We often hear that certain foods, such as processed meats, or activities, such as using a tanning bed, can increase people’s risk of getting cancer. What isn’t always clear is how these risks are determined and how substances, behaviors, and personal characteristics come to be thought of as “risk factors.”

Risk factors

Risk factors are the characteristics and behaviors that can increase people’s chances of getting cancer. There are four types of risk factors:

• Behavioral risk factors are behaviors or habits such as smoking, drinking alcohol, not exercising regularly, or not eating healthy foods.
• Biological risk factors are physical traits such as race, age, and sex.
• Genetic risk factors are specific gene mutations that people inherit from their parents. A person might have a genetic risk factor if he or she had several family members with the same type of cancer.
• Environmental risk factors are found in our surroundings, such as secondhand smoke, pollution, and pesticides. Viruses such as hepatitis B and C and the human papillomavirus (HPV) also are considered environmental risk factors.

What is risk?

When thinking of cancer specifically, “risk” is the chance of getting cancer for members of a particular group. The two types of risk are absolute risk and relative risk.

Absolute risk is the number of people who will be diagnosed with a type of cancer in a particular time period—for example, in the United States, about 120 of every 1,000 women will be diagnosed with breast cancer in their lifetime. The absolute risk of breast cancer for women in the United States is thus 12%.

Relative risk is a comparison of one group’s risk of getting a type of cancer to another group’s risk. The risk for each group is calculated using data from clinical trials or from agencies like the National Cancer Institute that keep track of cancer statistics. The groups could be assigned according to sex, age, or some other characteristic. A relative risk of 1.0 means the risk of developing cancer is the same for both groups—in other words, the characteristic being studied is not a risk factor for cancer.

A relative risk below 1.0 means people with the characteristic are less likely to get cancer than are those without the characteristic. In contrast, a relative risk above 1.0 means people with the characteristic have a greater risk of getting cancer than do those without the characteristic.

For example, if the relative risk for lung cancer is 20 for a group of smokers compared with a group of nonsmokers, we can conclude that smokers, as a group, are 20 times more likely to get lung cancer than nonsmokers. We can also conclude that smoking is a behavioral risk factor for lung cancer.

If the relative risk for colorectal cancer is 2.3 for a group of people with more than one first-degree relative (parent, brother, sister, or child) who had colorectal cancer compared with a group whose first-degree relatives did not have colorectal cancer, we can conclude that people in the first group are 2.3 times more likely to get colorectal cancer.

Risk factors and screening

It’s important to remember that these numbers don’t reflect any one person’s individual risk. Nevertheless, it’s important to be aware of the risk factors that might place you in a high-risk group for getting any type of cancer.

For example, keep note of your family’s cancer history, and keep your doctor informed of any changes in this history. This risk factor may affect your need to be screened for particular types of cancer. Likewise, being aware of behavioral or environmental risk factors, such as smoking or exposure to secondhand smoke, could help you avoid them. And people with exposure to environmental risk factors may be offered additional screening, such as low-dose computed tomography scans for people with a significant smoking history.

– M. Wade