Carcinogens do not cause cancer at all times, under all circumstances. In other words, a carcinogen does not always cause cancer in every person, every time there is any kind of exposure.
Some may only be carcinogenic depending on how one is exposed to a carcinogen (for example, swallowing it as opposed to touching it).
Some may only cause cancer in people who have a certain genetic makeup (sometimes also called genetic predisposition). A genetic predisposition results from specific genetic variations that are often inherited from a parent. These genetic changes contribute to the development of a disease but do not directly cause it. Some people with a predisposing genetic variation will never get the disease while others will, even within the same family.
And for any particular person, the risk of developing cancer depends on the length and intensity of the exposure. Some of these agents may lead to cancer after only a very small and short exposure, while others might only cause cancer after prolonged, high levels of exposure.
Determining whether or not something can cause cancer is often difficult. This is made more difficult by the fact It isn’t ethical to test a substance by exposing people to it and seeing if they get cancer from it. So, scientists must use other types of tests, such as lab tests on cell cultures and animals, or epidemiology studies which look at human populations; however, they are not always clear nor reliable.
Lab studies alone can’t always predict if a substance will cause cancer in people. For example, the effects seen in lab studies with very high doses of a substance may not be the same at much lower doses, or the effects of a substance when it is inhaled may not be the same as if it is applied to the skin. Also, the bodies of lab animals and humans don’t always process substances in the same way. Nonetheless, almost all carcinogens are first tested on and found to cause cancer in lab animals before they are later found to cause cancer in people.
Epidemiology studies looks at different groups of people to determine which factors might be linked to cancer. Although it also provides useful information, they have their limits. For example, humans don’t live in a controlled environment. People are exposed to all kinds of substances at any given time, including those they encounter at work, school, or home, in the food they eat and in the air they breathe. These make it very hard to determine which of these factors might be linked to cancer.
By combining data from both types of studies, scientists endevours to make an educated assessment of the potential of a substance to cause cancer.