Report from the Sub-comittee on the environment and health

Cancer
Mechanism behind <strong>the</strong>
development of cancer
Cancer cannot be
predicted
Findings in
epidemiological studies
124
infertility without it being possible to point directly to an oestrogenous
effect. As stated earlier, several studies of individuals with a high
occupational exposure have shown negative effects on <strong>the</strong> reproductive
system and particularly on fertility. However, in <strong>the</strong> case of <strong>the</strong> general
population, <strong>the</strong> data are much less certain.
In 1995, DEPA published a report that concluded that sperm quality in
o<strong>the</strong>rwise healthy men had been falling since <strong>the</strong> end of <strong>the</strong> 1930s
(DEPA 1995a). In <strong>the</strong> same period, a marked increase in testicular cancer
was recorded, particularly in younger men. An increased frequency of
certain deformities of <strong>the</strong> sexual organs of boys also seemed to have
occurred. However, <strong>the</strong>re was insufficient documentation to determine
whe<strong>the</strong>r (early) exposure to pesticides could have been a cause of <strong>the</strong>se
effects on <strong>the</strong> reproductive system. In <strong>the</strong> case of most chemical
pollutants in our environment, it is unknown 1) whe<strong>the</strong>r <strong>the</strong>y have an
oestrogenous effect or not, 2) how big <strong>the</strong>ir effect is, individually or
combined, and 3) <strong>the</strong> actual magnitude of <strong>the</strong> exposure (Editorial 1995).
For a number of pesticides previously in use <strong>the</strong>re is evidence of
carcinogenicity in animals, whereas arsenic and mixtures containing
arsenic are <strong>the</strong> only pesticides classified by IARC (International Agency
for Research on Cancer) for which <strong>the</strong>re is sufficient evidence of
carcinogenicity in humans, based on an increased risk of lung cancer and
skin cancer.
There are currently some few pesticides on <strong>the</strong> market that are classified
as carcinogenic in group 3 (Carc 3), see section 8.1 and Lindhart et al.
(1998).
The carcinogenic effect can be exercised in different ways. Some
pesticides affect <strong>the</strong> cells’ DNA and, in <strong>the</strong> worst scenario, induce
precisely <strong>the</strong> changes that enable <strong>the</strong> cells to develop malignant
properties (initiators). O<strong>the</strong>r pesticides can cause cancer to develop by
stimulating initiated cells to divide fur<strong>the</strong>r (promotors). In relation to
pesticides, attention has been paid particularly to <strong>the</strong>ir hormone-like
effect, since oestrogens, for example, can promote hormone-dependent
initiated cells (e.g. mammary gland cells). However, pesticides may also
affect <strong>the</strong> development of cancer in o<strong>the</strong>r and more indirect ways, for
example by inhibiting specific parts of <strong>the</strong> immune system.
One big difficulty in investigating a relationship between exposure to an
environmental factor and <strong>the</strong> risk of cancer is that cancer often takes a
very long time to develop – up to 20 years and perhaps even longer.
Despite intensive research, valid biomarkers for early stages of cancer
have not been identified with certainty.
There are only a few epidemiological studies of <strong>the</strong> relationship between
<strong>the</strong> risk of cancer and exposure to pesticides in <strong>the</strong> general population.
Most of <strong>the</strong> studies have focused on <strong>the</strong> relationship between <strong>the</strong> level of
organochlorine compounds in <strong>the</strong> blood and <strong>the</strong> risk of breast cancer. In
1993, a study <strong>from</strong> New York aroused some interest (Wolf et al. 1993).
In this study, an approximately 4 times higher frequency of breast cancer
was found in women with a high blood concentration of DDE and PCB.
Dieldrin was not measured. In 1994, a similar study was carried out in
California, and <strong>the</strong>re, <strong>the</strong> relationship was not nearly as strong, even