Day 022 - 12 Sep 94 - Page 38


     
     1        which are specially bred to show susceptibility to a
     2        certain chemical which is then perhaps given to the rat,
     3        because otherwise you would be carry out your experiment
     4        for many, many years.  What you are trying to do is to run
     5        an experiment where you get an answer quite quickly.
     6        Therefore, you are using animals which are particularly
     7        susceptible to certain factors to which you are going to
     8        expose them.
     9
    10        You are also exposing the animals to these chemicals, for
    11        example, in much greater concentrations than, you know,
    12        you or I would ever be exposed to in normal life.
    13
    14        So, what one does not know is, for example, with chemical
    15        exposure, is there a threshold level?  Do you need to have
    16        more than a certain level of exposure before you will get
    17        an effect, or is there a continuing spectrum of dose and
    18        response?  You know, this is a problem which we have not
    19        been able to answer in lots of things.  For example, in
    20        radiation exposure; we try to keep radiation exposure to
    21        members of staff or the population as low as possible.
    22        But yet we are only able to demonstrate effects from
    23        radiation once the level gets to a certain point, the
    24        level of exposure gets to a certain point.  We do not know
    25        if the spectrum continues down below that particular level
    26        of exposure.
    27
    28   Q.   I think you said before just on that subject, for example,
    29        from Hiroshima it can take 30 years for cancer to develop?
    30        A.  It may do.  I mean, some things develop rapidly.
    31        Hiroshima was an extreme example, but for people who got
    32        relatively low levels of exposure then it may be 20 or 30
    33        years before you see a discernible effect.
    34
    35   Q.   So you would not place a reliance on animal experiments on
    36        whether something was healthy or unhealthy?
    37        A.  No.  You know, it is like a jigsaw puzzle; there are
    38        lots of pieces that have to fit into place before you have
    39        an answer at the end of it.  I think population studies
    40        are one piece of the jigsaw; I think animal experiments
    41        are another piece of the jigsaw, but they are only parts
    42        and they can only give you pointers.  It is rare for them
    43        to actually give you the complete answer.
    44
    45   Q.   Before I come to cohort which is obviously something you
    46        feel quite positive about, just on the genetic side:  You
    47        say that the genetic causal process has not been
    48        established?
    49        A.  Only in exceptional circumstances and this applies to
    50        cancer as a whole.  I have mentioned polyposis coli which 
    51        is an autosomal dominantly inherited abnormality.  These 
    52        people will go on to develop bowel cancer usually after 
    53        the age of 30.  But we do see, for example, cancer running
    54        in families.  We know, as I said this morning, the risk of
    55        developing cancer is increased substantially if you have a
    56        blood relative, a direct blood relative, who has had the
    57        disease.
    58
    59        Now, we have not been able to identify specific genetic
    60        abnormality which is responsible for that.

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