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Classifying cancer-causing substances

Various international agencies follow a scientific process and consider the weight of scientific evidence to determine whether a substance or chemical causes cancer. They have different ways of describing how clear and strong the scientific evidence is, and how confident scientists are about the risk associated with exposure.

International Agency for Research on Cancer (IARC)

The World Health Organization, through its agency called IARC, uses 4 classes to describe its conclusions after reviewing the scientific evidence:  

  • Class 1: exposure causes cancer in humans
  • Class 2A: exposure probably causes cancer in humans
  • Class 2B: exposure possibly causes cancer in humans
  • Class 3: scientists are unable to determine or classify whether exposure does or does not cause cancer in humans
  • Class 4: exposure probably does not cause cancer in humans

IARC publishes its scientific evaluations of these substances in a series of in depth reports called monographs.

National Toxicology Program (NTP)

The NTP in the United States follows a similar process to describe whether a substance causes cancer in humans. Following the review of animal and human scientific information, NTP scientists classify substances as either: 

  • known to be human carcinogen
  • reasonably anticipated to be human carcinogen

The NTP publish their classifications in the regularly updated Report on Carcinogens.

United States Environmental Protection Agency (EPA)

The United States EPA reviews pesticides and other toxic chemicals for likelihood that they cause cancer. Based on reviews of animal studies, it classifies substances in one of 5 categories: 

  • carcinogenic to humans
  • likely to be carcinogenic to humans
  • suggestive evidence of carcinogenic potential
  • inadequate information to assess carcinogenic potential
  • not likely to be carcinogenic to humans


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