Carcinogens

What is definition of carcinogens?
What are mechanisms of actions?
What are the sources of carcinogens?
How can we identify and classify carcinogens?

What is definition of carcinogens?
It is any cancer-causing substance, chemical or physical, such as benzene, cadmium, asbestos or certain types of radiation. The term cancer refers to a type of neoplasm, defined as a new growth (plasia). Cancer is a type of neoplasm characterized by tissue undergoing rapid and abnormal growth. Tissues that grow faster tan the surrounding tissue form an enlarged mass of cells referred to as tumor.

What are mechanisms of actions?
How toxic chemicals or physical agents cause cancer is still not clearly understood. The toxic substance or agen may interact directly with the DNA or affect DNA replication. These types of toxic substances are referred to as genotoxic carcinogens.

The rate at which cells divide and multiply is determined by the genetic information of the cell. Genotoxic carcinogens interact with the DNA and produce mutation of the genes controlling cell division. As a result, uncontrolled growth of nondifferentiated cells occurs, forming a tumor in the affected tissue. The development and growth of tumors involves three distinct stages: initiation, promotion and progression. 

What are the sources of carcinogens?
Almost every aspect of life provides an opportunity to become exposed to chemical and physical agents that are carcinogens. We may encounter them in the workplace, household products we use, the food we eat or in our surrounding. Some of these substances are known to be carcinogenic, some others are potentially carcinogenic, while the remaining are unknown.

LIfestyle may also affect the development of cancer, say habit in using tobacco in cigarette. It contains tobacco. Tobacco products are known to be the best studied and documented substances in relation to their carcinogenesis. The number of individuals who develop some form of cancer as a result of the use of tobacco varies according to the study. However, it has been generally estimated that around 90% of all lung cancers are the result of tobacco use and around 30% of all cancer deaths can be attributed to use of tobacco products. Cessation of use results in decreased frequency of cancer occurence.

A number of chemical substances found in the workplace have been associated with the development of cancer. Exposure to asbestos fibers causes two kinds of cancer: bronchogenic carcinoma (a type of lung cancer) and malignant mesothelioma (associated with inhalation of asbestos fibers).

How can we identify and classify carcinogens?
The National Toxicology Program of the US Public Health Service produces a regularly updated list of carcinogens. The International Agency for Research on Cancer (IARC) was one of the first organizations to identify and classify chemicals according their carcinogenicity. Environmental Protection Agency (EPA) and the Chemical Manufacturers Association have also developed lists and classification schemes for carcinogenic susbtances. The American Conference of Governmental Industrial Hygienists (ACGIH) and the Occupational Safety and Health Administration (OSHA) are concerned with exposure of individuals to carcinogens in the workplace.

Because there are several organizations involved in identifying and classifying carcinogens, there are multiple lists. Often these lists vary in length for the same category (i.e., known carcinogen) but they may not contain the same substances. This complicates the process of effectively communicating the level of risk associated with chemical exposure.

The National Toxicology Program – Department of Health and Human Services
International Agency for Research on Cancer
Environmental Protection Agency
Chemical Manufacturers Association
The American Conference of Governmental Industrial Hygienists (ACGIH)
Occupational Safety and Health Administration (OSHA)

Reference:
Kent, C. 1998. Basics of Toxicology. John Wiley & Sons, Inc. New York