses of pesticides may have greater effects when people are also exposed to other chemicals to which we are all commonly exposed.
Unfortunately, when pesticides are applied in the home, they dont just kill bugs and disappear. Rather they often stick around (for years or decades) and are found in the air and on the dust we touch and breathe, meaning that we all have sustained and multiple exposures to these toxic chemicals. And when we apply them outside, pesticides enter the air we breath, fall on the lawn on which we walk and our children play, and eventually seep into our water. The result can be devastating for the wildlife with which we share our world, and also may have significant impact on rates of human diseases, including breast cancer.
The best way to minimize insects inside and outside the home is through careful and regular cleaning. Integrated Pest Management (also known as IPM) approaches provide chemical-free (or low-chemical) strategies for protecting home environments, yards and agricultural crops. The University of California at Davis has published a good resource for learning more about IPM applications for the home and garden.