Glossary

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Summary Statement

We live in a society in which our lives are made easier; our average lifespan is longer by many years; and our world is interconnected socially, politically and economically as a result of the development of (among other things) plastics, electronics, chemical additives, pesticides and herbicides, and medical advances of many types. Yet even as we embrace these new technologies, we must recognize that the development and implementation of many of them may have negative consequences.

One goal of this project has been to explore ways in which many commonly-found chemicals, as well as by-products of their synthesis and disposal, may interact to affect the likelihood that a woman may develop breast cancer, and that collectively, exposure to these chemicals may help to explain the increased incidence of this disease over the past several decades.

Another goal has been to explore the growing scientific evidence that suggests that there are critical periods of susceptibility to many of these compounds, and that we need to be particularly concerned about exposures to these chemicals, alone and in combination, for individuals during key times of development, from conception through adolescence.

In exploring the scientific evidence, we have tried to acknowledge controversies and disagreements among scientists, while also providing a framework for understanding some basic methodological issues in this complicated field. Of especial concern for us has been addressing individual differences in sensitivity to chemical exposures, reflecting on the role of age of exposure, genetic predisposition, age of diagnosis of cancer, and interactions with reproductive history. The sensitivity to exposures varies from person to person, and except under truly unusual conditions of massive exposure to single chemicals, it is very difficult to pinpoint the moments of exposure that contributed to the development of breast (or other) cancer.

We have also focused on the interactions between multiple chemicals, recognizing that rarely are we exposed to single substances in the absence of others. These interactions can be complex, sometimes being additive, sometimes being synergistic, and sometimes actually canceling out or diminishing effects of other compounds. What do we mean by additive or synergistic?

An additive effect is when two factors act independently of each other, but each leads to a similar outcome, such as an increased risk for breast cancer. Exposure to both factors would result in a simple addition of the two original risk values. As a very simple example, if exposure to Chemical A led to an increase of 2 additional cases of breast cancer per 1000 women and exposure to Chemical B led to an increase of 3 additional cases of breast cancer per 1000 women, an additive effect of exposure to both Chemical A and Chemical B would lead to 5 additional cases of breast cancer per 1000 women.

A synergistic effect is the simultaneous interaction of two or more risk factors that result in a greater effect than they would have had individually and also greater than the simple additive effects of the two factors. Many low-concentration chemicals can interact synergistically with each other to exert a larger combined effect. For example, if exposure to Chemical A led to an increase of 2 additional cases of breast cancer per 1000 women and exposure to Chemical B led to an increase of 3 additional cases of breast cancer per 1000 women, a synergistic effect of exposure to both Chemical A and Chemical B might lead to 7 additional cases of breast cancer per 1000 women.

It is important to remember that few if any of the risks outlined in this project may be sole causes of breast cancer across large populations of women. Instead, breast cancer is caused by a history of multiple exposures, some working additively or synergistically, leading to accumulated cellular damage that takes many years to be expressed as cancer. Our histories of exposures and our predispositions to respond are quite individual.

Hazards and risks are a daily part of life. The risks outlined in this project are not meant to discourage or scare people. Rather, every individual has the right to informed consent to the activities in which they are engaging. It is impossible to attempt to avoid all risks, but reassuringly, not every exposure to risk will have a negative effect. We believe that it is worthwhile to look for less harmful alternatives when possible, and to decrease the potential additive and synergistic effects that may come from these exposures.