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Carcinogens and you
“Cancer” is a word that everyone has heard, but is not a condition that everyone
fully understands. And that, unfortunately, includes the medical profession.
There are many reasons for this, including the fact that your body’s reaction to
‘carcinogens’ (the name for cancer producing substances) is not necessarily the
same as the reaction of the person sitting next to you. Individual differences
do exist, and may even be inherited (genetic) influences.
Cancer is caused by abnormalities in a cell’s DNA (its genetic blueprint).
Abnormalities may be inherited from your parents, or they may be caused by
outside exposures such as chemicals, radiation, or even infectious agents
including viruses. Some of these conditions do not act on DNA directly, but
cause cancer in other ways, such as causing cells to divide at a faster rate.
All of these substances that can cause changes that can lead to cancer are
called ‘carcinogens’.
The difficulties in studying them come from the fact that known carcinogens do
not cause cancer in every case, every time. Substances classified as carcinogens
may have different levels of cancer-causing potential. Some may cause cancer
only after prolonged, high levels of exposure (remember the words of Paracelsus:
“Dosage alone determines poisoning”). And for any particular person, the risk of
developing cancer will depend on many factors, including the length and
intensity of exposure to the carcinogen and the person’s genetic makeup and
general health.
So just how do we classify any compound as being a carcinogen? With difficulty,
is the simple answer. The research boffins get much of their data about whether
or not something might be carcinogenic from laboratory (cell culture and animal)
studies. However, you have also to remember that Man is not a Large Rat (even
though certain divorced ladies might attest differently). It is not possible, on
animal studies alone, to pin the carcinogen rap on any particular compound. It
does however, give us an indication. Although it isn’t possible to predict with
absolute certainty which substances will be carcinogenic to humans based on
animal studies alone, virtually all known human carcinogens that have been
adequately tested in lab animals produce cancer in these animals.
Another problem comes from the fact that most studies of potential carcinogens
in lab animals expose the animals to doses that are far higher than common human
exposures. For most carcinogens, it is assumed that those that cause cancer at
larger doses in animals will also cause cancer in people. This produces the
concept, in some quarters, that it is reasonable for public health purposes, to
assume that lowering human exposure will reduce risk. Understandable logic, but
far from absolute.
Another way to identify carcinogens is through epidemiologic studies, which look
at the factors that might affect the occurrence of cancer in human populations.
While these studies also provide useful information, they also have their
limitations. Humans do not live in a controlled environment. People are exposed
to numerous substances at any one time, including those they encounter at work,
school, or home; in the food they eat; and the air they breathe (including
secondhand cigarette smoke). It is usually many years (often decades) between
exposure to a carcinogen and the development of a cancer. Therefore, it can be
very difficult to single out any particular exposure as having a definite link
to cancer.
The most widely used system for classifying carcinogens comes from the
International Agency for Research on Cancer (IARC), which is part of the World
Health Organization (WHO). The IARC has evaluated the cancer-causing potential
of about 900 likely candidates in the last 30 years, 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
There are around 90 carcinogens in Group 1, with most being referred to by long
chemical names such as 1-(2-Chloroethyl)-3-(4-methylcyclohexyl)-1-nitrosourea
(Methyl-CCNU; Semustine), however, there are ones you will recognize like solar
radiation, alcoholic beverages, analgesic mixtures containing phenacetin, salted
fish (Chinese-style) and tobacco smoke.
Yes, tobacco smoke - yours or the person next door! Stop now is the logical
response!
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