What is a Carcinogen?

Types and Rating of Carcinogens in Our Environment

charred meat as an example of a carcinogen (heterocyclic amines)
What is a carcinogen and how can you know if a substance causes cancer?. istockphoto.com

You may have heard that a substance is a carcinogen or is carcinogenic. What does this mean?

Definition of a Carcinogen

A carcinogen is defined as something that can directly cause cancer. This can be a chemical substance, a virus, or even the medications and radiation we use to treat cancer. While many cancers are caused by a carcinogen or combination of carcinogens, the tendency to develop cancer may also be inherited as part of our genome.

Carcinogens may work in a few ways:

  • A carcinogen may directly damage the DNA in cells (cause mutations,) which in turn leads to a disruption in the normal process of cells.
  • The carcinogen may instead cause damage and inflammation which results in the cells dividing more rapidly. When cells divide there is always a chance that a change will occur (a mutation) which in turn increases the chance of developing a cancer.

Types of Carcinogens and Carcinogenicity

We are around carcinogens every day, whether at work, at home, or at play. Carcinogens do not cause cancer in everyone who is exposed; the ability of a carcinogen to cause cancer depends on many factors, including the amount of exposure, the length of exposure, the health of the individual, and other factors in the person's life that either raise or lower the risk of cancer. People also differ in personal susceptibility to a carcinogen based on their genetic makeup.

 In many cases cancer is multifactorial, meaning that there are several factors that work together to either cause or prevent cancer. Types of carcinogens include:

  • Certain chemicals in the home or workplace.
  • Environmental radiation – Ultraviolet radiation from the sun is a well-known cause of skin cancer. Radon emitted from the normal decay of uranium in the soil and then trapped in homes is a leading cause of lung cancer
  • Medical radiation - Both radiation used medically for diagnostic tests and that used to treat cancer are considered carcinogens. For example, women who receive radiation therapy after a mastectomy for breast cancer are at an increased risk of developing lung cancer due to the carcinogenicity of radiation.
  • Viruses - Viruses such as human papillomaviruses which cause oral cancer and cervical cancer, and hepatitis C which can cause liver cancer are considered carcinogens. Check out the other viruses which are thought to cause cancer.
  • Some medications - Some chemotherapy drugs and hormonal therapy can raise the risk of cancer.
  • Lifestyle factors – Smoking and obesity are both carcinogens in that they can be responsible for the mutations which result in cancer.

Latency Period

An important to understand is the concept of a latency period. This is the time between exposure to a carcinogen and the time a cancer develops and could be very short, such as exposures to radiation in a nuclear disaster, or decades depending upon the particular carcinogen.

Determining if a Substance or Exposure is a Carcinogen/Testing for Carcinogens

It is not always easy to determine if a substance or an exposure is a carcinogen.

A good example of this is smoking. It took many years of research and millions of dollars to determine the relationship of smoking to lung cancer. Many studies to evaluate substances for carcinogenicity are done on animals using high exposures. Prior to animal testing, many of these substances are first looked at in cell cultures in a lab. While it would be unethical to test substances for carcinogenicity in humans, retrospective studies looking at people with cancer, and evaluating prior exposures, are used to analyze substances or exposures to evaluate the ability to cause cancer.

Rating of Carcinogens/Classification

There are several systems in place that define carcinogens in slightly different ways:

The Environmental Protection Agency:

  • Group A: Carcinogenic to humans
  • Group B: Likely to be carcinogenic to humans
  • Group C: Suggestive evidence of being carcinogenic to humans
  • Group D: Inadequate information to assess carcinogenicity
  • Group E: Not likely to be carcinogenic to humans

International Agency for Research on Cancer:National Toxicology Program:

  •   Group A: Carcinogenic to humans
  • Group B: Likely to be carcinogenic to humans
  • Group C: Suggestive evidence of carcinogenic potential
  • Group D: Inadequate information to assess carcinogenic potential
  • Group E: Not likely to be carcinogenic to humans

National Toxicology Program

  • Known to be carcinogenic to humans
  • Reasonably anticipated to be carcinogenic to humans

Safety Precautions – When You are Uncertain

Keep in mind that every substance that is a potential carcinogen has been tested. Not only are there millions of possible carcinogens both in nature and industry, but it’s simply not practical to test every chemical on hundreds of thousands of people (or ethical.) For that reason, it's important to practice discretion with any potential carcinogens to which you may be exposed:

  • Read labels, and check out ingredients you are not familiar with.
  • Follow recommended procedures when working with chemicals on the job.
  • Consider alternatives to substances with long lists of ingredients: for example, I use vinegar, lemon juice, olive oil, and baking soda as the staples to clean my home.
  • Consider options.  For example, check out these ideas on how to grill food to reduce carcinogens.

Databases of Carcinogens

There are several databases in which you can look up chemicals and substances you are exposed to in order to determine their carcinogenicity.  


International Agency for Research on Cancer. Evaluation of Carcinogenic Risks to Humans. Accessed 05/22/16. http://monographs.iarc.fr/

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