Research has established links between cancer and various lifestyle factors, chemicals produced in the body, or that enter the human body, and chemicals in the environment.
Scientists have established several causal mechanisms for risk factors.
Below is an example of how risk factors have been found. This has come from studies on smoking cigarettes and lung cancer.
Historically, in the USA, a pattern can be seen between the number of cigarettes smoked and the number of lung cancer deaths.
As the number of cigarettes smoked has increased over the years, the incidence of lung cancer has increased also. Note that there is a time lag, because cancer usually takes some years to develop.
There is a clear association, called a correlation, between the variables. As one increases, so does the other.
With cancer and other non-communicable diseases, scientists have found correlations. They then try to find the specific cause of the correlation.
If there is a correlation between a particular factor and an outcome, it does not mean that the factor necessarily causes the outcome. Scientists must look for a possible causal mechanism by which the factor could be the likely cause.
In the case of lung cancer, scientific analyses of cigarette smoke have shown that at least 70 of the chemicals present in smoke will cause cancer in laboratory animals. This establishes a causal link.