The lists themselves say nothing about how likely it is that an agent will cause cancer. Carcinogens do not cause cancer at all times, under all circumstances. Some may only be carcinogenic if a person is exposed in a certain way (for example, swallowing it as opposed to touching it). Some may only cause cancer in people who have a certain genetic makeup. Some of these agents may lead to cancer after only a very small exposure, while others might require intense exposure over many years. Again, you should refer to the agencies' reports for specifics.
Even if a substance or exposure is known or suspected to cause cancer, this does not necessarily mean that it can or should be avoided at all costs. For example, exposure to ionizing radiation is known to cause cancer, with increased risks even at low levels of exposure. Yet there is no way to completely prevent exposure to natural sources of radiation such as cosmic radiation from the sun or radon in soil. These lists also include many commonly used medicines, particularly some hormones and drugs used to treat cancer. For example, tamoxifen increases the risk of certain kinds of uterine cancer but can be very useful in treating some breast cancers, which may be more important for some women. If you have questions about a medicine that appears on one of these lists, be sure to ask your doctor.
Looking at the list below can tell you whether or not something may increase your risk of cancer, but it is important to try to get an idea of how much it might increase your risk. It is also important to know what your risk is to begin with. Many factors can enter into this, including your age, gender, family history, and lifestyle factors (tobacco and alcohol use, weight, diet, physical activity level, etc.). As noted above, the type and extent of exposure to a substance may also play a role. You should consider the actual amount of increased risk when deciding if you should limit or avoid an exposure.
