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.
The World Health Organization, through its agency called IARC, uses 4 classes to describe its conclusions after reviewing the scientific evidence:
IARC publishes its scientific evaluations of these substances in a series of in depth reports called monographs.
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:
The NTP publish their classifications in the regularly updated Report on Carcinogens.
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: