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Identifying cancer-causing substances

To determine whether a substance increases the risk of cancer, scientists carefully review evidence from studies in people and in animals. Most often human studies involve exposures that occur at work, where exposure to potential cancer-causing substances is higher than in the community or at home.

Scientists doing this research usually look at 3 things:

  1. How much, how often and under what circumstances people or animals are exposed to a particular substance. Scientists are more confident that an exposure is directly related to cancer risk when:
    • the risk of cancer increases as exposure increases
    • the risk of cancer decreases as exposure decreases
  2. How strong and how consistent the relationship is between exposure and the risk of developing cancer. Evidence of a strong and consistent relationship with an exposure to a substance provides more support for its link to cancer. For example, studies that evaluate the relationship between tobacco use and lung cancer risk consistently show that frequent tobacco users are at much greater risk of developing lung cancer than those who do not use tobacco.
  3. How similar the findings are in human and animal studies. When human and animal studies come to similar conclusions, scientists have more confidence about the relationship between exposure to a substance and its risk of cancer.

Scientists consider all the scientific information to determine whether a substance causes, or could potentially cause, cancer.

When these 3 steps are followed, scientists often say they have considered the weight of scientific evidence or body of evidence.

After reviewing the scientific evidence, agencies like the World Health Organization’s International Agency for Research on Cancer and the United States National Toxicology Program draw a conclusion about the risk of cancer associated with exposure to a particular substance.

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