(NBC News) As many as 40 percent of cancer cases, and half of cancer deaths, come down to things people could easily change, researchers said Thursday.

While Americans often worry about whether chemicals, pollution or other factors out of their control cause cancer, the new analysis shows otherwise: People are firmly in charge of much of their own risk of cancer.

A man smoking a cigarette on December 3 2013. Jonathan Brady / PA Wire/Press Association Images

The team at Harvard Medical School calculated that 20 to 40 percent of cancer cases, and half of cancer deaths, could be prevented if people quit smoking, avoided heavy drinking, kept a healthy weight, and got just a half hour a day of moderate exercise.

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