by Dr. Iain Corness
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)
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
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