Methods and Responsible Agencies
A carcinogen is a substance that has been scientifically proven to cause cancer in humans. There are literally hundreds of known or suspected carcinogens in the environment around us, each of them with its own unique level of risk and types of cancer-causing exposures. Cataloging large numbers of carcinogens in a scientifically meaningful way is a task that has been undertaken by numerous private and governmental organizations, though, two of the most widely recognized carcinogen classifying agencies are the International Agency for Research on Cancer (IARC), and the United States National Toxicology Program (NTP).
Classifying a particular substance as a carcinogen requires a scientific understanding of the substance’s biological actions that cause malignant disease. This is a highly complex task that begins with the researcher’s knowledge of the fact that cancer occurs when the body experiences highly accelerated and uncontrolled cell division. Normal cell replication is interrupted by changes in the cell’s DNA, mutations that can be the result of exposures to environmental toxins. These exposures can be behaviorally related through voluntary actions such as tobacco use or occupational in nature if (for example) an individual worked for years in an asbestos mine. Behavioral and occupational exposures to carcinogens aside, cancer-causing toxins can also exist in the air, the water, food supplies, and elsewhere.
Carcinogen Classification Determinations
Labeling a particular substance as a carcinogen can be a difficult task. Obviously, scientists are unable to test a suspected substance on humans to determine if it causes cancer, therefore, other methods must be used. Much of the data on suspected or proven carcinogens is obtained in a laboratory where human cell culture and animal testing provides much of the clinical data that supports a contention that a particular substance causes cancer.
Laboratory studies of suspected carcinogens can result in inconclusive results due to an almost infinite number of variables related to exposure type, longevity, intensity, etc. Carcinogen studies can be further complicated by the myriad number of supporting environment and individual physiological variables that can dramatically affect the outcome of carcinogen related scientific investigations.
Epidemiology Plays a Limited Role
In some instances, cancer/carcinogen researchers will examine specific populations in order to determine if a suspected substance causes cancer. Studying classes of people while identifying and quantifying common exposures within that group can lead to valuable information, but, again, the sheer number of human and environmental variables will limit the overall value of such studies. Scientists can only gather as much data as they possibly can before reaching a reasonably supported conclusion relevant to a suspected cancer-causing agent. In many cases, scientists can only present what is a highly educated guess as to whether or not a particular substance should be classified as a carcinogen.
As part of the World Health Organization, the IARC is responsible for the development of the most widely accepted carcinogen classification system. Over the past 30 years, the IARC has studied over 900 suspected cancer-causing agents, each of which has been placed into one of the five groups that are noted below:
- Group 1: Definitively proven to be carcinogenic to humans
- Group 2A: Most likely or probably carcinogenic to humans
- Group 2B: Possibly causes cancer in humans
- Group 3: Not classifiable as a cancer causing agent in humans
- Group 4: Most likely not a carcinogen in humans
Allied with the federal Centers for Disease Control, National Institutes of Health, and the Food and Drug Administration, the NTP issues a carcinogen classification report every two years. The NTP Report on Carcinogens classifies substances in one of the two ways noted below:
- Known to be human carcinogens
- Reasonably anticipated to be a human carcinogen