Resources - Cancer FactFinder

The International Agency for Research on Cancer (IARC) determines whether exposures, compounds, nutrients, occupations, and other exposures are likely to cause cancer. They divide potential cancer-causing exposures into groupings based on the strength of the scientific evidence.

The IARC rating system is different than what is presented in the Cancer FactFinder because it considers a scientific review of specific exposures and occupations, while the FactFinder focuses on fewer topics that may be of interest to the general public, including topics that may represent misinformation.

The information provided by IARC may be difficult for the general public to understand, but it represents the best evidence available about whether a risk factor or exposure is related to cancer or not. Most of the topics studied by IARC are not in the FactFinder.

If you are exposed to anything on the IARC list that is likely to cause cancer, it does not mean you will get cancer. It only means is that there is evidence that you might be at increased risk of getting cancer, that we are not sure if the exposure causes cancer, and that more cancers would likely be seen in populations that are exposed. IARC does not have a category for “definitely does not cause cancer,” but the agency does note when a chemical has been well-studied without finding evidence that it causes cancer.

Usually, exposures are thought to cause one or more specific cancer types. For example, there is no doubt that cigarette smoking can cause lung cancer, so tobacco smoking is listed in “Group 1” as “carcinogenic to humans.”

Some things on the list are exposures we have all had: For example, sun exposure is known to cause skin cancer and is listed by IARC in Group 1. However, some sun exposure can have benefits (like getting enough vitamin D) and being out in the sun for exercise has health benefits, too. Therefore, the IARC groupings are not guidelines: They do not tell us if or when some limited exposure to things on the list may be OK, or how to guide our choices to weigh risks and benefits from the exposure.

International Agency for Research on Cancer’s carcinogen rating system

IARC group	IARC definition	Number of items per group	What it means	Examples
1	Carcinogenic to humans	121	Limit being exposed to these to reduce your cancer risk.	Alcoholic beverage consumption, diesel exhaust, sun exposure, air pollution, benzene, infection by HPV, tobacco smoking, and smokeless tobacco use
2A	Probably carcinogenic to humans	90	It is likely that being exposed can cause cancer.	Red meat consumption, night shift work, glyphosate (Round Up)
2B	Possibly carcinogenic to humans	322	There is some evidence that these cause cancer, but the data are not yet conclusive.	Gasoline, marine diesel fuel, firefighter occupation, dry cleaning occupation
3	Not classifiable as to its carcinogenicity to humans	498	There is not enough evidence to say whether these cause cancer or not. There may be evidence that suggests the exposure is not cancer-causing, or even reduces risk of some cancers (like coffee consumption). More work needs to be done to decide—-and there is not sufficient evidence to change your lifestyle.	Home hair coloring products, coffee consumption, caffeine, chlorinated drinking water, tea, table salt, fluorescent lighting

See IARC’s full list of carcinogens