carcinogens can be broadly defined as Substances and exposures that can lead to cancer.
It should be noted that the American Cancer Society does not determine if something causes cancer (that is, if it is a carcinogen), but we do look to other respected organizations for help with this.
When a substance or exposure has been labeled a carcinogen, it means it has been studied extensively by researchers, and one or more agencies have evaluated the evidence and determined it to be a cause of cancer.
Cancer is the result of changes in a cell’s DNA – its genetic “blueprint.” Some of these changes may be inherited from our parents. Others may be caused by outside exposures, which are often referred to as environmental factors. Environmental factors can include a wide range of exposures, such as:
Carcinogens do not cause cancer in every case, all the time. Some clearly raise a person’s risk of one or more types of cancer. But even the strongest carcinogens don’t raise the risk of all types of cancer.
Substances labeled as carcinogens can have different levels of cancer-causing potential. Some might increase cancer risk after only a short exposure, but others might only cause cancer after prolonged, high levels of exposure. And for any particular person, the risk of developing cancer depends on many factors, including how they are exposed to a carcinogen, the length, and intensity of the exposure, and the person's genetic makeup.
Testing to see if something can cause cancer is often difficult. It isn’t ethical to test a substance by exposing people to it and seeing if they get cancer from it. Instead, scientists must use other types of tests, such as lab tests on cell cultures and animals, or epidemiology studies, which look at human populations. These types of tests might not always give clear answers.
There are far too many substances (both natural and man-made) to test each one, so scientists use what is already known about chemical structures, results from other types of lab tests, the extent of human exposure, and other factors to select chemicals for testing. For example, they can often get an idea about whether a substance might cause a problem by comparing it to similar chemicals that have already been studied.