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Understanding the Causes of Cancer

There are a lot of rumors and myths about cancer that make it hard to know what’s true. Here are answers to some of the questions you may have. You'll also learn how potential carcinogens are tested and classified, and find a list of known and probable carcinogens.

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Answers to Common Questions

When we hear something might cause cancer our first reaction is often to think that we need to avoid it at all costs. But often it’s not that simple.  Get answers to common questions about the causes of cancer, and find out what questions you should ask when you see news about something that could cause cancer. Asking questions can help you better understand the information and decide if it is something you might need to act on.

Determining if Something Is a Carcinogen

Substances and exposures that can lead to cancer are called carcinogens.  Learn how researchers study the causes of cancer and how expert agencies classify known, probable, and possible carcinogens.

Known and Probable Human Carcinogens

See the lists of substances and exposures that are known or suspected to cause cancer from the International Agency for Research on Cancer (IARC) and the US National Toxicology Program (NTP).

Cancer Clusters

A cancer cluster is a “greater-than-expected number of cancer cases that occurs within a group of people in a defined geographic area over a period of time.”

More than 1,000 suspected cancer clusters are reported to state health departments each year. But most are determined not to be true clusters.

Cancer Warning Labels Based on California's Proposition 65

Labels warning that a product contains compounds that may cause cancer, birth defects, or reproductive harm are now required on many household items sold in California. But people in other states may see them as well.