The leading causes of an increased risk of cancer are lifestyle factors that we can
control, such as smoking, obesity, alcohol, and lack of physical activity. Another
important cause is random genetic mutations or “bad luck.” However, many people
are more afraid of exposure to carcinogens—the substances and exposures in our
environment that can lead to cancer. We tend to exaggerate risks that are not under
our control, for example, carcinogens in drinking water, as was dramatized in the
movie Erin Brockovich; even though these risks may be less important than risks
we can do something about, like quitting smoking or not drinking alcohol.

There is no way to avoid exposure to all known or suspected carcinogens. We encounter
ionizing radiation from natural sources, such as cosmic radiation from the
sun, and we use common medicines and chemicals that are probable carcinogens.
Carcinogens increase the risk of cancer, but their cancer-causing potential is quite
variable. Often, exposure to carcinogens does not cause cancer, but some carcinogens
may cause cancer after very small exposures, and others only after prolonged,
high levels of exposure. Many times, it is difficult to find out if a substance will
cause cancer in humans. Although, as a precaution, it is usually assumed that exposures
that cause cancer at larger doses in laboratory animals can also cause cancer in
humans, some chemicals have been classified as carcinogens erroneously.
When the evidence is conclusive, the substance is classified as a carcinogen. When
the available evidence is compelling but not considered conclusive, the substance
may be classified as a probable carcinogen. Because it is impractical to study all of
the thousands of chemical entities that exist, often there just isn’t enough information
to know how to classify a specific chemical and, of carcinogen candidates, most
are listed as being of probable, possible, or unknown risk.

The principal agencies responsible for determining the cancer-causing potential of
various substances are the WHO’s International Agency for Research on Cancer
(IARC) and in the U.S. the National Toxicology Program (NTP), the Environmental
Protection Agency (EPA) and several other U.S. federal agencies. The lists prepared
by these agencies only include those agents that have been evaluated, and the lists
do not present information about how likely it is that an agent will cause cancer.
IARC has evaluated the cancer-causing potential of more than 900 likely candidates,
placing them into one of the following groups:

• Group 1: carcinogenic to humans
• Group 2A: probably carcinogenic to humans
• Group 2B: possibly carcinogenic to humans
• Group 3: unclassifiable as to carcinogenicity in humans
• Group 4: probably not carcinogenic to humans
The National Toxicology Program (NTP) Report on Carcinogens has placed about
240 substances and exposures into two groups of agents:
• Known to be human carcinogens
• Reasonably anticipated to be human carcinogens

The U.S. Environmental Protection Agency (EPA) maintains the Integrated Risk
Information System (IRIS), an electronic database that contains information on human
health effects from exposure to certain substances in the environment. The EPA
uses a rating system similar to that of IARC:

• 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
Reports from the IARC and NTP have more detailed information and the American
Cancer Society website lists many chemicals known or probable carcinogens at
A few of the known human carcinogens that many people are familiar with are:

• Aflatoxins (from mold)
• Alcoholic beverages
• Asbestos (all forms) and mineral substances (such as talc or vermiculite) that
contain asbestos
• Engine exhaust, diesel
• Infection with Epstein-Barr virus
• Fission products, including strontium-90
• Formaldehyde
• Outdoor air pollution and the particulate matter in it
• Pesticides
• Radon-222 and its decay products
• Tobacco
• Ultraviolet (UV) radiation, including UVA, UVB, and UVC rays
• Ultraviolet-emitting tanning devices
• X- and gamma-radiation

There is evidence that pesticides used in agricultural, commercial, home, and garden
applications are associated with increased cancer risk. Pesticides include a
diverse group of chemical structures, and their uses include for control of insect
pests (insecticides), molds (fungicides), and unwanted plants (herbicides). Specific
chemical entities have been identified that are likely to increase the risk of
lymphoma, leukemia, multiple myeloma, prostate and breast cancers. The current
approach to reducing the risk of cancer is to minimize or eliminate exposure (e.g.,
by protecting farmworkers and choice of organically grown food) and to continue
research to better identify and eliminate the most dangerous pesticides.

This blog presents opinions and ideas and is intended to provide helpful general information. I am not engaged in rendering advice or services to the individual reader. The ideas, procedures and suggestions in that are presented are not in any way a substitute for the advice and care of the reader’s own physician or other medical professional based on the reader’s own individual conditions, symptoms or concerns. If the reader needs personal medical, health, dietary, exercise or other assistance or advice the reader should consult a physician and/or other qualified health professionals. The author specifically disclaims all responsibility for any injury, damage or loss that the reader may incur as a direct or indirect consequence of following any directions or suggestions given in this blog or participating in any programs described in this blog or in the book, The Building Blocks of Health––How to Optimize Your Health with a Lifestyle Checklist (available in print or downloaded at Amazon, Apple, Barnes and Noble and elsewhere). Copyright 2021 by J. Joseph Speidel.