Sorry, I was way behind in my e-mail and did not get around
to reacting to the thread on chemicals and PD.
As some might recall, I posted a long-ish message on
pesticides and PDs on October 30. The bottom line: there are
three pesticides that have substantial evidence of causing
PD: paraquat, rotenone, and maneb. There is also substantial
evidence from epidemiological studies that something about
rural living is associated with higher rates of PD, but
there is nothing that convincingly shows that it is exposure
to pesticides that is the cause, rather than any number of
other possibilities.
Because I was concentrating on pesticides, I did not mention
two other chemicals known to cause Parkinson's Disease: MPTP
(an impurity in street drugs) and manganese. But that's it:
there are no other chemicals for which there is reliable
evidence that they are a cause of PD. It may well turn out
that there are others, but we have no evidence what they
are.
In the current thread, folks are writing in to say: I (or my
spouse, parent, etc.) was exposed to X, and I'm convinced
that that's what caused my/his/her PD. Candidates for X that
have
been mentioned include:
Nerve agents
Fertilizer
Bug spray
Radiation (type unspecified)
Working in a gas station
Any chemicals capable of crossing the blood-brain
barrier
DDT
Dyestuffs
Anesthetics
Antidepressant
Lindane
Artificial sweetener
This long and varied list has one thing in common: they all
have
an extensive scientific literature on their effects on human
health, none of which demonstrates that they cause PD to any
substantial degree.
It could turn out in the future, of course, that some of
them may be significant causes of PD, but there is no
evidence for that.
The people who tell themselves "It must have been X" have
committed one of the classical errors in logic, so old that
there is a Latin phrase for it: post hoc, ergo propter hoc
("after this, therefore because of this").
There is only one way we can have reliable knowledge of
causation, through the scientific method, which requires
replicated studies with a reasonably large and unbiased
exposed group and control group (not a complete description,
but you get the idea). If you observe one individual develop
a disease after a particular exposure, you can formulate a
hypothesis that that may be the cause, but without
well-designed studies confirming it, it will remain just a
hypothesis.
So, except for those who have a family history of PD or who
have been exposed to one of the handful of chemicals listed
earlier, none of us have any basis for believing that this
or that is the cause of our PD or even a significant
contributor to development of the disease. Of course, that
does not stop us (myself included) from trying to answer the
question, why did this happen to me? There is a
psychological need to try and answer this question, but we
need to recognize that we do not have a scientific basis for
the answers we develop for ourselves. We need to resist the
temptation to base public policy on these subjective
grapplings.
I will try to comment on the database issue in the next few
weeks.
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