Cancer is caused by changes in a cell's DNA – its genetic "blueprint." Some of these changes may be inherited from our parents, while 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:
Medical treatments (radiation and medicines including chemotherapy, hormone drugs, drugs that suppress the immune system, etc.)
Substances and exposures that can lead to cancer are called carcinogens. Some carcinogens do not affect DNA directly, but lead to cancer in other ways. For example, they may cause cells to divide at a faster than normal rate, which could increase the chances that DNA changes will occur.
Carcinogens do not cause cancer in every case, all the time. Substances labeled as carcinogens may have different levels of cancer-causing potential. Some may cause cancer only 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.
Unfortunately, many of the substances and exposures on the lists below can often go by different names. This can make it hard to find a particular substance on one or both of these lists, which are in alphabetical order and may not always use the most common term.
These lists include only those agents that have been evaluated by the agencies. These agencies tend to focus on substances and exposures most likely to cause cancer, but there are many others that have not been studied fully yet.
Some of these agents may lead to cancer after only a very small exposure, while others might require intense exposure over many years.