When scientists talk about risk, they're referring to a probability — the chance that something may occur, but not a guarantee that it will. For example, if you flip a coin, there is one chance in two, or a 50 percent chance, that the coin will land heads up.
Risk estimates for cancer and other diseases are determined by studying large groups of people. Researchers focus on the probability that any person or category of people will develop the disease over a certain period of time. They also look to see what characteristics or behaviors are associated with increased or decreased risk.Most cancer risk (and protective) factors are initially identified in epidemiology studies. In these studies, scientists look at large groups of people and compare those who develop cancer with those who don’t. These studies may show that the people who develop cancer are more or less likely to behave in certain ways or to be exposed to certain substances than those who do not develop cancer.Such studies, on their own, cannot prove that a behavior or substance causes cancer. For example, the finding could be a result of chance, or the true risk factor could be something other than the suspected risk factor. But findings of this type sometimes get attention in the media, and this can lead to wrong ideas about how cancer starts and spreads
When many studies all point to a similar association between a potential risk factor and an increased risk of cancer, and when a possible mechanism exists that could explain how the risk factor could actually cause cancer, scientists can be more confident about the relationship between the two.
The list below includes the most-studied known or suspected risk factors for cancer. Although some of these risk factors can be avoided, others—such as growing older—cannot. Limiting your exposure to avoidable risk factors may lower your risk of developing certain cancers.It is usually not possible to know exactly why one person develops cancer and another doesn’t. But research has shown that certain risk factors may increase a person’s chances of developing cancer. (There are also factors that are linked to a lower risk of cancer. These are sometimes called protective risk factors, or just protective factors