An official website of the United States government.

This is not the current EPA website. To navigate to the current EPA website, please go to This website is historical material reflecting the EPA website as it existed on January 19, 2021. This website is no longer updated and links to external websites and some internal pages may not work. More information »

Norwood, Pennsylvania

Why is it difficult to establish a link between cancers or other illnesses and environmental exposures?

The complex nature of cancer makes it inherently challenging to identify, interpret, and address cancer clusters. Cancer is a term describing different diseases that share a similar characteristic: uncontrollable cell growth and division. As a group, cancers are very common. Cancers are the second leading cause of death in the United States, exceeded only by diseases of the heart and circulatory system. One of every four deaths in the United States is due to some form of cancer. For more information, please visit