What are the main risk factors for cancer?

The National Cancer Institute lists the most-studied known or suspected risk factors for cancer as:

• Age.
• Alcohol.
• Cancer-causing substances.
• Chronic inflammation.
• Diet.
• Hormones.
• Immunosuppression, meaning a weakened immune system.
• Infectious agents.
• Obesity.
• Radiation.
• Sunlight.
• Tobacco.

Some of these can be avoided. Others, such as growing older, cannot. Limiting your exposure to the avoidable ones may lower your risk of certain cancers.

What a risk factor is doing

Cancer starts with changes in genes. The NCI traces those changes to three sources. Some are errors that happen as cells divide. Some come from things in the world that harm DNA, such as the chemicals in tobacco smoke and the sun's rays. Some are inherited from your parents.

Nearly every item above works through one of the first two. Tobacco, radiation, sunlight, and cancer-causing substances harm DNA directly. Long-term swelling and extra body weight push cells to divide more often. Each division is one more chance for a copying error. A weak immune system removes some of the watch that clears odd cells.

Age tops the list for a plain reason. More years mean more cell divisions. They also mean more time for exposures to add up.

Risk is not the same as cause

This is the part that gets lost. Having one or more risk factors does not mean you will get cancer. Having none does not mean you will not. As the NCI puts it, it is usually not possible to know why one person develops cancer and another does not.

These lists come from studies of large groups over many years. They do not explain any one case. They also cannot be run backward to assign blame after a diagnosis.

How to read the next headline

Cancer risk gets reported badly with real regularity. The NCI itself warns against drawing wrong conclusions from news coverage of research.

Two questions cut through most of it. First, is the number a relative increase or an absolute one? Doubling a very small risk still leaves a very small risk. Second, did the study follow people forward in time, or ask them later to recall what they ate and did? The second kind is much weaker evidence.

Where family history fits

A family history of certain cancers can be a sign of an inherited cancer syndrome. Take it seriously if relatives were diagnosed young. Take it seriously if the same cancer shows up in several relatives, if one person had more than one cancer, or if a rare cancer appears.

If that sounds like your family, ask your doctor about genetic counseling. An inherited syndrome changes screening schedules. It can also open preventive options, for you and for your relatives.

What to do with the list

Do not try to fix all twelve. Pick the one or two with the biggest lever in your own life. The avoidable factors are not weighted the same for everyone.

Ask which vaccines apply to you, since some infections linked to cancer can be prevented. Keep up with the screening tests recommended for your age and history, because screening catches cancers that risk reduction does not prevent. And bring your family history to your next visit in writing, with ages at diagnosis where you know them.